Chapter 4 Exam

4.1 The classification of marine organisms One way to make sense of the enormous biotic variety found in our oceans is using a taxonomic hierarchy. A taxonomic hierarchy classifies aquatic life into taxonomic groups based on the features, relationships and evolutionary pathways that they share. Taxonomic hierarchy sorts each species into a series of similar groups of organisms or taxa. Similar species are grouped into a genus (plural: genera) which are in turn linked into a family, an order, a class, a phylum (plural: phyla), a kingdom, and then a domain. Figure 4.1 shows the taxonomic hierarchy for the blue shark, Prionace glauca. Figure 4.1: Taxonomic hierarchy for the blue shark, Prionace glauca. We give each marine organism a name in Latin composed of two parts, the genus followed by the species. This naming system is called binomial nomenclature. It was first formulated in 1736 by Carolus Linnaeus, a Swedish botanist. The genus is given a capital letter, whereas the species is always lower case. In print, a binomial name always appears in italic. For example, the Latin binomial name for the Galápagos penguin is Spheniscus mendiculus. When you write a binomial name by hand, you should underline it (for example Spheniscus mendiculus). When marine scientists try to determine which species a specimen belongs to, they often use a dichotomous key. A dichotomous key, such as Table 4.1, is a series of choices between alternative characteristics, with a direction to another stage in the key. Choices are made until the species is identified. You can use Table 4.1, together with Figure 4.2 which shows the characteristic features of a blue shark, to identify the blue shark as shark species F. Step Characteristic Common name 1a 1b Kite like shape to body … Go to 2 Non kite like shape to body… Go to 3 2a 2b Hornlike appendages on snout No hornlike appendages on snout Shark spp A Shark spp B 3a 3b Six gill slits Five gill slits … Go to 4 Shark spp C 4a 4b Spines on dorsal fin No spines on dorsal fin … Go to 5 Shark spp D 5a 5b Long point on end of snout No long point on end of snout … Go to 6 Shark spp E 6a Big eyes surrounded by a small ring Shark spp F 1 2 a b c d e • • • 6b Eyes on end of hammer-like projection Table 4.1: Dichotomous key for shark identification. Figure 4.2: (a) Photograph of a blue shark; (b) Biological drawing of a blue shark, Prionace glauca. CORE PRACTICAL ACTIVITY 4.1: MARINE BIOLOGY OBSERVATIONS AND DRAWINGS Introduction When investigating life in the ocean, it is important that marine scientists can make biological drawings of either live specimens or from photographs of a specimen. To do this you do not have to be a great artist as the aim is simply to produce a clear outline of the main biological features. Before you start Discuss in pairs why marine scientists make biological drawings. Make a list of the external features that you may be able to label from each of the following marine groups: Echinoderm Bony fish Cartilaginous fish Macroalgae Marine plant. Equipment Before you start, ensure you have all the equipment you will need: a sharp hard (minimum HB) pencil eraser, pencil sharpener and ruler plain paper. Method Use the photograph of a stickleback fish in Figure 4.3 to make a biological drawing: Figure 4.3: Photograph of a stickleback fish. 1 2 3 4 5 6 7 8 9 1 2 1 2 a Write the title of the specimen at the top of a piece of plain paper. Begin drawing by using a sharp HB pencil to make an outline of the specimen. The drawing should be at least half the size of the space provided, with room on each side for labels if required. No shading is permitted so only use smooth continuous bold lines to show the features. If a drawing error is made then an eraser can be used to make corrections. Try to ensure that the scale of the image is appropriate (for example, the length of the tail is in the correct ratio to the length of the whole fish). You need to look very carefully at the specimen if you are to be scientifically accurate with your drawing. An example could be whether the stickleback has two or three spines. Try to also ensure that the position and size of an observable feature is biologically correct (for example, where do the dorsal fins start compared to the anal fin?) Labels are a good way to name and describe characteristics visible in the specimen. Labels should be printed clearly and in ink. The labels should be to the sides of the image and the label lines should be horizontal. Label lines should be drawn in pencil with a ruler and you need to ensure they touch the site being identified. Add the following labels to your diagram: Operculum, Lateral line, Pectoral fin, Caudal fin, Anal fin, Pelvis fin, Dorsal fin, Dorsal spines, Eye, Mouth. If you are simply asked to ‘add labels’ then the name of an observed feature is all that is required. In annotated diagrams, the label should however also include an explanation for how that feature is an adaptation for living in a marine ecosystem, such as: ‘The operculum is essential for ventilation as it is a bony flap that covers and protects the gills.’ Results The results for this experiment should be a labelled diagram. It should be large, drawn in pencil and labelled with ruled label lines. There should be no sketching or shading. Evaluation and conclusions Explain what makes a good biological drawing. Suggest why biological drawings are different from an artistic representation of the organism. REFLECTION After completing Core Practical Activity 4.1, swap your biological drawing with another member of the class. How do they compare? How could they be improved? Test yourself Complete the spaces in a copy of Table 4.2 to describe the taxonomic hierarchy for the Galápagos penguin, Spheniscus mendiculus. Taxonomic hierarchy Galápagos penguin Eukarya Animalia Chordata Aves Sphenisciformes Spheniscidae Table 4.2: Figure 4.4 is a biological drawing of a crayfish. List three features of the drawing that are good and three features that could be improved. b Figure 4.4: Biological drawing of a crayfish. Draw a diagram of a coral polyp from Figure 4.5. Add labels to indicate the location of the following structures: mouth, tentacle, nematocyst. Figure 4.5: Coral polyp. • • • • • • • • 4.2 Key groups of marine organisms There is an enormous variety of marine organisms ranging in size from microscopic viruses, bacteria and plankton, through to the largest animal on Earth, the blue whale, that can weigh up to 180 000 kg and is more than 30 m in length. Marine producers include phytoplankton, macroalgae and marine plants. They are crucial to the marine food web as they photosynthesise to transform light energy from the Sun into chemical energy (food energy) used by marine consumers. Each trophic group plays a crucial ecological role within the marine web of life. In this section we will explore a sample of aquatic life by comparing the characteristics of eight groups of marine organisms: phytoplankton zooplankton echinoderms crustaceans bony fish cartilaginous fish macroalgae marine grasses. We will also explore the ecological and economic importance of each group. We will use a range of specific examples of organisms that play pivotal roles in the ecology of their aquatic habitats: crown of thorns starfish (COTS), Antarctic krill, Peruvian anchoveta, blue shark, kelp and seagrass. The abundance and distribution of these keystone species is indicative of not only the health of the ecosystem but also mankind’s impact on the marine environment. Keystone species are consumers that affect biodiversity to a greater extent than would be expected from their population numbers. Keystone species control other species by means of grazing, predation and competition. They may also be capable of ecosystem engineering by physically modifying the habitat. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether. Keystone species are important in conservation programmes that focus resources on maintaining such species, rather than attempting to protect and manage all endangered species in a habitat that is at risk. Keystone species Plankton Plankton are microscopic organisms that have limited mobility. They simply float with the current in water. (The word plankton is derived from Greek, and means ‘wandering’.) Plankton are divided into two groups: phytoplankton and zooplankton. Phytoplankton are producers that obtain their nutrition by using the energy in light through photosynthesis. Zooplankton are consumers that gain their nutrition from the food energy in producers or consumers. Phytoplankton There are many different species of phytoplankton. In the open ocean, most are free floating and are found in surface water where light intensity is highest. They have no need to settle on a substrate and are carried wherever the ocean currents take them. Phytoplankton can be divided into three groups depending on their size: picoplankton (0.2–2 µm); nanoplankton (2–20 µm); and microplankton (20–200 µm). Each group of phytoplankton exist with slightly different adaptations and niches. Diatoms (Figure 4.6a) are unicellular phytoplankton found in the oceanic surface water. There are more than a hundred different genera but all have intricate cell walls of silica, which often have extraordinarily beautiful designs. They are able to reproduce very rapidly when conditions are optimal and blooms are often seen in spring. This is when light intensity and temperature are rising and upwelling of mineral ions into the surface waters occurs. The blooms tend to appear and then rapidly disappear because of consumption by primary consumers such as planktonic crustaceans and krill, and depletion of mineral ions. Phytoplankton are important in removing carbon dioxide from the atmosphere and form the base of many marine food webs. Dinoflagellates (Figure 4.6b) are also unicellular protoctists but do not have the silica cell wall that diatoms have. Like diatoms, they live in the upper surface waters of oceans and can undergo rapid reproduction to produce algal blooms when conditions are optimal. The blooms of some species of dinoflagellate produce toxins that can poison fish and accumulate in shellfish. Contaminated shellfish that are eaten by other organisms, including humans, can cause poisoning. Blooms of dinoflagellates that produce toxins are called harmful algal blooms (HABs) and include red tides, which cause areas of the ocean to turn red. Pollution caused by the run-off of fertiliser from fields is the source of many blooms of dinoflagellates and, as farming has become more intense, the number of harmful algal blooms has increased. (Human impact on marine waters will be discussed further in Chapter 9.) Some dinoflagellates bioluminesce, and this can often be seen at night on the ocean or at the coast, when impressive displays can occur in the evening. They bioluminesce as a defence mechanism since it tends to attract large predators into the area which then consume predators of the dinoflagellates. Figure 4.6: Phytoplankton: (a) Diatoms; (b) Dinoflagellate. Zooplankton Zooplankton are important consumers and include larvae, copepods and larger animals such as jellyfish. Zooplankton migrate vertically in the water column each day, to feed on phytoplankton in the photic surface layer. Zooplankton provide a critical link in the food chain between the primary producers and larger consumers including shellfish, fish and whales. Zooplankton are sensitive to environmental change and can be killed by pollution, microplastics, ocean acidification or increases in water temperature due to global warming. Many marine organisms have a complex life cycle where the eggs develop into larvae before turning into juveniles and then adults. Larvae include the planktonic stage of nearly all species of fish and invertebrates, such as seastar (Figure 4.7a). The larvae are adapted to life floating in the ocean (for example, some species of fish larvae have oil globules that give them added buoyancy). Copepods (Figure 4.7b) are crustaceans and comprise the most abundant and diverse group of zooplankton. They are small herbivores that feed on diatoms. Copepods bodies are divided into three sections: the head, thorax and abdomen. Two antennae protrude from the head and aid in swimming, while two to four pairs of appendages extend from the thorax. Their tough exoskeleton is made of calcium carbonate and can include spikes that provide protection as well as an increased surface area : volume ratio for better flotation. Jellyfish species (Figure 4.7c) live in every part of the ocean and belong to the same group (cnidarian) as corals and sea anemones. Many of these soft-bodied, fragile organisms are made of two parts: a transparent bell, called a medusa, with tentacles dangling from it. The medusa can be as large as 1 m with the tentacles extending to 65 m. In contrast to larvae, jellyfish remain planktonic throughout their entire life and the pulsing of the medusa provides a limited mobilty. Jellyfish are predators that survive by using stinging cells called nematocysts on their tentacles to kill other plankton and larval fish. Figure 4.7: Zooplankton: (a) seastar larvae; (b) copepod; (c) jellyfish. Other examples of zooplankton include krill. Krill are shrimp-like carnivores that feed on other zooplankton species and phytoplankton. Krill are important food sources for birds, fish, seals and baleen whales. Echinoderms Echinoderms are a phylum of marine invertebrate and include creatures such as seastars (also known as starfish), sea urchins, sea cucumbers, sea lilies and brittle stars (see Figure 4.8). There are more than 7000 species of echinoderms, which can be found in a range of sea floor habitats from the intertidal zone to the benthic depths. Figure 4.8: The five main classes of echinoderms. Their name comes from the Greek for their characteristic ‘spiny skin’ which is composed of a thin layer of skin over a hard calcium carbonate skeleton (see Figure 4.9). Echinoderms have planktonic larvae that develop into adults with pentaradial symmetry: five arms (or fans) radiating from a central body cavity which contains a mouth and anus. Echinoderms move via a system of water-filled tubes that increase and decrease the hydraulic pressure in the tube feet arranged in grooves on the underside of each arm. The tube feet (Figure 4.9b) are used by seastar to open the shells of bivalve molluscs such as oysters and clams. The seastar’s stomach is then everted (turned inside out) and enters the opened shell. The stomach releases enzymes that digest the prey before the nutrients are absorbed. The sticky tube feet act as tiny suction cups that help it adhere to the seafloor. The tube feet also help in gaseous exchange for respiration by taking in oxygen and releasing carbon dioxide. Figure 4.9: (a) Main features of an adult echinoderm;) (b) tubed feet. Ecological importance of echinoderms Some echinoderms are keystone species forming unique and crucial ecological components within marine ecosystems. Examples of echinoderm species can be found in a variety of habitats including coral reefs, kelp forests and sandy shores. Coral reefs The crown-of-thorns starfish (COTS) (Acanthaster planci) is an example of an echinoderm that is a keystone species in coral reef communities. The COTS feeds on the fastest growing coral species. By doing so, it supports the colonisation of slower growing coral species. This increases the coral biodiversity on the reef, which in turn provides more niches for other marine organisms to occupy, increasing the overall biodiversity of a reef. The removal of the COTS predators can, however, lead to an explosion in their population. These COTS outbreaks can lead to destruction of large areas of coral which in turn severely damages the coral reef biodiversity. COTS are key to the delicate relationship between other organisms within the coral reef community and can promote and damage the reef’s biodiversity. (You will explore this topic further in the COTS case study in Chapter 5.) Kelp forests In kelp forests, echinoderms such as sea urchins feed on the kelp holdfast and are predated by sea otters. When the predator–prey balance is not in equilibrium (for example, if sea urchins are overharvested) then ecosystems can become overrun by kelp. Similarly, if local sea otter populations are hunted to extinction for their fur, then sea urchins proliferate, leading to the destruction of kelp forests (kelp barrens). Sandy shores On sandy shores, echinoderms such as sea cucumbers filter seawater, and burrow into the substrate, providing increased oxygen to organisms that live hidden in the sand. Sea cucumbers’ planktonic larvae form an important part of the food chain for plankton-eating (planktivorous) fish. They also ingest sand along with their food and the nitrogenous waste produced by sea cucumbers is an important nutrient for marine habitats. Economic importance of echinoderms Echinoderms can have both positive and negative effects on coastal economies. Sea cucumbers and sea urchins can be a source of income when they are used in agriculture, fishing, food and scientific industries. COTS can, however, damage the ecotourism industry when they destroy the kelp beds or coral reefs frequented by dive companies. Sea cucumbers (Figure 4.10) are considered a delicacy in Chinese cuisines and their commercial harvesting can play a pivotal role in local economies. In the Galápagos Islands, fishermen began large-scale harvesting of sea cucumbers in 1992. Initially, there was no control and up to 150 000 per day were collected. This led to an 80% decrease in the sea cucumber population. In 1999, a fishing season and minimum catch size were introduced but failed to prevent continued over-harvesting. In 2005, the resultant collapse of the sea cucumber population led to a political crisis on the islands as the local fishermen reliant on the industry lost their means of income. Sea cucumbers are also the source of pharmaceutical drugs that inhibit the growth rate of some cancers. Figure 4.10: Sea cucumber. Sea urchins are eaten in Japan, New Zealand, Peru, Spain and France. Farmers also add sea urchin’s calcium carbonate rich endoskeleton to acidic soil to raise the soil’s pH. Research scientists also use sea urchins as model organisms to help study developmental biology. Crustaceans Aquatic crustaceans have adapted to live in salt, brackish or fresh water and can be found in almost all aquatic habitats. They include crabs, crayfish, lobster, krill, shrimp (Figure 4.11), prawns, barnacles, copepods, amphipods and fish lice. Crustaceans have a distinctive planktonic larval form called a nauplius. Figure 4.11: Biological drawing of a shrimp. The name crustacean comes from the Latin for ‘hard shell’. They are characterised by a hard exoskeleton made of calcium and a polysaccharide called chitin. The outside of the exoskeleton provides protection from predators and water loss, while the inner part provides support for the attachment of the muscles. It is divided into a cephalothorax and a segmented abdomen. The cephalothorax is a combined head and thorax (as crustaceans have no neck) which contains the heart, gills, and stomach. The dorsal side is protected by a carapace. Crustaceans have two pairs of antennae: one pair is shorter, called antennules. The head has two compound eyes and three pairs of mouthparts (one pair of mandibles and two pairs of maxillae) which can be used for defence and feeding. Crustaceans, like all arthropods, have jointed legs. Unlike insects (three pairs) and arachnids (four pairs), crustaceans have at least five pairs of legs so can be described as decapods or ten-legged. Another distinguishing feature that separates them from other groups of arthropods, is that the legs have two-parts limbs. The cephalothorax has claws (chela) as well as walking legs (pereiopods) that can also be used to gather food. The abdomen is split into separate sections (segmented) and contains swimmerets (pleopods) which are swimming legs that can also assist in reproduction (for example, the first pair may be enlarged in males and used to pass sperm to the eggs on the female’s swimmerets). The abdomen ends in ends in a fan-shaped tail (uropod and telson). Ecological importance of crustaceans Crustaceans play an important role in many marine ecosystems, as they can be found ranging from deep sea benthic habitats (for example amphipods), through to surface pelagic waters, where the nauplius larvae drift in the zooplankton soup. Scavenging crustaceans are crucial to the recycling of mineral nutrients as they often occupy the niche of detritivores consuming dead and decaying matter. By helping in the breakdown of detritus, they accelerate the conversion of complex organic matter into inorganic ions by decomposers such as bacteria. Crustaceans also assist in maintaining the health of seagrass meadows where shrimps graze on the algae that grow on the blades of these marine plants. Without these crustacean primary consumers, the algae would block sunlight preventing photosynthesis and so reducing the growth rate of seagrass beds. Crustaceans are also the food source for many marine organisms. Marine birds feed on mud crabs commonly found in intertidal mangroves forests. Similarly, smaller crustaceans, such as copepods and krill, are crucial trophic component on marine food chains. Krill form a biomass of 110 billion kg in the Southern Ocean and are a vital part of the Antarctic food chains, being consumed by a variety of organisms, including fish, birds, squid, whales, seals and penguins. Krill are a group of about 80 different species of crustaceans. Antarctic krill (Euphausia superba) are one of the most important krill species in these polar waters of the Southern Ocean (Figure 4.12). Figure 4.12: Antarctic krill. Antarctic krill have a high reproductive capacity: the females can lay up to 10 000 eggs that can result in swarms that contain 30 000 individuals per cubic metre. Melting ice in spring and summer produces a layer of less-saline water on the Southern Ocean’s surface, together with increased nutrients and sunlight. These changes in abiotic conditions lead to a massive increase in phytoplankton, known as a bloom. The phytoplankton blooms feed krill, which subsequently become very abundant. Global warming can result in a decrease in krill population due to the rise in ocean temperatures leading to a change in ocean currents and a retreat in the Antarctic sea ice. Krill are also a target species for commercial fisheries and the annual global krill harvest is up to 20 million kg. Over-harvesting of krill can lead to a decrease in the krill population. The lower krill abundance reduces their grazing on phytoplankton. This can result in even larger phytoplankton blooms which can produce toxins harmful to other organisms in the ecosystem. Krill are also highly susceptible to overfishing of other Antarctic fauna, such as Antarctic squid. Despite krill being only one component of the diet of squid, the dramatic fluctuations in squid numbers due to their overfishing can directly affect the krill abundance. Economic importance of crustaceans Crustaceans are of great economic importance to humans. The larger crustaceans, such as prawns, shrimps, krill, crabs, crayfish and lobsters, are consumed by humans. The small zooplanktonic crustaceans, such as copepods and krill, form the largest animal biomass on Earth and are a crucial trophic link in maritime food webs that support the seafood we eat. Crabs, lobster and krill fishing • • • • • ○ ○ are major industries, as they are considered a delicacy all over the world. Krill is eaten in certain countries (for example in Japan where it is known as okiami). Krill is also caught for use as an aquaculture and aquarium feed or as bait in sport fishing or in the pharmaceutical industry. Krill oil contains omega-3 fatty acids and antioxidants and is sold as a dietary supplement to lower blood lipid and help alleviate arthritis pain. Crustaceans can also be harmful to aquaculture industry as they can slow the growth rate of the fish or molluscs being farmed. Water fleas are considered a pest as they eat smaller zooplankton that are an important food for juvenile fish in fish farms. Fish lice are copepods that are parasites on fish. Some species of fish lice suck blood while clinging to scales on the outside of the fish, while other species enter the fish via its gills and are internal parasites. Pea-crabs enter their oyster hosts as larvae and remain there as they feed and grow. Bony fish 96% of fish have a bony skeleton and belong to the Class: Osteichthyes. They can be recognised by their external features (Figure 4.13a). Gills Delicate pink membranous structures that provide a large surface area for exchange of oxygen and carbon dioxide between seawater and the blood in fish. They are supported by bony structures called gill arches. Operculum A thin bony flap of skin covering and protecting the gills. As it opens and closes, it allows sea water containing dissolved gases to flow over the gills and thus enables ventilation and gaseous exchange. Lateral line A canal that can be seen on the head and the side of the body in bony fish. It contains sense organs that can detect changes in the electric field (electroreceptors), as well as vibrations in the water (mechanoreceptors). This assists in shoaling behaviour, navigation and detecting prey. Figure 4.13: (a) the external features of bony fish; (b) the internal features of bony fish. Scales Made of bone. Covered by skin and mucus. Scales help protect the fish as well as reducing drag and hence increasing the hydrodynamic efficiency of the fish. The majority of bony fish have thin, overlapping flexible scales with either a smooth edge (cycloid) or a toothed edge (ctenoid). Like trees, these fish scales form growth rings and the thickness and number of rings can help scientists determine the age and growth rate of a fish. Some fish have much harder, heavier and thicker scales (ganoid). Fins Protrude from the body surface and assist in movement, stabilising position, reproduction and protection. There are five main types of fins found in bony fish: Pectoral fins: Found in pairs on either side of the body, just behind the operculum. Assist in turning, balance, stopping and swimming. Red Handfish (Figure 4.14a) have adapted their pectoral fins to walk while Flying fish (Figure 4.14b) use them to leap above the waves Caudal fin: At the tail of the fish. The main source of power for propulsion. There are five main shapes, depending on the swimming style of the species (Figure 4.15). Rounded fins are found in relatively slow swimmers that can swim for long periods of time. Truncated fins are useful for ambushing prey as they allow the fish sudden changes in direction as well as providing quick acceleration. Forked fins are found in continuous swimmers. Lunate fins can maintain fast speeds for long durations. The last type is heterocercal shaped. This is the fastest tail fin however it is most commonly found in sharks rather than bony fish. ○ ○ ○ Figure 4.14: Bony fish adaptations: (a) Red handfish; (b) Flying fish; (c) Hairy frogfish. Figure 4.15: Types of caudal fin. Pelvic fins: Found in pairs, one on each side of the fish. They are found on the front of the fish so are also known as ventral fins. They assist with stability and can be adapted to assist with walking on, or sticking to a benthic substrate. Anal fin: Found on the ventral surface behind the anus/cloaca. They help stabilise the fish while swimming. Dorsal fins: Found on the back surface of the fish. There can be up to three dorsal fins. They assist with steering and balance, protecting the fish from rolling and enabling sudden stops and turns. They may also have spines that give protection against predators. Hairy frogfish (Figure 4.14c) use a spine on their dorsal fin as a lure to attract prey. Bony fish also contain a variety of adaptations that can only be seen when we dissect a fish. As their name suggests, these internal features (Figure 4.13b) include a jaw and skeleton made of bone and cartilage. Many also contain a swim bladder, which is a specialised buoyancy organ. By adding or releasing gas between the swim bladder and the blood, the swin bladder allows bony fish to stay in midwater without the need to continuously swim. Ecological importance of bony fish The 27 000 known species of bony fish found in our rivers and oceans today are responsible for storing a significant proportion of marine organic nutrients in their tissue. When bony fish migrate, feed and reproduce they play a pivotal ecological role by linking the nutrient cycles of the different habitats they live in. When bony fish excrete, they release nitrates, phosphates and other dissolved nutrients in a form that can be readily absorbed by primary producers. Primary producers are the nutrient and energy base of all marine ecosystems. Fish also play an important trophic link by linking the plankton eaten by planktivorous fish to higher order consumers in the food web. Not only are they a food source for an abundance of fish-eating (piscivorous) marine species, they also link the aquatic and terrestrial ecosystems. Dead fish that are washed up onto the shore transport and redistribute essential nutrients between the open ocean and coastal habitats. One example of the ecological importance of bony fish are salmon species that migrate up river from the sea to spawn and die. During this salmon run, bears feast on the breeding stock (Figure 4.16). When the bears return to the surrounding woods, their faeces are important source of nitrates to the soil. The salmon carcasses and eggs that remain in the stream contribute much-needed nutrients at the end of summer that help ensure many freshwater species survive winter. A reduction in salmon populations would jeopardise the productivity and survival of many other species. For this reason, salmon are considered keystone species; their abundance is indicative of the health and water quality of the whole wetland ecosystem. • • • Figure 4.16: Grizzly bear hunting a sockeye salmon. The erosion of bony fish biodiversity is likely to have detrimental effects on land-based wildlife, including humans. If fish are harvested above sustainable levels then the resultant collapse of fish stocks will deprive us of an essential fatty acid and protein source in our daily diet. Economic importance of bony fish Throughout history, coastal communities have relied on harvesting and trading marine bony fish to provide nutrients and calories in our diet. Fish meat is an important source of protein and five essential amino acids that humans cannot synthesise. The liver of some fish, such as cod, contains oils that are often sold as nutritional supplements as they are rich in iodine, vitamin A and D. Russian sturgeon eggs are called caviar; they are an expensive delicacy in some countries. It is, however, not just ocean fisheries and their related service industries that provide income and employment. It is predicted that, in the future, fish farming will play an increasing contribution to our diet, especially if overfishing continues to deplete ocean stocks. The non-edible parts of fish also have economic importance. They are rich in nitrogen and calcium phosphate. They are ground into fish meal for animal feed and fertiliser. The remaining oil can be used to create soap, candles, varnish and paint. The skin can be made into lanterns or turned into glue. The air-bladders can make a powder called isinglass, which is used in wine and beer making. The scales of fish can be used to create jewellery. Other economic benefits from bony fish include recreational saltwater sport fishing; scuba diving and snorkelling by ecotourists; the catching and breeding of tropical fish for the pet industry; and the use of fish for scientific teaching and research. CASE STUDY 4.1: THE ECOLOGICAL AND ECONOMIC IMPORTANCE OF PERUVIAN ANCHOVETA Peruvian anchoveta are a species of anchovy fish that live for up to three years and can grow to be 20 cm in length. Anchoveta are filter feeders: water is taken in through the mouth and zooplankton is filtered out by the gill rakers which are bony extensions attached to gill arches. They are small forage fish found in the open ocean away from the seabed or the shore. A forage fish is a prey fish that is consumed by predators for food. Forage fish are an ecologically important part of the food chain as they provide food for: larger fish, such as tuna and salmon mammals, such as dolphins and whales sea birds, such as gulls and pelicans. One important factor that controls the number of young Peruvian anchoveta surviving to maturity is the effect of El Ni?no events (introduced in Chapter 2). El Niño is a change in the trade winds in the Pacific Ocean that stimulate upwelling of cold, nutrient-rich waters into the warmer sunlit water above. During an El Ni?no year, the thermocline deepens, which effectively blocks the cooler water underneath from mixing with the warm water at the top. Lack of nutrients therefore limits the primary productivity in the sunlit zone, leading to reductions in the numbers of fish. The Peruvian anchoveta fishery is the biggest single species fishery in the world and is of pivotal importance to the economy of local communities. In the 1960s, annual catches of anchoveta off the coast of Peru were more than 10 million tonnes per year. In 1971, the catch was 13.1 million tonnes. In 1972, the industry collapsed because of overfishing and the El Niño phenomenon. After 1 2 3 the collapse of the anchoveta populations, the industry shifted to fishing sardines for several years until the waters cooled again and anchoveta numbers increased. There was another reduction in numbers during an El Niño event in the early 1980s, but by the mid-1990s the catch was back up to 12.5 million tonnes. In 2014, only 2.2 million tonnes were caught after the second fishing season was cancelled because of the El Niño phenomenon. It is not just the fishing industry that is affected by El Niño. Guano (excrement from sea birds) is an important fertiliser. When there are fewer fish, there is less food for the sea birds, which are their predators, and, therefore, less guano is produced. Questions Explain how a reduction in the nutrients in the upper layers of the ocean could lead to a decrease in the numbers of forage fish such as anchoveta. Anchoveta feed on large zooplankton whereas sardines feed mainly on phytoplankton. Use this information to suggest why sardine numbers are less likely to collapse during an El Niño event. Look at the graph in Figure 4.17. Suggest two years, other than 1971, that could have had El Niño events. Explain your answer. Figure 4.17: Anchoveta and sardine catches between 1950 and 1998. Cartilaginous fish The second main type of marine fish is the Class Chondrichthyes, which includes sharks, skates, rays and chimaeras. These are cartilaginous fish as their jaws and skeletons are made of cartilage only. Cartilage has less calcium in it, and is softer and more flexible, than bone. Most sharks have eight fins: two pectoral, one caudal, two pelvic, one anal and two dorsal. The caudal fin of many sharks is larger on the dorsal than the ventral side. This hetercercal shape (Figure 4.14) provides a greater surface area for muscle attachment on the dorsal side, which provides more speed and acceleration. In contrast, rays get their forwards thrust through use of their pectoral fins. Cartilaginous fish also have a tough skin covered with unique tooth-like scales called denticles. These overlapping scales are all in one direction and, when rubbed the wrong way, give them their characteristic sandpaper-like feel. Denticles provide protection, as well as improving swimming efficiency by improving the streamlining. The lateral line in cartilaginous fish is not externally visible as it is under the skin. Unlike bony fish, cartilaginous fish have a number of separate openings that help ventilate the gills. Sharks, skates and rays have five to seven pairs of gill slits just behind the head, plus a modified slit, called a spiracle, which lies just behind the eye. They take water in through the mouth, over the gills, and out via the gill slits. They can use two methods, known as ‘ram ventilation’ and ‘pumped ventilation’. Cartilaginous fish do not have swim bladders and so to keep buoyant they must keep swimming in a forward direction. Ecological importance of cartilaginous fish Cartilaginous fish occupy a wide variety of marine habitats ranging from hunting in estuarine inlets and lagoons to cruising the continental shelf, and even diving deep into benthic waters to feast on whale falls. Their ecological niche is invariably one of a top predator, but there are also some cartilaginous scavengers and filter feeders. Cartilaginous fish play a pivotal ecological role with marine food webs. Their abundance is often linked to their evolutionary cousins, bony fish. For example, overfishing of bony fish, such as haddock in New England, led to an increase in cartilaginous fish including dogfish and skates. This increase is thought to be due to reduced interspecific competition between the two groups of fish. The decline of one fish group does not, however, always lead to a rise in the other. An example is when fishermen removed cartilaginous fish such as tiger sharks from tropical waters, hoping that this would lead to an increase in tuna catches. The opposite occurred, with a fall in tuna numbers and densities. This was believed to be because the absence of sharks led to less shoaling by tuna, and an increase in the abundance of other smaller predators of fish. The loss of cartilaginous fish can also detrimentally affect commercial shellfish stocks. For example, a decrease in sharks along the US Mid-Atlantic coast resulted in an increase in their prey, including the cownose ray population. The rays then decimated the bivalve species including oysters, scallops and quahog clams. As a result of this, the North Carolina scallop fishery collapsed and restaurants were forced to remove clam chowder from their menus. The most significant ecological impact is arguably due to sharks as they comprise more than 500 of the approximately 1250 species of cartilaginous fish. An example is the blue shark (Figure 4.2) which was once one of the most common sharks with a large geographic distribution inhabiting temperate and tropical waters between Norway and New Zealand. It has large pectoral fins which allow it to conserve energy on its long migrate in search of food and mates. The fins are highly prized when caught either intentionally by shark finning operations or as bycatch in gillnet and longline commercial fisheries. When caught, the fins are cut off and kept, while the rest of the shark is often thrown back overboard as waste. Through harvesting and loss of habitat, it is estimated that up to 20 million blue sharks are killed each year. This is especially alarming as sharks take a long time to reach sexual maturity and do not produce many offspring. This makes their recovery as a species from overfishing more difficult. As a result, some populations have fallen by 80% and the International Union for Conservation of Nature and Natural Resources (IUCN), have classified blue sharks as ‘near threatened’ with extinction. As apex predators, the only threats blue sharks face are being fed on by larger sharks, such as great white and tiger sharks, and due to culling by man. Blue sharks feed on a wide variety of species including squid, cuttlefish, octopuses, lobster, shrimp, crab, bony fishes, small sharks and sea birds. Sharks play a crucial role in the ecosystem by promoting biodiversity through helping maintain a balance between the predator and prey species below them in the food web. As such, they are keystone species as they serve as an indicator of ocean health. When shark populations are normal, they keep the prey population constant by removing weak and diseased individuals within a coral reef ecosystem (Figure 4.18). When sharks are over-harvested, the larger predatory fish, such as groupers, increase in abundance and feed more voraciously on the herbivores, such as yellow tang. This results in a drop in the abundance of primary consumers. With less herbivores, the algae that they graze on, increase their interspecific competition with coral, resulting in the reef becoming covered by algae. Sharks also predate and play an important role in the control of invasive species, such as lionfish, which causes a significant decrease in the population of juvenile native fish. The loss of sharks can hence indirectly lead to the decline in the biodiversity of coral reef habitat. Figure 4.18: Blue Shark as apex predator in a coral reef. Economic importance of cartilaginous fish The declining global stock of bony fish has led to a surge in the commercial fishing of a range of cartilaginous fish species. Game fishing provides an even greater economic return as there is the additional economic benefits of the coastal communities charging tourists for charter boat hire, food and lodging. This intense harvesting is causing the reduction of many shark and ray populations and the reduction in these top-level predators may now be negatively affecting marine ecosystems. Since ancient times, cartilaginous fish have been harvested for more than just their strongly flavoured meat. The gelatin-rich cartilaginous fins have been a culinary delicacy in China. Shark liver oil was originally harvested as a nutritional source rich in vitamin A. It is now more commonly used in preserving leather and wood, as a lubricant, in cosmetics, and for its range of medicinal properties from treating arthritis to cancer. The denticles provide an abrasive surface that is traditionally favoured in Japan for covering sword hilts. Shark leather is more durable than cowhide and can be used in the manufacture of boots, belts and wallets. In Greenland, some Inuit make rope from shark skin, while the Maori of New Zealand use shark teeth as earrings. Many indigenous cultures used the spines of stingrays as needles and spear tips, and the ancient Greeks and Romans used the electric shock of electric rays to ‘cure’ headaches. In more recent times the commercial value of ecotourism has flourished. In the Bahamas, a reef shark when caught by a fisherman is worth US$ 50. In contrast, dive tourism and photography can generate US$ 250 000 to the local community. Similarly, one whale shark in Belize can bring in US$ 2 million over its lifetime. Diving with sharks and rays has also promoted a better understanding of the beauty of these creatures, as well as the ethical and biological importance inherent in their preservation and conservation. Chordates Bony fish and cartilaginous fish are both chordates (in the Phylum Chordata). All organisms in this phylum share common features at some point in their development (Figure 4.19). Some of these features are listed below. • • • Figure 4.19: Common features of chordates. Notochord is a flexible, rod-shaped organ that extends the length of the body and allows the body to bend during muscle contractions. Dorsal neural tube is a tube-shaped organ that extends the length of the body. During development, the forwards (anterior) end becomes the brain while the rear (posterior) end becomes the spinal cord. Pharyngeal slits link to the mouth cavity and the digestive system. In primitive chordates, they permit the release of water taken in by the mouth for filter feeding. In bony or cartilaginous fish, the pharyngeal slits develop into gill arches that support ventilation across the gills. In most marine mammals and birds, pharyngeal slits are present only during embryonic development and develop into the jaw and inner ear bones. Post-anal tail is mainly used for swimming and is located to the rear of the fish. Macroalgae As well as phytoplankton there are also much larger marine producers such as macroalgae, including kelp and seaweeds; and marine plants, such as seagrass. These organisms are all photoautotrophs: they make their own food (chemical potential energy) using light energy from the Sun. Most macroalgae species have a similar structure that enables them to survive in the water of shallow seas and oceans (Figure 4.20). The whole body of the macroalgae is known as a thallus and has three main parts: Holdfast This is a strong, root-like structure that anchors the kelp to the seabed, preventing it from being moved by strong ocean currents or storms. It is for anchorage only and has no function in absorbing minerals. Stipe This is a long, tough, vertical stalk similar to the stem of plants. It extends from the holdfast and reaches up to the blades. It is very tough, to prevent breakage. Blades The blades are broad leaf-like structures that ‘hang’ in the water. They have a large surface area to absorb light and minerals. Figure 4.20: Kelp showing the features of macroalgae. Some macroalgae species also have gas bladders (also known as pneumatocysts) found underneath the blades. These act as floatation aids to keep the producer upright and the blade in the top of the photic layer to where there is increased light intensity for photosynthesis. Because many species live at depths where exposure to red light is restricted, they contain accessory pigments such as xanthophyll and fucoxanthin to absorb additional wavelengths of light. Ecological importance of macroalgae Macroalgae are particularly important in the intertidal (littoral) and subtidal (sublittoral) coastal habitats. One major underwater ecosystem is giant kelp forests (Figure 4.21) which are composed of brown macroalgae species. They require nutrient-rich clear water and a temperature of between 8 °C and 16 °C. When conditions are optimal, they have a very high rate of growth. One genus, Macrocystus, is able to grow up to 0.5 metres a day and can reach lengths up to 80 m. Large kelp forests proliferate on continental shelves and serve as crucial habitats for a diverse range of fauna. Kelp also increases the productivity of the nearshore ecosystem by generating large quantities of detritus. Kelp forests are an important global marine ecosystem because they are the base of many food chains, generating vast species biodiversity. Figure 4.21: Giant kelp forests. CASE STUDY 4.2: THE IMPORTANCE OF SEA OTTERS IN MAINTAINING KELP FOREST ECOSYSTEMS Kelp beds can be damaged by sea urchins, which are echinoderms that feed on kelp and detach them from their holdfasts that anchor them to their rocky substrate. Biotic factors that control sea urchin populations include predators such as sea otters, Enhydra lutris. The kelp forest food web (Figure 4.22) shows that the life in the ocean is, however, far more complicated than a simple food chain such as: kelp → sea urchin → sea otter In reality, many species, including abalone, crabs and herbivorous fish, feed on kelp. There is also a range of additional predators that feed on sea urchins, including starfish and larger bony fish (such as cod and sheepshead). Predator–prey relationships are crucial for keeping a healthy balance of populations within the kelp ecosystem. Sea otters prey on sea urchins, and where sea otter populations have decreased, kelp density has also decreased because of a dramatic increase in the sea urchin population. If these predators did not protect kelp forests from damage by the sea urchins, kelp barrens would eventually form. This reduced productivity in the ecosystem would result in less chemical energy (food) for other species in the food web, causing a reduction in biodiversity. Sea otters are a keystone species for kelp forests because many other populations are dependent on them. The loss of sea otters would lead to a significant decrease in overall biodiversity. Reductions in sea otter populations can be caused by abiotic factors including climatic fluctuations (such as storms, the El Niño Southern Oscillation and global warming). Natural biotic factors can 1 2 3 also negatively affect sea otter abundance. An increase in killer whale numbers can result in increased predation on sea otters. Similarly, a decline in the populations of seals and sea lions may cause killer whales to switch their diets to include more sea otters. Figure 4.22: A kelp forest food web. Sea otter populations face a number of threats from humans. Sea otters often get caught in discarded nets and traps, which injure or kill them. Oil in the ocean from oil spills clogs sea otters’ fur, decreasing the fur’s ability to shield against cold water, leading to death by hypothermia. Polluted waters have also contributed to diseases in some sea otter populations. Hunting for fur historically had huge impacts on sea otter populations: European and North American fur traders hunted sea otters to the brink of extinction in the 1800s. Commercial fishing decreases the number of fish resulting in reduced food for the sea otters. Questions Why do kelp forests only grow in shallow waters? Explain what effect an decrease in kelp density could have on fish abundance. Using the kelp ecosystem food web in Figure 4.22, suggest reasons for potential positive and negative affects that a decrease in the kelp density could have on the local abalone shellfish industry? Economic importance of macroalgae Humans have harvested macroalgae, such as kelp and seaweeds, for thousands of years. Depending on the species it can be harvested (Figure 4.23) in three different ways: attached to a substrate, free floating or cast up on the shore. Concerns, however, have been raised as to the impact of this widescale harvesting on the marine environment. Despite production doubling since 2000, the demand exceeds the supply and many parts of the world, such as China and Japan, have begun commercial farming (mariculture). Macroalgal mariculture has grown to become an international industry with eight million tonnes produced annually and an estimated value of US$ 6 billion (£4.75 billion). Seaweed is seeded onto nets or ropes, which are then tethered in an area of lagoon that is not shaded, ideally with a temperature between 25 °C and 30 °C. After a period of growth, it is collected and hung out to dry. It is then used for a rich variety of products. • • • • • • • • Figure 4.23: Harvesting of seaweed in the Philippines. Cooking Seaweeds have been used as food by coastal communities for years in many countries such as Japan, Korea, Iceland and Wales. In Japan, more than 20 species of seaweed are used as food and the red algae species Porphyra is dried to make sheets of nori, commonly used as the wrapping for sushi rolls. Nutritionally, seaweed is rich in protein, many vitamins and mineral salts, especially iodine, and is very low in fat. Food industry Seaweed is used as a source of biochemicals that are used to make solid gels and emulsifiers that hold food substances in suspension, including: Alginate: a substance extracted from seaweed and used to form a gelatinous substance; it is used as an additive in many foods such as ice cream and has recently been used to make small gelatinous capsules that contain different flavours, like a synthetic caviar; alginate gel is also used in burns plasters and firemen’s clothing Agar: used to make vegetarian jellies and also the agar plates used frequently in microbiology to grow bacteria and test antibiotic resistance Carrageenan: used to make food with a range of different textures, including chocolate milk drinks and milk chocolate bars, because it helps to hold the chocolate in suspension. Cosmetics and herbal medicine Seaweed extracts are often found in moisturising skin creams and herbal remedies for a range of conditions including arthritis, tuberculosis and the common cold. Fertiliser This is added as a rich source of nutrients to farmland. Aquaculture This is processed into pellets which can be used to feed abalone in aquaculture farms. Marine plants Flowering plants are less common in estuarine and marine environments than they are in land-based (terrestrial) ones. Like other aquatic producers, they can come in three variants: floating, emergent and submergent. Floating plants include the water cabbage, a type of floating herb found in the Brazilian wetlands at the mouth of the Amazon river. Emergent plants are those that are rooted in the substrate and project above the water surface. Estuarine examples of these include the mangrove forest (that we will discuss in Chapter 5) and the herbs and shrubs that make up saltmarsh communities. Submergent plants are rooted in the substrate but remain beneath the waterline. These include 72 species of seagrasses which can be found in large beds, called meadows, on the shallow continental shelf. They are the most common marine plants and will be the main focus of this section. Seagrasses, which are marine plants, are not to be confused with seaweeds, which are macroalgae. Seagrasses are flowering marine plants that form underwater meadows in shallow waters ranging from 4 °C to 24 °C (Figure 4.24). Seagrasses are, therefore, not found in subpolar regions and are also under threat from global warming. Figure 4.24: Location of seagrass beds. Marine plants have many specific adaptations to life on the continental shelf. They have well-developed root systems, with thick, horizontal rhizomes that lie up to 25 cm deep in the substrate (Figure 4.25). The root system anchors the seagrass into the seabed so that it is not moved by the shifting water currents and wave actions. The rhizomes also enable seagrasses to reproduce asexually. Figure 4.25: Seagrass showing the features of marine plants. The leaf structure is unusual. It has an epidermis layer with chloroplasts (these are absent in the epidermis of most terrestrial plants) to maximise photosynthesis, no stomata, and a very thin waxy cuticle so that the leaf cells can obtain mineral ions directly from the water. Seagrasses have very few vascular bundles, as there is no need to transport water or minerals through the plant. The leaves are also very flexible so they do not get broken by water currents. The leaves and roots are physiologically adapted to seawater so that their cells are able to exist in saltwater without losing water by osmosis. They also contain a specialised tissue of ‘air cells’ within stems called aerenchyma. This tissue delivers air containing oxygen for aerobic respiration to all the submerged areas of the plant. They are able to reproduce both sexually and asexually. When reproducing sexually, they produce flowers that release pollen that is carried in the water to other flowers. Ecological importance of marine plants Marine plants are the foundation of aquatic communities with a high biodiversity. Their sensitivity to • • • water quality makes them important keystone species that can help indicate the overall health of coastal ecosystems. The different types of marine plants all share the ability to perform numerous crucial ecological roles in coastal areas. Marine plants and phytoplankton are the dominant producers in estuarine habitats and are a direct source of food energy for marine primary consumers. For example, if a massive amount of seagrasses die, it will reduce the overall energy available for the consumers in the ecosystems. Consumers that rely on seagrass species are especially harmed. One such species is turtle grass , Thalassia testudinum, which is found in the Gulf of Mexico, Caribbean Sea and Bermuda, and is a vital food source for turtles, manatees (Figure 4.26) and herbivorous fish. Detritus from dead seagrass provides organic energy for worms, sea cucumbers, crabs, echinoderms and sea anemones. Some fish, such as seahorses and lizardfish, can be found in seagrasses throughout the year, while for many other vertebrate and invertebrate species the seagrass provide a nesting and nursery habitat for only their larval and juvenile stages. Bottlenose dolphins are often found feeding on fish, crabs and squid. Figure 4.26: The ecological importance of seagrass meadows. Seagrass meadows occupy less than 0.2% of the area of the world’s oceans but hold about 15% of the ocean’s total carbon. They also act as oxygen producers, increasing the dissolved oxygen. Seagrasses typically grow as long, thin leaves which help absorb wave motion and slow currents. Seagrasses further reduce turbidity and improve water clarity by trapping silt suspended in the water column. Their dense intertwined root systems bind the soft sediment reduce the stirring up of silt while filterfeeders, such as bryozoans, sponges and forams, help maintain the quality of coastal areas by absorbing the nutrient and heavy metal rich run off from land-based ecosystems before they can reach and damage more sensitive marine habitats such as coral reefs. Research in India and Florida has suggested that conservation programmes for individual species within seagrass ecosystems can have mixed results. For example, when sea turtle populations are too low, seagrass dies off and is replaced by algae. To combat this, sea turtle conservation projects use turtle hatcheries to increase the local populations of sea turtles. (Figure 4.27). This in turn encourages good seagrass growth, increases the fertility of the seabed and helps disperse the seagrass. However, if turtle populations become too high, seagrass is overconsumed and dies off. Sharks play a key ecological role by consuming juvenile sea turtles but, in many parts of the world, shark numbers have plummeted due to overfishing. A seagrass conservation programme that focusses only on turtles and does not also protect the natural predators, such as sharks, can lead to further ecological damage due to an overabundance of populations of herbivores that eat seagrass. So, damaged seagrass habitats need a holistic restoration programme to protect these ecologically sensitive natural refuges combining: turtle breeding and nesting protection a ban on shark fishing restricted fishing, aquaculture and coastal land use practices. 3 4 a b Figure 4.27: A sea turtle conservation project. Economic importance of marine plants It may appear that marine plants have less economic importance to humans than their terrestrial counterparts, as they care rarely used as a food source and cannot be harvested as easily. However, marine plant’s main economic value should be measured through the industries that they support indirectly. As nursery grounds for marine invertebrates and vertebrates, for example, marine plants are the biotic cornerstone of the next generation of commercial and recreational fisheries. The habitats also provide incalculable economic support to coastal communities through their physical protection from weathering, erosion and flooding during storms. Traditionally, mangroves provided wood for fires and building, and seagrasses were used to insulate houses, weave furniture, thatch roofs, make bandages, fill mattresses and fertilise fields. More recently, ecotourism ventures which promote scuba diving to observe manatees are also becoming popular conservation adventure holidays. Test yourself Copy and complete Table 4.3 to show how each species is classified. Purple seastar (Pisaster ochraceus) are an example of an echinoderm that is a keystone species in coral reef communities. Purple starfish are not top predators, because they are prey for sharks, rays and sea anemones. Purple starfish feed on a range of species, including sea urchins and mussels. Explain the effect on the coral reef if purple starfish are removed. Discuss why purple seastar can be considered a keystone species. Taxonomic hierarchy Crown of thorns starfish (COTS) Antarctic krill Blue shark Peruvian anchoveta domain Eukarya Eukarya kingdom Animalia Animalia phylum Arthropoda class Asteroidea Actinopterygii order Valvatida Malocostraca Carcharhiniformes Clupeiformes family Acanthasteridae Euphausiacea Carcharinidae Engraulidae genus species Table 4.3: Classification of marine species. 4.3 Biodiversity Biodiversity describes the degree in variation of organisms and ecosystems on Earth. Life evolved in the maritime environment and our global seas and oceans have an extremely high biodiversity. Biodiversity can be explored at three levels: species diversity, genetic diversity and ecological diversity. These three levels work together to describe the complexity of life on Earth. This is the first step to starting to understanding the importance of maintaining the biodiversity of marine environments. Types of biodiversity Species diversity Species diversity is a measure of the abundance and richness of a species in a given place at one time. Species abundance is the number of individuals per species. Species richness measures the number of species in an area. Coral reefs have many different ecological niches which are exploited by a multitude of different species, hence are one of the most species-rich areas on Earth. Other marine habitats, such sandy shores, have a far lower number of species. Genetic diversity Genes are made up of nucleic acids (DNA or RNA) and are the developmental code for life on Earth. The forms of genes are called alleles and they are responsible for both the similarities and the differences between species. Genetic diversity describes the variety of forms of gene alleles within a species. Each species has a set number of genes. Individuals within a species have their own particular gene allele composition. A population is all the individuals within one species in one area at one time, and each population has a different gene allele frequency. Genetic diversity within a species can be assessed by finding out the allele frequency per gene. Understanding the genetic biodiversity of different populations of a species is crucial as some may be inbred and have a smaller range of alleles. A reduction in the number of alleles may result in populations being less able to adapt when there are changes in the environment. Ecological diversity Ecological diversity is the variation of ecosystems or habitats on a regional or global level at one time. An ecosystem is a community of organisms (biota) and their interaction with the abiotic environment. An ecosystem can cover a large area, such as the Great Barrier Reef, or be as small as a hydrothermal vent. Measuring ecological diversity is however difficult because each of Earth’s ecosystems merges into the ecosystems around it. Importance of marine biodiversity Marine ecosystems with a high biodiversity include kelp forests, seagrass meadows, mangroves and coral reefs. These habitats each provide crucial services to the coastal communities that they support. Maintaining stable ecosystems The more species there are, and the more evenly the number of organisms are distributed among the different species, the greater the species diversity. Ecosystems with high species diversity are considered important as they tend to be more stable and resistant to ecological change. The environment is a major factor influencing the biodiversity of a habitat. Environments that are either unstable or extreme tend to have a lower biodiversity than environments that are stable and not extreme. Stable ecosystems support a rich biodiversity which helps maintain the complex interactions between the community of organisms and the physical environment. Some common marine examples are coral reefs, rocky shores, hydrothermal vents, and reef slopes. Coral reefs Coral reefs occupy less than 1% of the ocean floor, but contain more than 25% of known marine life. This high biodiversity is the result of a stable and non-extreme environment that provides abiotic conditions that are close to optimum for the producers. A vibrant community of producers provides the foundation for long food chains and a diverse food web. Rocky shores Rocky shores are also stable, non-extreme environments. The rock provides a good attachment surface for molluscs and seaweeds, so there is less chance of the organisms being washed away. Rocky shores also provide protective habitats, such as rock pools and crevices. They are less porous than sandy shores so organisms are less prone to dying from drying out (desiccation). Rocky shores hence support a greater biodiversity than sandy shores. Hydrothermal vents Hydrothermal vents communities are located in an environment that is extreme because the abiotic conditions, including toxins, temperature, pH, hydrostatic pressure and light, are outside the zone of tolerance for most organisms. Because very few organisms are adapted to live in the extreme conditions, hydrothermal vents have a low biodiversity. Reef slopes Reef slopes are the steep, sometimes vertical, walls at the front (fore) of a reef. These fore-reef zones absorb most of the energy and damage from incoming waves and stormy seas. This means the sandy substrate of a reef slope is easily eroded by currents, waves and wind, and it is difficult for marine plants to grow there. Lack of biomass in primary producers and the usual loss of energy between trophic levels means that food chains are short and the environment cannot support species at higher trophic levels. The organisms that can successfully use the reef slope sand habitat include animals that burrow into the sand, such as worms, clams, sand fleas and crabs. Although the physical conditions are not extreme, they are constantly changing so the environment is described as unstable. Not many marine organisms are adapted to survive in such conditions, so biodiversity is low. Protection of the physical environment Ecosystems with a high marine biodiversity lead to increased protection of the coastal environmental. For example, the root systems of marine plants such as seagrasses and mangroves, stabilise the muddy substrate and so reduce the erosion of estuaries and mud flats during heavy storms. Similarly, fringing reefs of coral provide a physical barrier to the destructive action of waves during tropical storms. The reefs absorb the power of waves and storms before they hit the mainland. This protects the coastline (for example, sandy beaches) from damage due to weathering and erosion. Climate control The producers which underpin high biodiversity marine ecosystems play a pivotal ecological role for our planet as they regulate atmospheric increases in carbon dioxide concentration that can lead to climate change. An example is the high productivity of phytoplankton and seagrasses which results in them being able to store twice as much carbon dioxide per hectare as terrestrial rainforests. Photosynthesis by these carbon sinks helps reduce atmospheric carbon dioxide and the man-made effects of ocean acidification and global warming. A by-product of this process is the production of oxygen which is required by marine organisms for aerobic respiration. Providing food sources High biodiversity ecosystems include the primary producers which provide food and shelter for many other species in a vibrant food web. By doing so, they nurture the growth of the range of ocean species – such as macroalgae, crustaceans and fish – which humans depend upon as a staple part of our nutrition. Source of medicines Marine biodiversity is an exciting source of new medicinal drugs derived from marine plants, animals, fungi or bacteria. Figure 4.28 shows the proportional uses of beneficial chemicals derived from marine organisms. Discoveries from our aquatic pharmacy that are currently undergoing medical trials include: • • • • 5 a Figure 4.28: The use of beneficial chemicals derived from marine organisms. a pain-relief drug harvested from marine cone snails schizophrenia medication from marine worms wound-healing drugs from corals anti-cancer drugs from a range of marine bacteria, bryozoans, fungi, tunicates and nudibranchs. A variety of marine ecosystems have proven lucrative in the global hunt for new ‘superdrugs’. The littoral zone of rocky shores in western North America are home to the giant keyhole limpet, Megathura crenulata (Figure 4.29). A protein extracted from this species, called keyhole limpet hemocyanin (KLH), has been proven to stimulate the immune system. KLH’s immunotherapy properties have meant that it is in high demand to treat a range of autoimmune, inflammatory and infectious diseases. It is also being tested as a potential vaccine for skin, breast and bladder cancer. Over-harvesting of the species has, however, resulted in a significant decline in their populations and research is now focussing on the potential aquaculture of this pharmaceutically important mollusc. Figure 4.29: Giant keyhole limpet (Megathura crenulata). Deep-sea trenches are the largest unexplored habitat on Earth. Marine scientists are searching a number of these ocean trenches either by deep-sea submarines or dropping a long coring device to the ocean floor. Researchers extract and test the bioactive compounds produced by these organisms for medicinal properties. Dermacoccus abyssi is a bacterium retrieved from sediment in the Mariana Trench. This organism produces dermacozines, a biochemical that may help protect against the parasite that causes African sleeping sickness. Marine organisms that live closer to the surface can also be the source of pharmaceuticals. For example, three species of Australian sea sponges have been found to produce chemicals called chondropsins. The drug potential of chondropsins is related to their ability to inhibit certain enzymes that play a role in the development of bone cancer, Alzheimer’s disease, viral infections, diabetes and cardiovascular disorders. Test yourself With reference to a mangrove ecosystem, define the following terms: species abundance b c 6 a b c species richness genetic diversity of mud crabs. The giant keyhole limpet, Megathura crenulata, occurs in intertidal and sub-littoral waters along the rocky coast of Eastern Pacific ocean from Southern California in USA to the Baja California peninsula in Mexico. Explain the importance of maintaining the biodiversity of this habitat with regards to: medicine food climate stability. • • • 4.4 Populations and sampling techniques Humans have been trying to measure, describe and understand life on Earth for millennia. As our understanding has progressed, scientists have created a biological vocabulary to help describe the structure and interdependency of marine ecosystems. This section will start by introducing some of the vocabulary that is key to sharing this knowledge: habitat, niche, species, population and community. Life on Earth can be divided into subunits called ecosystems. An ecosystem is all the living organisms in an area plus the non-living environmental factors that act on them. Key examples of five marine ecosystems will be discussed in greater detail in Chapter 5: the open ocean, tropical coral reef, rocky shore, sandy shore and mangrove forest. The biological, chemical and physical parts of an ecosystem are linked by energy and nutrient flows that were explored in Chapter 3. A habitat is the natural environment where organisms live. Habitats are areas in which organisms can find food, protection, shelter and a mate. Marine environments form a range of habitats in estuaries, on the shoreline and in shallow and deep ocean water. Estuaries are brackish areas where fresh and salt water mix. Sediments from streams often settle in estuaries, creating a number of important habitats where marine species can feed and breed. These habitats include swampy areas called wetlands, mangrove forests and salt marshes. The habitat an organism occupies can be defined by where it lives and how it moves. For example, organisms that: drift in ocean currents (planktonic) include phytoplankton and zooplankton can actively swim (nektonic) include fish, marine reptiles and mammals live on the seabed (benthic) include starfish, crabs and sea cucumbers. Some organisms cross from one habitat to another during their life cycles. For example, crabs and clams both start out as planktonic larvae but become benthic adults. Habitats are not always geographical (for example, parasitic worms live inside their host species). A species is defined as a group of similar organisms that can interbreed naturally to produce fertile offspring. Endemic species are organisms that are found in only one area. For example, the Galápagos marine iguana only lives and hunts for food on the shores of the Galápagos Islands (Figure 4.30). Figure 4.30: Galápagos marine iguana (Amblyrhynchus cristatus). A population is all the organisms of the same species that live at the same place at the same time, and are able to reproduce. For example, the squat lobsters living off Otago, New Zealand, are a population. Similarly, all the salmon in the Atlantic Ocean make up the Atlantic salmon population. The number of individuals in any population often increases and decreases. Population increases are caused by reproduction or by new individuals joining the population area. Population decreases are caused by death or by individuals leaving the population area. The largest population that can be sustained by the available resources is called the carrying capacity. If some resources are less than optimal, or get completely used up, they are called limiting factors and result in reduced growth in the population. Limiting factors can be either biotic or abiotic. Biotic limiting factors include competition and predation. Abiotic limiting factors affect growth, survival and reproduction, and include living space, food, water temperature, pH and light intensity. • • • A community is an association of all the different populations of species occupying a habitat at the same time. An example is the mollusc community on a Californian rocky shore, which would include all the different species of molluscs living in this habitat. Biomes are communities that extend over large areas of the globe and are classified according to the predominant vegetation. Marine biomes include intertidal, rocky, sandy and muddy shores, coral reefs and the seabed. Each biome has a characteristic community. A niche is defined as the role of a species within an ecosystem. There are a variety of roles, or niches, that organisms perform in their aquatic environment. These were described in Chapter 3 and include producers, consumers, decomposers, predators and prey. The term niche also takes into account interrelationships with other organisms: Feeding relationships Both sperm whales and killer whales are top predators. Sperm whales predominantly consume squid, whereas killer whales consume a wider variety of prey, including elephant seals and baleen whales. These two species of whale therefore occupy different ecological niches. Spatial relationships Two species may have the same feeding relationships but occupy that niche in different parts of the ocean. For example, if a prey species is found throughout the water column, one predator may feed on it within the surface photic zone (where there is light) while another feeds deeper down in the aphotic zone (where there is no light). Temporal relationships Two species may have the same feeding relationships but occupy the niche at different times. For example, if a prey species is found in the same location throughout each day, one predator may feed at night (nocturnal) while another feeds in the daytime (diurnal). Biotic and abiotic factors in marine ecosystems The distribution and abundance of organisms in the marine environment are affected by both biotic and abiotic factors. The biotic factors of an ecosystem are the links between living organisms, and include the feeding relationships between producers, consumers and decomposers. These trophic level connections can be shown as food chains or food webs and include predator–prey relationships. The distribution of a predator is reliant on there being prey species for it to hunt, kill and eat. An example on a rocky shore is that predators such as oyster borers are reliant on the abundance of prey species, such as oysters, mussels and barnacles. Other interspecies interactions that were discussed in Chapter 3 include the three main types of symbioses (an interaction between two different organisms living in close physical association): parasitism, commensalism and mutualism. Symbiosis means ‘living together’ and refers to two or more organisms from different species living in close physical association. The abundance of the hosts will directly affect the symbiont (for example, the growth rate of hard corals is far slower without mutualistic zooxanthellae). Diseases are illnesses characterised by specific signs and symptoms and can be caused by pathogens such as viruses and bacteria. Diseases result in a decrease in a marine population’s distribution and abundance. Florida is currently experiencing a widespread and lethal coral disease outbreak caused by stony coral tissue loss disease (SCTLD). This results in small colonies of hard corals dying within a few weeks, while larger colonies can take more than a year to perish (Figure 4.31). SCTLD was first sighted in 2014 and by 2019 had spread to affect nearly half of the stony coral species found on the Florida Reef Tract. Figure 4.31: Destruction of a brain coral by stony coral tissue loss disease (SCTLD). Competition is a biotic relationship between organisms that strive for the same resources in the same place. There are two different types of competition: • • • • • intra-specific competition, which occurs between individuals of the same species (for example, two male fish of the same species competing for female mates in the same area) inter-specific competition, which occurs between members of different species (for example, predators of different species competing for the same prey). Inter-specific competition can lead to overlap between ecological niches. The species that is less well adapted has access to fewer resources and is less likely to survive and reproduce: this could lead to the species becoming extinct. However, inter-specific competition more often leads to greater niche adaptations and niche specialisation. The fundamental niche is the niche of a species when that species experiences no competition with others. The fundamental niche can also be defined as the tolerance range for all important abiotic conditions, within which individuals of a species can survive, grow and reproduce. But all organisms are part of complex food webs, sharing the ecosystem with other species, and all competing for the same biotic and abiotic resources. As these resources are limited, this leads to inter-specific competition. The competitive exclusion principle predicts that, in a stable ecosystem, no two species can be in direct competition with each other. If the niches for two species are identical, one species will die out as a result of inter-specific competition. For example, if an introduced species is added to an existing marine habitat then the new species may have the same niche as a native species (for example, two top predators feeding on the same prey at the same time in the same habitat). Inter-specific competition between the two species will occur, as they compete for this niche. One of the two species will be better at hunting the prey and, therefore, will thrive and increase in population size. The population of the other species will be less ecologically or reproductively viable, eventually dying out. Abiotic factors that affect the distribution abundance of marine organisms include: geological features include substrate type and shape of the seafloor (topography) physical features include temperature, exposure to air, wind and sunlight, wave action, tides, currents, hydrostatic pressure, water turbidity (murkiness), as well as light availability. The light available can be measured by both its brightness (intensity) and colour (wavelength). chemical features include pH, salinity, oxygen concentration, carbon dioxide concentration and nutrient availability (for example, nitrate/phosphate concentration). Each of the above biotic and abiotic factors can potentially reduce the growth, survival and reproductive success of organisms with marine ecosystems. If so, they are called limiting factors. In a deep sea hydrothermal vent, limiting factors include the lack of light, the acidic pH and the extremely high hydrostatic pressure and temperature. The abiotic factors (the environment’s geological, physical and chemical features; the non-living part of an ecosystem) of a marine ecosystem are the environment’s non-living components. The hard coral species can only grow in the surface waters of reefs as they are limited by the need for low turbidity clear waters which allow light to penetrate for photosynthesis by the mutualistic zooxanthellae. Coral reefs are also limited to growing between 30 °N and 30 °S of the equator as they require warm waters. A reduction in sea level caused by either climate change or low tides can also kill coral. This is due to the polyps’ inability to survive exposure to air. A lack of nutrients (such as calcium required for corallite) limits the polyps’ ability to grow. An excess of nutrients, such as nitrate and phosphate ions in the run-off from land-based fertilisers, can also limit growth due to excessive algal growth that smoother the polyps (eutrophication). In contrast, soft corals are usually only found in deeper waters as they are out-competed by the faster growing coral species. The environment here is colder and darker, which results in a slower growth rate. In a rocky shore ecosystem, the organisms that survive at high tide must be able to resist large changes in daily salinity and temperature due to the seawater becoming more concentrated on a hot summer day due to evaporation, while on a cold and rainy day, the salinity and temperature will plummet due to the influx of cold fresh water. The organisms must also be able to cling firmly to the rocky substrate in order to resist dislodging by wave action and tides. Measuring marine biodiversity The majority of all aquatic life in the world’s oceans is thought to be unknown. Universities and research institutes use a range of scientific methods and statistical models to discover, sample and estimate this unknown biodiversity. In this section we will introduce how fieldwork techniques can be used to measure the fauna and flora within the littoral zone. We will also use maths skills to help us estimate population sizes and analyse the relationship between biotic and abiotic factors. Mark–release–recapture For mobile species where it is not practical to count all the individuals in the population, one approach to estimating the population size is by using the mark–release–recapture method. Using this approach, the population size can be estimated from as few as two visits to the study area, but increased accuracy is gained by more than two visits. • • • • • • • 1 2 Estimation of the size of rocky shore populations of dog whelks can be made using the mark–release– recapture method. The molluscs are captured and all marked with a small dot of non-toxic waterproof paint. The number captured and marked is recorded. One to two days later another sample of the dog whelk population at the same location is taken and the total number captured on this second visit, both marked and unmarked, is counted. The number of marked individuals within this second sample is also recorded. Numbered tags are another way of marking animals and these are commonly used for assessing fish, whale, turtle or shark populations. In addition to population estimates, the data from tagging can provide information on migration routes, feeding patterns, seasonal variation in numbers and birth and mortality rates. This data is useful for conservation scientists as well as those employed in managing the fishery industry. All biological surveys are limited by their methodology. For example, the population estimate calculated by the mark–release–recapture method is based on a number of assumptions: Marked individuals are unaffected by the tagging process. Marked individuals disperse throughout the unmarked population. All animals have the same probability of being marked initially. Markings are not lost in the time between the two samples. The second sample is a random sample. The effects of emigration, immigration, mortality and recruitment are negligible. All marked animals seen in the second sample are reported. MATHS SKILLS 4.1 ESTIMATING POPULATION SIZE USING THE LINCOLN INDEX The Lincoln index is one of a number of mathematical equations that can use the mark–release– recapture data to estimate the population size. The Lincoln index uses the following formula and symbols: N=n1×n2m2 where N = estimate of population size n1 = number of individuals captured in first sample n2 = number of individuals (both marked and unmarked) captured in second sample m2 = number of marked individuals recaptured in second sample Worked example The mark–release–recapture method was performed on dog whelks found a rocky shore. The results were: n1 = number of dog whelks captured and marked on the first visit: 120 n2 = number of dog whelks (both marked and unmarked) captured on the second visit: 200 m2 = number of recaptured dog whelks that were marked: 40 Use the Lincoln index equation to estimate the total population size of dog whelks, N. N=n1×n2m2=200×12040=600 Questions A biologist wants to estimate the size of a population of turtles on an island. He captures 20 turtles on his first visit, and marks their backs with red paint. Two weeks later he returns to the island and captures 25 turtles. Five of these 25 turtles have red paint on their backs. Using the Lincoln index equation, calculate the estimated population size. Overfishing in Antarctica has resulted in a steep decline in Patagonian toothfish. Fishery scientists are monitoring the population size to see if it is recovering by using the mark– release–recapture method and the Lincoln index equation. On their initial visit, the scientists caught and marked 100 specimens. When they returned on a second visit, they captured a total of 70 Patagonian toothfish. If the scientists estimated the population size was 700, how many of the recaptured toothfish were marked? If any of these assumptions are not true, the accuracy of the predicted total would decrease. Investigating distribution and abundance of organisms in the littoral zone There are a number of methods that can be used to investigate the distribution and abundance of organisms in the littoral zone. A frame quadrat is traditionally a plastic or metal square that sets a standard unit of area for study of the distribution marine organisms (Figure 4.32). They can be of any size but for most field work they are 10–100 cm in length. They are one of the most common tools used by marine ecologists and are best suited to estimate populations for sessile (immobile) marine species. In marine ecology field studies, a transect is any rope marked at regular intervals. For rocky or sandy shore this may often be a 30–50 m tape measure. The regular interval can vary (for example, on a rocky or sandy shore this may be 2–5 m). Two common population sampling methods that use these tools are line transects and belt transects. Figure 4.32: Frame quadrat. Line transects are a quick way to record data on distribution of species; in other words, which species are present. The transect is placed in a straight line between two points in the direction of the environmental gradient you wish to study (for example, between the high and low tide mark on shore). In continuous sampling, any species that touches the line is recorded along the whole length of the transect. Alternatively, if systematically sampling, then the species presence is only recorded at regular intervals on the transect, say, every 0.5 m. Belt transects are similar to the line transect method but give additional data on species abundance. A frame quadrat is placed at regular intervals on one side of the line and the frequency or percentage cover of organisms is recorded. You will investigate this method further in Core Practical Activity 4.2. The belt transect method is an example of systematic sampling as samples are taken at fixed intervals along the transect. It is used to gather data on the distribution and abundance when investigating the zonation of marine species (dependent variable) along an abiotic gradient, such as exposure from high tide to low tide (independent variable). In contrast, frame quadrats can also be used for random sampling. This is often chosen when the environment under study is fairly uniform or time is limited and the habitat size is very large (for example, a flat area of seagrass meadow). In the most basic random sampling method the quadrat is placed anywhere within the sample site. A more rigorous method is to generate a numbered grid of the sample site. A computer program is then used to select which squares to sample. This ensures that there is no observer bias in the selection of the sample locations. When planning a field experiment to sample natural populations of organisms, it is crucial that we design an ethical and safe method. An ethical method is one that evaluates and chooses ways to protect the natural habitat under investigation. For example, an investigation that involves students snorkelling on a coral reef could elect for students to all access the reef from the beach in a single file to reduce the area damage by students walking over and damaging the coral. Designing a safe method involves identify potential hazards and then reducing their risk of occurring. For the coral reef study, this could involve checking the tides and weather forecast to plan the safest time for the sampling. 7 a b c d 8 • • • • • • • 1 2 Test yourself Explain what is meant by each of the following terms: population community species ecosystem. Blue bat star (Patiria pectinifera) are a species of seastar that live in Japanese waters. They were marked with a branding iron as part of a mark–release–recapture method to estimate their populations near Japan. Suggest one positive and one negative effect of this marking process. CORE PRACTICAL ACTIVITY 4.2: INVESTIGATING THE DISTRIBUTION AND ABUNDANCE OF ORGANISMS IN THE LITTORAL ZONE Introduction The littoral zone of a rocky shore has distinct intertidal zones (for example, low tide, mid-tide, high tide and splash zones) where specific groups of organisms are found. The species commonly found in each zone are those that are best adapted to the abiotic and biotic conditions of that zone. Field work using a quadrat and transect line can be performed to investigate these zonation patterns. A typical quadrat for this habitat is a square approximately 50 cm in length with a marked grid of ten squares by ten squares. By collecting quantitative and qualitative data about the relative abundance of species between high- and low-tide marks, you can determine which organisms are found in each zone of a rocky shore (Figure 4.33). This method can also be used to study sandy and muddy shores. Equipment You will need: 30 m tape measure as a transect quadrat 50 cm in length species identification chart recording sheets in a waterproof folder. Safety considerations Check weather forecast to ensure no storms or bad weather. Check tide times to ensure beach study at low tide. Complete a safety and risk assessment before starting the practical. Before you start Research either online or using a species identification chart (field guide) the organisms found on a rocky shore in your local area or elsewhere. In pairs, test each other to see if you can recognise and name them. Discuss the difference between safe and ethical practices. As a group, name three of each and discuss together why they are a benefit. 1 2 3 4 5 • • • • • • • Figure 4.33: Students taking a rocky shore transect of Buckleton’s Beach, New Zealand. Method Look at local tide charts and time the field trip for one hour either side of low tide. Lay out a tape measure to mark your transect route. The transect should be perpendicular to the water’s edge, from high tide to low tide. Use a quadrat to sample the rocky shore organisms. Use a species identification chart to identify common organisms on your rocky shore. Choose four species to collect data for. Choose the number of sampling points. For example, if you are doing a 20 m transect line, you may decide to take 11 quadrat samples – one for every 2 m, starting at high tide (0 m). Recording data There are a number of ways to record your data. For each species you must use the same method for all the transect lines sampled. To be able to calculate averages, it is important to repeat the transect three to five times at different points along the rocky shore. For large fauna (for example, chitons) the simplest method is to count the total number of organisms in your quadrat. Alternatively, if numbers are large (for example, periwinkles or barnacles), then an estimate can be made by counting the number in a quarter of the quadrat and multiplying this value by 4. For plant species (for example, seaweed) vegetation percentage cover can be estimated using the 10×10 quadrat grid. Alternatively, abundance can be recorded using the ACFOR scale: A = abundant (greater than or equal to 30%) C = common (20–29%) F = frequent (10–19%) O = occasional (5–9%) R = rare (1–4%) Abiotic data (for example, temperature, pH or salinity) may also be recorded along the transect line. You may wish to use a data logger probe for this. On your record sheet there should be a column for any features that may affect the abundance of species (for example, if the quadrat is in or near a rock pool or crevice). You may wish to record the height of each quadrat above low tide. The collected data can then be used to plot the topographical shape (profile) of the rocky shore. A sketch map of the rocky shore should also be drawn showing the position of your transect lines. This may be useful when comparing zonation patterns with different coastal characteristics (for example, more exposed versus less exposed to wave action). School-based simulation If you cannot get to a coastline, you can simulate the above method within your school. One way is to use a sports field or school gym. One side of the pitch is designated as high tide while the other end is low tide. Four sets of coloured cones, each representing a different littoral species, are then placed across the surface of the pitch. You can then complete the method above using a 30 m tape measure, as a transect, together with large 1 m×1 m plastic-framed quadrats. Alternatively, a table in a classroom can be used to represent a rocky shore. Four differently coloured self-adhesive stickers or paperclips can be scattered over the table to represent the four littoral organisms being sampled. You can use a miniature frame quadrat, such as 10 cm×10 cm together with a 1 m ruler as a transect. Results and analysis Record your results in a table such as the example for a rocky shore shown in Table 4.4. Figure 4.34 shows how a kite graph can be used to represent the distribution and abundance of each organism (y-axis) against the distance from low to high tide (x-axis). Note that the y-axis represents different methods of estimating abundance. For example, seaweed and periwinkles are both being measured in percentage cover, while mussels and barnacles are measured in number of specimens. Species Quadrat distance from high tide to low tide / m 0 2 4 6 8 10 12 14 16 18 20 periwinkles % cover 1 a b c 2 a b c 3 a b i ii c 4 5 a b 1 periwinkles % cover mussels number barnacles number seaweed % cover Table 4.4: Rocky shore data. The scale chosen for plotting abundance may also vary between each species and does not have to be the same. Kite graphs allow zonation patterns between organisms in the littoral zone to be easily seen. For example, in Figure 4.34, periwinkles are found most commonly towards high tide, mussels and barnacles are found most commonly in mid-tide, seaweed is most commonly found further towards low tide. Line plots of either profile or abiotic data can also be included at the bottom of kite graphs. This may be also useful when discussing reasons why an organism’s niche may be at a certain zone on the littoral habitat. Figure 4.34: A kite graph representing zonation patterns on a rocky shore. Evaluations and conclusions Identify and describe each of the following: independent variable (variable that is changed) dependent variable (variable that is measured) control variables (variables that are kept constant). When using a quadrat, suggest a time when the percentage cover for the species present may total to more than 100%. Quadrats can come in different sizes. It is important to ensure the correct quadrat is used in field work. Discuss the problems associated with using a quadrat that is either too large or too small. A common question asked by students is how far apart should quadrats be placed in a belt transect. Discuss this in a group and see if you can reach a recommendation. For each of the three methods of measuring population size, state whether the data is quantitative or qualitative. Comparing quantitative data with qualitative data, suggest: one benefit of each method one limitation of each method If the tide was coming in and the weather turning, suggest which method of measuring population size would be most appropriate. Collate you class data from this activity. Analyse it to see what zonation patterns you found. Look at the species that are found closest to high tide and explain adaptations that they have that allow them to survive here. Explain common adaptations shared by species that are located at low tide. REFLECTION After completing Core Practical Activity 4.2, think about these questions: Discuss how the findings of this school-based investigation reinforce your understanding of the effects of tidal exposure on the zonation of marine organisms. What were the benefits and limitations of this practical approach? 2 • • • • • • • • • • What problems did you have carrying out this practical activity? How could you avoid them in the future? MATHS SKILLS 4.2 CALCULATING BIODIVERSITY There are a variety of ways to quantify the biodiversity of a marine community. The two main factors to consider are richness and evenness: Richness is the number of species in a community. The more species present in a sample, the ‘richer’ the sample. Richness does not take into account the population size of each species, so gives equal rating to those species that have very few, and those species that have many, individuals. One whelk therefore has as much influence on the richness of a rocky shore as 1000 barnacles. A community with many species is considered to be richer than a community with a lower number of species present. Evenness is a measure of the relative abundance of the different species making up the richness of an area by comparing the population size of each of the species present. A community dominated by one species is considered to be less diverse than a community in which several different species have a similar abundance. A biodiversity measure that accounts for both species richness and species evenness is Simpson’s index of diversity (D). It can be calculated using the following formula and symbols: D=1−(∑(nN)2) where Σ = sum of (total) n = number of individuals of each different species N = the total number of individuals of all the species What do indices of biodiversity mean? A low species index of diversity suggests that: there are relatively few successful species in the habitat the environment is extreme or unstable with relatively few ecological niches, and only a few species are really well adapted to that environment food webs are relatively simple a change in the environment would probably significantly reduce the biodiversity. A high species index of diversity suggests that: There is a greater number of successful species and a more stable ecosystem. More ecological niches are available and the environment is less likely to be hostile. There are complex food webs. Environmental change is less likely to be damaging to the biodiversity of the ecosystem. Species biodiversity may be used to investigate the biological health of a particular habitat. For example, a decrease in the biodiversity index of a coral reef indicates that something has had a negative effect on the reef’s health (for example, an oil spill or overfishing), which marine ecologists and managers need to mitigate. Alternatively, an increase in the biodiversity index may signify that conservation efforts have been effective. Worked example Scientists who discovered a recent whale fall counted the species in Table 4.5. Use this data to calculate Simpson’s index of biodiversity for this habitat. Species Number (n) sharks 2 amphipods 8 whales 1 hagfish 1 crabs 3 total N = 15 1 2 3 Organism Number (n) nN (nN)2 sharks 2 0.133 0.018 amphipods 8 0.533 0.284 whales 1 0.067 0.004 hagfish 1 0.067 0.004 crabs 3 0.200 0.040 Totals N = 15 ∑(nN)2=0.350 Table 4.5: Whale fall data. Putting the figures into the formula for Simpson’s index of biodiversity: D=1−(∑(nN)2)=1−0.350=0.650 Questions Calculate Simpson’s index of diversity for the following data in Table 4.6 for a reef on the Baa Atoll in the Maldives. Species Number of individuals whale shark 1 surgeon fish 12 butterfly fish 3 parrot fish 2 stingray 3 anemone fish 6 Table 4.6: Reef species data. Calculate Simpson’s index of diversity for the mangrove swamp data in Table 4.7. Species Number of individuals mangrove crabs 82 water snake 2 manatee 1 grey snapper 8 loggerhead turtle 2 alligator 2 Table 4.7: Mangrove swamp data. Calculate Simpson’s index of diversity for the rocky shore survey data in Table 4.8. Species Number of individuals barnacle 63 hermit crab 8 black nerita 33 sea slaters 55 4 • • • periwinkles 22 rock snails 11 limpets 6 Table 4.8: Rocky shore survey data. Two prospective marine reserve locations are being investigated for the release of captive-bred turtles. Site A has a Simpson’s index of diversity of 0.821 while site B has an index of 0.411. Based on this information, suggest and explain which location is better suited as a marine reserve. MATHS SKILLS 4.3 SPEARMAN’S RANK CORRELATION While population sampling on a rocky shore you may observe that the distribution of an organism is linked to variations in an abiotic factor, such as time exposed above tide. Alternatively, two different species may seem to occur in the same littoral zone and you may consider if there could be a predator–prey association between them. The null hypothesis (H0) is that there is no correlation between the two species. To test if there is any potential association, you first plot a scatter graph. Three types of relationships are shown in Figure 4.35. In graph A, there is a positive correlation relationship: as x increases, y increases. In graph B, there is a negative correlation relationship: as x increases, y decreases. In graph C, there is no association: an increase in x is not linked to y. If the scatter graph suggests there is a correlation, then you can use different types of correlation coefficient calculations to test the strength of the relationships between the distribution and abundance of species and abiotic or biotic factors. Spearman’s rank correlation is commonly used in marine science to find out if there is a correlation between two sets of variables, when they are not normally distributed. Correlations exist between −1 (perfect linear negative correlation), 0 (no correlation) and +1 (perfect linear positive correlation). In Figure 4.35, graph A is close to a perfect linear positive correlation, so it will have a correlation of just less than 1. The relationship in graph B is a negative correlation but it is not as linear, so it will have a correlation value between 0 and −1. The Spearman’s rank correlation coefficient (rs ) is determined using the following formula and symbols: rs=1−(6×∑D2n3−n) where rs = Spearman’s rank correlation coefficient ΣD2 = sum of the difference between each pair of ranked measurements n = number of pairs of items in the sample The Spearman’s rank correlation coefficient involves ranking the data for each variable and assessing the difference between the ranks. It can be used when you have collected either quantitative data (for example, distance from high tide, light intensity, number of animals or percentage cover of plant species in quadrats) or qualitative data (for example, ACFOR abundance scale for animals plant species in quadrats). Figure 4.35: Types of scatter graph. Worked example A marine scientist collecting rocky shore data wants to find out if there is a relationship between the distribution and abundance of two species (spp). The scientist sampled 10 quadrats and collected the data shown in Table 4.9. Quadrat # 1 2 3 4 5 6 7 8 9 10 spp A % cover 0 1 2 10 8 9 11 7 4 3 spp B % cover 0 5 14 25 28 30 40 32 20 8 Table 4.9: Species percentage cover. The first step is to draw a scatter graph either by hand or by using the graphing facility of a spreadsheet program. The results, shown in Figure 4.36, suggest that there may be a positive correlation. To test the strength of this correlation, we calculate the Spearman’s rank correlation coefficient. First, we count the number of pairs of data sets in the sample. This is 10, as each of the 10 quadrats sampled contains one pair of data sets (one set for species A and one set for species B). Next, we rank each data set for species A, and then rank each data set for species B. For example, for species A, the percentage cover in quadrat 7 is highest (11%), so we rank it 1 for species A. The next highest is in quadrat 4 (10%), so we rank this 2. For species B, the highest percentage cover is also in quadrant 7 (40%), so that is ranked 1 for species B, and so on. Once we have ranked the data sets for both species, we calculate the differences in rank, D, by subtracting the rank of species B from the rank of species A. Each of these differences is then squared to give D2. Figure 4.36: Scatter graph of percentage cover species A vs species B. Last, you add all the D2 values to find ΣD2. This is shown in Table 4.6. Quadrat # 1 2 3 4 5 6 7 8 9 10 spp A % cover 0 1 2 10 8 9 11 7 4 3 spp A rank 10 9 8 2 4 3 1 5 6 7 spp B % cover 0 5 14 25 28 30 40 32 20 8 spp B rank 10 9 7 5 4 3 1 2 6 8 D 0 0 1 −3 0 0 0 3 0 −1 D2 0 0 1 9 1 0 0 9 0 1 ΣD2 21 Table 4.10: Spearman’s calculation table. Substituting into this formula, we can calculate the Spearman’s rank correlation coefficient: rs=1−(6×∑D2n3−n)=1−(6×211000−10)=1−(126990)=1−0.13=0.87(to two decimal places) To test if rs is significant, we use Table 4.11 to compare our calculated rs (0.87) with the critical values of rs at 0.05 (5%) probability for n = 10 (0.65). n 5 6 7 8 9 10 11 12 14 16 critical value rs 1.00 0.89 0.79 0.76 0.68 0.65 0.60 0.54 0.51 0.51 Table 4.11: Critical values of rs at 0.05 probability. Because 0.87 > 0.65, we can conclude that the null hypothesis (that there is no correlation between the two species) is rejected. We can, therefore, accept the alternative hypothesis that there is a significant correlation between the abundance of species A and species B on a rocky shore. A correlation does not necessarily imply a causal relationship. For example, the number of species A and species B may have a positive correlation but this does not mean that there is a direct 1 2 a b c 3 4 • • • • • association (in other words, a change in A does not cause a change in B, or vice versa). For example, there may be an independent abiotic or biotic factor that is causing both to occur in the same rocky shore littoral zone, such as tidal exposure time. Questions Marine science students decided to investigate if there was a correlation between the abundance of green turtles and the percentage cover of seagrass. State a null hypothesis (H0) for this field experiment. Population samples were studied at eight sites and the results are recorded in Table 4.12. To test whether there was a correlation between the abundance of the two organisms, a Spearman’s rank correlation was performed. Complete the ranking for seagrass in Table 4.12. Calculate ΣD2 Calculate rs to three decimal places. Show your working. Table 4.13 shows part of a Spearman’s rank probability table. Use the critical values in Table 4.12 to state what the calculated rs says about the relationship between green turtle and seagrass abundance. Explain if the results prove that sea turtles cause a change in seagrass density. Site # Green turtle / number Rank Seagrass / % cover Rank D − rank difference D2 1 3 5 57 2 0 8 52 3 2 6 30 4 12 2 20 5 1 7 55 6 6 4 34 7 14 1 22 8 9 3 25 Table 4.12: Spearman’s rank calculation table N (number of pairs) 8 9 10 11 12 critical value, rs at significance level 5% 0.738 0.700 0.648 0.618 0.618 at significance level 1% 0.881 0.883 0.794 0.755 0.727 Table 4.13: Spearman’s rank (rs ) probability table. PROJECT: A MARINE SKIT In pairs, choose a marine organism that fascinates you or that you have seen recently in a local or international news article. Prepare a three minute dramatic performance to give in class on this species to cover the following areas: the species’ classification ecological role of the species including its predators and/or prey economic value of the species human impact on the species conservation of the species. Thinking about your project Produce a written multiple choice test with five questions based on your presentation (one for each of the points above) to give to the class. Look at the results from the multiple choice tests and • • analyse which questions classmates failed to answer correctly. Reflect on how you could change your presentation next time to make it better. Reflect on the other presentations made by your classmates: Which ones were the most informative? Where these the same ones you enjoyed the most? Suggest three ways that you could you have improved your own presentation to make it more informative and interesting. 1 a b c d e 2 a b i EXAM-STYLE QUESTIONS Figure 4.37 is a biological drawing of a sea cucumber, one type of echinoderm. Figure 4.37: Biological drawing of a sea cucumber. State one feature seen in Figure 4.37 that is characteristic of echinoderms. [1] State one feature from Figure 4.37 that is not characteristic of other echinoderms. [1] Sea cucumbers are considered to be a keystone species in the Galápagos Marine Reserve. State what is meant by a keystone species. [1] Describe the ecological importance of the planktonic larvae and adult sea cucumbers. [2] Sea cucumbers are considered a delicacy in Asia and commercial fishing of sea cucumbers around the Galápagos Islands began in 1992. In 1999, a fishing season and minimum catch size were introduced but failed to prevent overfishing and the collapse of sea cucumber stocks in 2005. Predict the economic and social impact of this on the local community. [2] [Total: 7] Green crabs, Carcinus maenas, are an invasive species of crustacean now found in Maine, USA. State two characteristics of crustaceans. [2] Figure 4.38 shows the ecological interdependence of ocean temperature, green crabs and clams. Figure 4.38: The interdependence of ocean temperature, green crabs and clams. Describe the relationship between ocean temperature and the green crabs. [2] ii 3 a i ii b c i ii iii 4 a b c 5 Describe the relationship between the distribution of green crabs and clams and suggest an explanation for this. [3] [Total: 7] Kelp are important marine producers often found growing in coastal waters in dense forests. Explain how the gas bladders ensure rapid growth of the kelp. [2] Explain the role of the holdfast. [1] State two uses from the harvesting of macroalgae such as seaweed or kelp. [2] In an area of sea off the coast of Scotland, sea urchins graze on kelp and are in turn eaten by sea otters. Draw this food chain. [1] Fishing and pollution have caused a reduction in the population of sea otters in some areas. Predict how this could affect the abundance of kelp. [2] Suggest how and why a reduction in kelp density could affect future harvests of fish. [2] [Total: 10] Describe three differences between the two species of seagrass in Figure 4.39. [3] Figure 4.39: Two species of seagrass. Describe and explain the adaptations that seagrass possess to enable them to survive in their particular habitat. [5] Tiger sharks live in the shallow coastal waters off Hawaii where they hunt, kill and eat sea turtles. Sea turtles, in turn, graze the seagrass beds. Predict the ecological impact of a collapse in the tiger shark population due to over fishing in the Pacific. [3] [Total: 11] The predominant krill species is the Arctic ocean is the Northern krill, Meganyctiphanes norvegica. Figure 4.40 shows the trophic niche of krill in a food web for the Arctic. a b c i ii iii 6 a b Figure 4.40: Arctic food web. Beluga whales are hunted in Canadian, Alaskan, and Russian Arctic regions for their meat, blubber and skin. Explain how this could affect the population numbers of northern krill in the food chain: phytoplankton → northern krill → small fish → beluga whale. [2] Predict the effect of global warming on northern krill. [3] A study of the stomach contents of ringed seals found that they included a variety of invertebrate prey: 73 crabs, 55 clams, 47 snails, 32 amphipods and 18 northern krill. Calculate, to the nearest whole number, the mean of invertebrates in the seals’ stomachs. Show your working. [2] Use Simpson’s index of diversity (D) to calculate the biodiversity in the seals’ diet. D=1−(∑(nN)2) where: Σ = sum of (total) n = number of individuals of each different species N = the total number of individuals of all the species. [2] A different population of ringed seal was found to have a higher species index of diversity. What does a high species index of diversity suggest about the habitat? [2] [Total: 11] Two rocky shores were sampled using quadrats. Transect lines from rocky shore A found 320 oysters, 335 whelks and 345 barnacles. Transect lines from rocky shore B found 20 oysters, 39 whelks and 941 barnacles. What is the species richness for each rocky shore? [1] Predict which rocky shore has the greatest biodiversity. [3] c d 7 8 Use Simpson’s index of diversity (D) to calculate the biodiversity of rocky shore A. Show your working and give your answer to 3 d.p. D=1−(∑(nN)2) where: Σ = sum of (total) n = number of individuals of each different species N = the total number of individuals of all the species. [2] Simpson’s index of diversity for rocky shore B is 0.113. Justify which rocky shore has the lower biodiversity. [1] [Total: 7] Describe a systematic sampling method to investigate the distribution and abundance of littoral zone organisms living between high tide and low tide on a rocky shore. [10] [Total: 10] Marine copepods are a type of zooplankton that feed on phytoplankton and are eaten by juvenile fish and other plankton-eating organisms (planktivores), including some whales. Many copepods are parasites of other marine organisms (for example worms and snails). Explain how biotic and abiotic factors influence the copepod population in a marine ecosystem. [10] [Total: 10]