4.1 Species, Communities and Ecosystems

Essential Idea

• The survival of living organisms including humans depends on sustainable communities

Understandings

• Species are groups of organisms that can potentially interbreed to produce offspring

• Members of a species may be reproductively isolated in separate populations

• A community is formed by populations of different species living and interacting with each other

• A community forms an ecosystem by its interactions with the abiotic environment

• Species are either autotrophs or heterotrophs

• Autotrophs obtain inorganic nutrients from the abiotic environment (or make their food)

• Consumers are heterotrophs that feed on living organisms by ingestion

• Detritivores are heterotrophs that obtain organic nutrients from detritus (waste or debris of any kind) by internal digestion

• Saprotrophs are heterotrophs that obtain organic nutrients from dead organisms by external digestion

• The supply of inorganic nutrients is maintained by nutrient cycling

• Ecosystems have the potential to be sustainable over long periods

Species

• A species is a group of organisms that can interbreed to produce fertile, viable offspring

  • Members of a single are unable to produce fertile, viable offspring with members from a different species

  • When two different species do produce offspring by cross-breeding, these hybrids are reproductively sterile (or they cannot reproduce) (e.g. liger, mule)

• A population is a group of organisms of the same species that are living in the same area at the same time 

  • Organisms that live in different regions (i.e. different populations) are reproductively isolated and unlikely to interbreed, however are classified as the same species if interbreeding is functionally possible

• Community: 

  • A group of populations living together and interacting with each other within a given area

• Habitat: 

  • The environment in which a species normally lives, or the location of a living organism

• Ecosystem: 

  • A community and its abiotic environment (i.e. habitat)

• Ecology: 

  • The study of the relationship between living organisms, or between living organisms and their environment

How do organisms obtain energy?

• Autotrophs

  • Synthesizes its organic molecules from simple inorganic substances (e.g. CO2)

  • Energy for this process is derived from sunlight (photosynthesis) or the oxidation of inorganic molecules (chemosynthesis)

  • Because autotrophs synthesize their organic molecules they are commonly referred to as producers

    • Most autotrophs derive the energy for this process from sunlight (via photosynthesis)

      Some may derive the needed energy from the oxidation of inorganic chemicals (chemosynthesis)

• Heterotrophs

  • Obtains organic molecules from other organisms (either living / recently killed or their nonliving remains and detritus)

  • Because heterotrophs cannot produce their organic molecules and obtain it from other sources, they are called consumers

• Mixotrophs

  • Certain unicellular organisms may on occasion use both forms of nutrition, depending on resource availability

• Species can be classified according to their mode of nutrition

  • Autotrophs produce their organic molecules using either light energy or energy derived from the oxidation of chemicals

  • Heterotrophs obtain organic molecules from other organisms via one of three methods:

    • Consumers ingest organic molecules from living or recently killed organisms

    • Detritivores ingest organic molecules found in the non-living remnants of organisms (e.g. detritus, humus)

    • Saprotrophs release digestive enzymes and then absorb the external products of digestion (decomposers)

• Consumers

  • Consumers are heterotrophs that feed on living organisms by ingestion

    • Herbivores are consumers that feed principally on plant matter (e.g. cows, sheep, rabbits)

    • Carnivores are consumers that feed principally on animal matter (e.g. crocodiles, wolves, tigers)

    • Omnivores are consumers that have a principal diet composed of both plant and animal matter (e.g. pandas, humans)

• Scavengers

  • Scavengers are a type of consumer that principally feed on dead and decaying carcasses rather than hunting live prey

    • Examples of scavengers include hyenas, vultures and carrion birds (such as crows)

• Detritivores

  • Detritivores are a type of heterotroph that obtains nutrients from non-living organic sources, such as residue and humus

    • Detritus is dead, particulate organic matter – such as decaying organic material and faecal matter

    • Humus is the term given specifically to the decaying leaf litter intermixed within the topsoil

    • Detritivores include dung beetles, earthworms, woodlice, snails and crabs

• Saprotrophs

  • Saprotrophs live on (or in) non-living organic matter, secrete digestive enzymes into it and absorb the products of digestion

• Unlike other types of heterotrophs, saprotrophs do not ingest food

• Are commonly referred to as decomposers

  • Examples of saprotrophs include bacteria and fungi

Nutrient Cycling

• Nutrients refer to the material required by an organism and include elements such as carbon, nitrogen and phosphorus

  • The supply of inorganic nutrients on Earth is finite – new elements cannot be created and so are in limited supply

• Chemical elements are constantly recycled after they are used:

  • Autotrophs obtain inorganic nutrients from the air, water and soil and convert them into organic compounds

  • Heterotrophs ingest these organic compounds and use them for growth and respiration, releasing inorganic byproducts

  • When organisms die, saprotrophs decompose the remains and free inorganic materials into the soil

  • The return of inorganic nutrients to the soil ensures the continual supply of raw materials for the autotrophs 

The water cycle

• The water cycle shows the continuous movement of water within the Earth and atmosphere. 

• It is a complex system that includes many different processes.

• Liquid water evaporates into water vapour, condenses to form clouds, and precipitates back to earth in the form of rain and snow.

The phosphorus cycle

• Phosphorus is a primary component of DNA and energy-storing molecules (ATP), and is present in membranes (phospholipids)

• Phosphorus-based compounds are usually solid and hence phosphorus is not found as a gas in the atmosphere

• Phosphorus (as phosphates) is incorporated and fixed to soil particles but may be released by the weathering of rocks

• Phosphates may enter the waterways via erosion and leaching

The sulfur cycle

• Sulphur is an essential component of living organisms, being apart of many proteins and enzyme cofactors

• Sulfur in the air and soil may be oxidized to form sulfates (SO42–)

• Plants and bacteria reduce sulfates and hence sulfur becomes incorporated into organic molecules

• Sulfur within the soil can also be mineralised into inorganic forms and incorporated with metals

• Burning of fossil fuels releases sulfur as sulfur dioxide (SO2), which is an enabling component of acid rain

Nitrogen Cycle

• The majority of the Earth’s atmosphere is composed of nitrogen gas however it is chemically inert in this form

• Atmospheric nitrogen must be chemically processed by nitrogen-fixing bacteria to be used by plant

• Plants absorb nitrogen from the soil as nitrate ions, nitrate ions or ammonium, while animals consume these products from plants

• When organisms die, nitrogen is in an organic form (proteins) and must be converted back into an inorganic form (ammonification)

• Nitrogen in the soil is converted back into inert nitrogen gas by denitrifying bacteria

Mesocosms

• Ecosystems describe the interaction between biotic components (i.e. communities) and abiotic components (i.e. habitat)

  • They are largely self-contained and can be self-sustaining over long periods

• There are three main components required for sustainability in an ecosystem:

  • Energy availability – light from the sun provides the initial energy source for almost all communities

  • Nutrient availability – saprotrophic decomposers ensure the constant recycling of inorganic nutrients

  • Recycling of wastes – certain bacteria can detoxify harmful waste byproducts

Chi-Squared Test

• The presence of two species within a given environment will be dependent upon potential interactions between them

  • If two species are typically found within the same habitat, they show a positive association

• Species that show a positive association include those that exhibit predator-prey or symbiotic relationships

• If two species tend not to occur within the same habitat, they show a negative association

  • Species will typically show a negative association if there is competition for the same resources

    • One species may utilize the resources more efficiently, precluding the survival of the other species (competitive exclusion)

    • Both species may alter their use of the environment to avoid direct competition (resource partitioning)

• If two species do not interact, there will be no association between them and their distribution will be independent of one another

Quadrat Sampling

• The presence of two species within a given environment can be determined using quadrat sampling

• A quadrat is a rectangular frame of known dimensions that can be used to establish population densities

  • Quadrats are placed inside a defined area in either a random arrangement or according to a design (e.g. belted transect)

  • The number of individuals of a given species is either counted or estimated via percentage coverage

  • The sampling process is repeated many times to gain a representative data set

• Quadrat sampling is not an effective method for counting motile organisms 

Species Interactions

• No species is there total isolation, all organisms interact with both the abiotic environment and other organisms

  • If two species interact directly within a shared environment, they share a positive association (they co-exist)

  • If interactions within an environment are mutually detrimental, they share a negative association (do not co-exist)

• Positive Associations

  • Predator-Prey Relationships

    • Predation is a biological interaction whereby one organism (predator) hunts and feeds on another organism (prey)

    • Because the predator relies on the prey as a food source, their population levels are intertwined

      • If the prey population drops (e.g. due to overfeeding), predator numbers will dwindle as competition increases

      • If the prey population rises, predator numbers will increase as a result of the overabundance of a food source

  • Symbiotic Relationships

    • Symbiosis describes the close and persistent (long-term) interaction between two species

    • Symbiotic relationships can be obligate (required for survival) or facultative (advantageous without being strictly necessary)

    • Symbiotic relationships can be beneficial to either one or both organisms in the partnership:

      • Mutualism – Both species benefit from the interaction (anemone protects clownfish, clownfish provides fecal matter for food)

      • Commensalism – One species benefits, the other is unaffected (barnacles are transported to plankton-rich waters by whales)

      • Parasitism – One species benefits to the detriment of the other species (ticks and fleas feed on the blood of their canine host)  

• Negative Associations 

  • Competition

    • Competition describes the interaction between two organisms whereby the fitness of one is lowered by the presence of the other

    • Competition can be intraspecific (between members of same species) or interspecific (between members of different species)

    • Limited supplies of resources (e.g. food, water, territory) usually triggers one of two types of responses:

      • Competitive exclusion – One species uses the resources more efficiently, driving the other species to local extinction

      • Resource partitioning – Both species alter their use of the environment to divide the resources between them

Niches

• An ecological niche describes the functional position and role of an organism within its environment

  • An ecological niche will be comprised of various components, including:

    • The habitat in which the organism lives

    • The activity patterns of the organism (e.g. periods during which it is active)

    • The resources it obtains from the environment

    • The interactions that occur with other species in the region

• Types of Niches

  • Some species may not be able to occupy their entire niche due to the presence or absence of other species

  • Niche differentiation describes the way by which competing species use the environment differently to exist

4.2 Energy Flow

Essential Idea:

• Ecosystems require a continuous supply of energy to fuel life processes and to replace energy lost as heat

Understandings:

• Most ecosystems rely on a supply of energy from sunlight

• Light energy is converted to chemical energy in carbon compounds by photosynthesis

• Chemical energy in carbon compounds flows through food chains using feeding

• The energy released from carbon compounds by respiration is used in living organisms and converted to heat

• Living organisms cannot convert heat to other forms of energy

• Heat is lost from ecosystems

• Energy losses between trophic levels restrict the length of food chains and the biomass of higher trophic levels

Energy Source

• All green plants, and some bacteria, are photoautotrophic – they use sunlight as a source of energy

  • This makes light the initial source of energy for almost all communities

  • In a few ecosystems, the producers are chemoautotrophic bacteria, which use energy derived from chemical processes

• Light energy is absorbed by photoautotrophs and is converted into chemical energy via photosynthesis

  • This light energy is used to make organic compounds (e.g. sugars) from inorganic sources (e.g. CO2)

  • Heterotrophs ingest these organic compounds to derive their chemical energy (ATP)

  • When organic compounds are broken down via cell respiration, ATP is produced to fuel metabolic processes

Energy Flow 

• Energy enters most ecosystems as sunlight, where it is converted into chemical energy by producers (via photosynthesis)

• This chemical energy is stored in carbon compounds (organic molecules) and is transferred to heterotrophs via feeding

Trophic Levels

• The position an organism occupies within a feeding sequence is known as a trophic level

  • Producers always occupy the first trophic level in a feeding sequence

  • Primary consumers feed on producers and hence occupy the second trophic level

  • Further consumers (e.g. secondary, tertiary, etc.) may occupy subsequent trophic levels

Food Chains

• A food chain shows the linear feeding relationships between species in a community

  • Arrows represent the transfer of energy and matter as one organism is eaten by another (arrows point in the direction of energy flow)

  • The first organism in a food chain is always a producer, followed by consumers (primary, secondary, tertiary, etc.)

Energy Loss 

• Energy stored in organic molecules (e.g. sugars and lipids) can be released by cell respiration to produce ATP

  • This ATP is then used to fuel metabolic reactions required for growth and homeostasis

  • A by-product of these chemical reactions is heat (thermal energy), which is released from the organism 

• Not all energy stored in organic molecules is transferred via heterotrophic feeding – some of the chemical energy is lost by:

  • Being excreted as part of the organism’s feces

  • Remaining unconsumed as the uneaten portions of the food

• The chemical energy produced by an organism can be converted into a number of forms, including:

  • Kinetic energy (e.g. during muscular contractions)

  • Electrical energy (e.g. during the transmission of nerve impulses)

  • Light energy (e.g. producing bioluminescence)

• All of these reactions are exothermic and release thermal energy (heat) as a by-product

  • Living organisms cannot turn this heat into other forms of usable energy

  • This heat energy is released from the organism and is lost from the ecosystem (unlike nutrients, which are recycled)

  • Hence ecosystems require a continuous influx of energy from an external source (such as the sun)

Energy Efficiency

• When energy transformations take place in living organisms the process is never 100% efficient

• Most of the energy is lost to the organism – either used in respiration, released as heat, excreted in faeces or unconsumed

• Typically energy transformations are ~10% efficient, with about 90% of available energy lost between trophic levels

• The amount of energy transferred depends on how efficiently organisms can capture and use energy (usually between 5 – 20%)

• As energy is lost between trophic levels, higher trophic levels store less energy as carbon compounds and so have less biomass

  • Biomass is the total mass of a group of organisms – consisting of the carbon compounds contained in the cells and tissues

  • Biomass diminishes along food chains with the loss of carbon dioxide, water and waste products (e.g. urea) to the environment 

• Because energy and biomass are lost between each level of a food chain, the number of potential trophic levels is limited

  • Higher trophic levels receive less energy/biomass from feeding and so need to eat larger quantities to obtain sufficient amounts

  • Because higher trophic levels need to eat more, they expend more energy hunting for food

  • If the energy required to hunt food exceeds the energy available from the food eaten, the trophic level becomes unviable

Pyramids of Energy

• A pyramid of energy is a graphical representation of the amount of energy at each trophic level of a food chain

• They are expressed in units of energy per area per time (e.g. kJ m–2 year–1)

• Pyramids of energy will never appear inverted as some of the energy stored in one source is always lost upon transfer

  • Each level should be roughly one-tenth of the size of the preceding level (as energy transformations are ~10% efficient)

  • The bottom level will always represent the producers, with subsequent levels representing consumers

• Ecological Productivity

  • In ecology, production (or productivity) refers to the rate of generation of biomass in an ecosystem

    • It is usually expressed in units of mass per area per time (e.g. kg m–2 day–1)

• Primary Production

  • Primary production describes the production of chemical energy in organic compounds by producers

  • The main source of energy for primary production is sunlight, but a fraction may be driven by chemosynthesis by lithotrophs

  • Primary production may be categorized as one of two types:

    • Gross primary production (GPP) is the amount of chemical energy as biomass that a producer creates in a given length of time

    • Net primary production (NPP) is the amount of chemical energy that is not consumed by respiration (NPP = GPP – respiration)

• Secondary Production

  • Secondary production describes the generation of biomass by heterotrophic organisms (consumers)

  • This biomass generation is driven by the transfer of organic compounds between trophic levels via feeding

  • Secondary production may also be categorized according to gross (total) and net (usable) amounts of biomass

• Food Webs

  • A food web is a diagram that shows how food chains are linked together into more complex feeding relationships

  • A food web is more representative of actual feeding pathways within an ecosystem because:

    • Organisms can have more than one food source

    • Organisms can have more than one predator

  • This means that, unlike a food chain, organisms in a food web can occupy more than one trophic level

    • When constructing food webs, position organisms at their highest trophic level (keep all arrows pointing in the same direction)

Ecological Pyramids

• Ecological pyramids show the relative amounts of a specific component at the different trophic levels of an ecosystem

  • The three main types of ecological pyramids measure species numbers, biomass and energy:

    • Pyramid of Numbers

      • A pyramid of numbers shows the relative number of organisms at each stage of a food chain

      • These are usually shaped like pyramids, as higher trophic levels cannot be sustained if there are more predators than pre

      • However, the shape may be distorted if a food source is disproportionately large/biomass compared to the feeder

        • For example, a large number of caterpillars may feed on a single oak tree and many fleas may feed off a single dog host

    • Pyramid of Biomass

      • A pyramid of biomass shows the total mass of organisms at each stage of a food chain

      • These pyramids are almost always upright in shape, as biomass diminishes along food chains as CO2 and waste is released

      • An exception to this rule is found in marine ecosystems, where zooplankton have a larger total biomass than phytoplankton

        • This is because phytoplankton replace their biomass at such a rapid rate and so can support a larger biomass of zooplankton

Biomagnification

• Because energy transformations are only ~10% efficient, higher trophic levels must consume more prey to meet energy needs

  • If a pollutant is ingested by living organisms, it will become concentrated at higher trophic levels as they eat more exposed prey

  • The increase of a substance (such as a pollutant) in a particular organism is called bioaccumulation

    • The increase in the concentration of a substance at a particular trophic level is called biomagnification

    • Bioaccumulation refers to how pollutants enter a food chain, whereas biomagnification refers to the tendency of pollutants to concentrate as they move from one trophic level to the next

  • Because pollutants become concentrated by biomagnification, higher trophic levels are more susceptible to their toxic effects

    • The pesticide DDT cause egg-shell thinning and population declines in species of birds that fed on exposed insects 

    • Heavy metals (like mercury) released into waterways via industrial processes may become concentrated in fish

Ocean Acidification

• The ocean is the largest active carbon sink on Earth

• Some CO2 may remain as dissolved gas within the water, however the majority will combine with water to form carbonic acid

• The solubility of CO2 in seawater is inversely proportional to oceanic temperature (i.e. more soluble in cooler temperatures)

• It is therefore a concern that global warming could limit carbon storage in oceans, exacerbating climate change

• As a result of deforestation and the increased burning of fossil fuels, atmospheric carbon dioxide concentrations have increased

• With more CO2 being absorbed by the oceans, there is an associated increase in the production of H+ ions

  • These H+ ions lower the pH of the ocean, causing acidification 

  • The H+ ions will also combine with carbonate ions, reducing the amounts available to marine organisms

  • This will result in the formation of thinner, deformed shells and reduce the population numbers of reef-building corals

  • The reduction in pH will also dissolve calcium carbonate structures, enhancing the damage to shells and corals

4.3 Carbon Cycling

Main understandings

• Autotrophs convert carbon dioxide into carbohydrates and other carbon compounds

  • In aquatic ecosystems, carbon is present as dissolved carbon dioxide and hydrogen carbonate ions

• Carbon dioxide diffuses from the atmosphere or water into autotrophs

  • Carbon dioxide is produced by respiration and diffuses out of organisms into water or the atmosphere

• Methane is produced from organic matter in anaerobic conditions by methanogenic archaeans and some diffuses into the atmosphere or accumulates in the ground

  • Methane is oxidized to carbon dioxide and water in the atmosphere

• Peat forms when organic matter is not fully decomposed because of acidic and/or anaerobic conditions in waterlogged soils

  • Partially decomposed organic matter from past geological eras was converted into either coal or into oil and gas that accumulates in porous rocks

• Carbon dioxide is produced by the combustion of biomass and fossilized organic matter

  • Animals such as reef-building corals and Mollusca have hard parts that are composed of calcium carbonate and can become fossilized in limestone

The carbon cycle

• The carbon cycle is a biogeochemical cycle whereby carbon is exchanged between the different spheres of the Earth

• The four spheres are the atmosphere (air), lithosphere (ground), hydrosphere (water/oceans) and biosphere (living things)

• Carbon is exchanged in a variety of forms, including:

  • Atmospheric gasses – mainly carbon dioxide (CO2), but also methane (CH4)

  • Oceanic carbonates – including bicarbonates dissolved in the water and calcium carbonate in corals and shells

  • As organic materials – including the carbohydrates, lipids and proteins found in all living things

  • As non-living remains – such as detritus and fossil fuels

  • Different processes facilitate the cycling of carbon between the different forms (e.g. feeding, combustion, etc.)

• Combustion

  • When organic compounds rich in hydrocarbons are heated in the presence of oxygen, they undergo a combustion reaction

• This reaction is exergonic (produces energy) and releases carbon dioxide and water as by-products

• The carbon dioxide is typically released into the atmosphere, increasing the concentration of the gas in the air

4.4 Climate Change

Understandings:

• Carbon dioxide and water vapor are the most significant greenhouse gases

• Other gasses including methane and nitrogen oxides have less impact

• The impact of a gas depends on its ability to absorb long-wave radiation as well as on its concentration in the atmosphere

• The warmed Earth emits longer wavelength radiation (heat)

• Longer wave radiation is absorbed by greenhouse gasses that retain the heat in the atmosphere

• Global temperatures and climate patterns are influenced by concentrations of greenhouse gases

• There is a correlation between rising atmospheric concentrations of carbon dioxide since the start of the industrial revolution 200 years ago and average global temperatures

• Recent increases in atmospheric carbon dioxide are largely due to increases in the combustion of fossilised organic matter

Greenhouse gasses

• Greenhouse gasses absorb and emit long-wave (infrared) radiation, thereby trapping and holding heat within the atmosphere

• Greenhouse gasses collectively make up less than 1% of the Earth’s atmosphere

  • The greenhouse gasses which have the largest warming effect within the atmosphere are water vapour (clouds) and carbon dioxide

    • Water vapour is created via evaporation of water bodies (e.g. oceans) and transpiration – it is removed via precipitation (rain)

    • Carbon dioxide is made by cell respiration and burning fossil fuels – it is removed via photosynthesis and absorption by oceans

• Other greenhouse gasses include methane and nitrogen oxides – these have less impact on the overall warming effect

  • Methane is emitted from waterlogged habitats (like marshes) and landfills – it is also a gaseous waste produced by ruminants

  • Nitrogen oxides are released naturally by certain bacteria and also is emitted in the exhaust by certain vehicles

• Water vapour is the most abundant greenhouse gas in the atmosphere, but is not produced as a product of the human activity

Conservation

• Conservation involves the protection and maintenance of natural resources – such as trees, water and wildlife

  • Conservation can be either in situ (on-site) or ex situ (off-site)

• In situ conservation is the preservation of plant and animal species within their natural habitat

  • This typically involves the designation of protected areas of land as either nature reserves or national parks

  • These areas may require active management to ensure that an appropriate and sustainable ecological balance is maintained

• Ecological monitoring of species may be required to ensure viable population levels are maintained

• Interventions may be required to prevent habitat degradation or competition from invasive species

  • Legislation may be necessary to ensure adequate funding for policing and education

  • In situ conservation offers several advantages when protecting endangered species from extinction:

  • It allows species to live in the environment to which they are adapted and to occupy their natural position in the food chain

• It maintains the animal's normal behaviour (offspring usually acquire skills from parents and peers around them)

• Retaining the natural habitat prevents its eventual loss and ensures it remains available for other endangered species

• Such areas provide a place to return animals from breeding programs as they provide realistic conditions for reintegration

• Reserves in different areas can share information and provide a place for scientific study and developing public awareness

• Ex situ conservation involves the preservation of plant and animal species outside their natural habitats

• Ex situ conservation may typically be required for critically endangered species when urgent intervention is required

  • There are several advantages associated with ex-situ conservation:

  • It allows for greater control of essential conditions 

  • It can improve the chances of successful breeding by utilizing artificial methods

• Ex-situ conservation is also associated with several disadvantages:

  • Such conservation methods do not prevent the potential destruction of their natural habitats

  • Species raised in captivity are less likely to be successfully reintroduced into the wild (loss of autonomous survival)

  • Ex-situ conservation increases inbreeding by restricting the gene pool and restricts the evolution of the species

• There are many ex-situ conservation measures employed around the world, including:

  • Captive breeding – animals are raised and bred in containment (e.g. zoos) to ensure survival prospects

  • Botanical gardens – areas devoted to the collection, cultivation and display of a wide variety of plant species

  • Seed banks – secure sites that store and catalogue seeds, to preserve plant genetic diversity