Chapter 4 Part 2: The Living World: Ecosystems and Biodiversity
The Carbon Cycle
Now, let's talk about carbon. The key events in the carbon cycle are respiration, in which animals (and plants!) breathe in oxygen and give off carbon dioxide, and photosynthesis, in which plants take in carbon dioxide, water, and energy from the sun to produce carbohydrates. In other words, living things act as exchange pools for carbon.
When plants are eaten by animal consumers, the carbon locked in the plant carbohydrote passes to other organisms and continues through the food chain (more on this later in the chapter). In turn, when organisms-both plants and animals-die, their bodies are decomposed through the actions of bacteria and fungi in the soil; this releases CO, back into the atmosphere.
One aspect of the carbon cycle that you should definitely be familiar with for the exam is this: when the bodies of once-living organisms are buried deep and subjected to conditions of extreme heat and extreme pressure, this organic matter eventually becomes oil, coal, and gas. Oil, coal, and natural gas are collectively known as fosil fuels, and when fossil fuels are burned, or combusted, carbon is released into the atmosphere. Finally, carbon is also released into the atmosphere through volcanic action.
There are three major reservoirs of carbon: the first is the world's oceans, because CO, is very soluble in water. The second large reservoir of CO, is the Earth's rocks. Many types of rocks—called carbonate rocks-contain carbon, in the form of calcium carbonate. Finally, fossil fuels are a huge reservoir of carbon.
The Nitrogen Cycle
The Earths atmosphere is made up of approximately 78 percent nitrogen and 21 percent oxygen. (The other components of the atmosphere are trace elements and the greenhouse gases.) Nitrogen is the most abundant element in the atmosphere. For this reason, it might not seem like living organisms would find it dificult to get the nitrogen they need in order to live. But it is! This is because atmospheric N, is not in a form that can be used direcly by most organisms. Thus, while the atmosphere is the major reservoir in this cycle, a step is needed to convert that nitrogen into usable forms. Most of the other reservoirs in the nitrogen cycle hold nitrogen compounds for relatively short periods of time.
In order to keep this rather complicated cycle straight, let's look at it in steps.
Step 1: Nitrogen fixation In order to be used by most living organisms, nitrogen must be present in the form of ammonia (NH,) or nitrates (NO). Atmospheric nitrogen can be converted into these forms, or "fixed," by atmospheric effects such as lightning storms, but most nitrogen fixation is the result of the actions of certain soil bacteria. Fixing is the process that allows nitrogen to be made biologically available, much as photosynthesis makes carbon biologically available. One important soil bacteria that participates in nitrogen fixation is Rbizobium. These nitrogen-fixing bacteria are often associated with the roots oflegumes such as beans or clover. In the future, we may be able to insert the genes for nitrogen fixation into crop plants, such as corn, and reduce the amount of fertilizer that is used.
Step 2: Nitrification-In this process, soil bacteria converts ammonia (NH) or ammonium (NH) into nitrites (NO,) and then to one of the forms that can be used by plants nitrate (NO,).
Step 3: Assimilation-In assimilation, plants absorb ammonium (NH), ammonia ions (NH), and nitrate ions (NO,) through their roots. Heterotrophs, or organisms that receive energy by consuming other organisms, then obtain nitrogen when they consume plants' proteins and nucleic acids.
Step 4: AmmonificationIn this process, decomposing bacteria convert dead organisms and other waste to ammonia (NH) or ammonium ions (NH), which can be reused by plants or volatilized (released into the atmosphere).
Step 5: Denitrification gas (N,O). In denitrification, specialized bacteria (mostly anaerobic bacteria) convert ammonia back into nitrites and nitrates, and then into nitrogen gas (N) and nitrous oxide. These gases then rise to the atmosphere.
The Phosphorus Cycle
The phosphorus cycle is perhaps the simplest biogeochemical cycle, mostly because phosphorus does not exist in the atmosphere outside of dust particles. Phosphorus is necessary for living organisms because it’s a major component of nucleic acids, ATP (cellular energy), cell membranes, and other important biological molecules. One important idea for you to remember about the phosphorus cycle is that phosphorus cycles are more local than those of the other important biological compounds.
For the most part, phosphorus is found in soil, rock, and sediments; it’s released from these rock forms through the process of chemical weathering. Phosphorus is usually released in the form of phosphate (PO43-), which is soluble and can be absorbed from the soil by plants. Symbiotic relationships that form between fungi and plants are known as mycrrohizae. In these relationships, mychorrhizal fungi colonize the root system of a host plant, which increases the water and nutrient absorption capabilities of the plant, while the plant provides the fungi with carbohydrates formed from photosynthesis. You should know that phosphorus is often a limiting factor (any factor that controls a population’s growth—food, space, water) for plant growth, so plants that have little phosphorus are stunted. Phosphates that enter the water table and travel to the oceans can eventually be incorporated into rocks in the ocean floor. Through geologic processes, ocean mixing, and upwelling, these rocks from the seafloor may rise up so that their components once again enter the terrestrial cycle. Take a look at the phosphorus cycle shown in the diagram. Humans have affected the phosphorus cycle by mining phosphorus-rich rocks in order to produce fertilizers. The fertilizers placed on fields can easily leach into the groundwater and find their way into aquatic ecosystems where they can cause eutrophication. Eutrophication occurs when a body of water receives excess nutrients. The abundance of nutrients can cause an overgrowth of algae and deplete the water of oxygen.
Sulfur
The last biogeochemical cycle we’ll talk about is sulfur. Sulfur is one of the components that make up proteins and vitamins, so plants and animals both need sulfur in their diets. Plants absorb sulfur when it is dissolved in water, so they can take it up through their roots when it’s dissolved in groundwater. Animals obtain sulfur by consuming plants. Most of the Earth’s sulfur is tied up in rocks and salts or buried deep in the ocean in oceanic sediments, but some sulfur can be found in the atmosphere. The natural ways that sulfur enters the atmosphere are through volcanic eruptions, certain bacterial functions, decomposition in estuaries, and the decay of once-living organisms. When sulfur enters the atmosphere through human activity, it’s mainly via industrial processes that produce sulfur dioxide (SO2) and hydrogen sulfide (H2S) gases. We’ll talk more about sulfur and how it contributes to air pollution in Chapter 9.
FOOD CHAINS AND FOOD WEBS
Now that we’ve reviewed the abiotic elements essential to ecosystems, it’s time to begin our study of the living, biotic components of the Earth. Together, all of the living things on Earth constitute the biosphere. All living things can be classified by how they obtain food. You might recall that plants and some cyanobacteria are capable of making their own food through photosynthesis, and that some animals (for example, mice) eat plants. Some animals (for example, humans) eat both plants and animals, and some animals (for example, wolves) eat only other animals. There are actually two fancy terms that are normally used to describe these broad categories of organisms: autotrophs are those organisms that can produce their own organic compounds from inorganic chemicals, while heterotrophs obtain food energy by consuming other organisms or products created by other organisms.
Finally, as unpleasant as it might be to think about, some animals feed only on the remains of other plants and animals! All of these different types of living things fall into specific categories—and you will definitely need to memorize all of these terms before the test, if you don’t already know them!
Producers
Producers are organisms that are capable of converting radiant energy, or chemical energy, into carbohydrates. The group of producers includes plants and algae, both of which can carry out photosynthesis. While most producers make food through photosynthesis, a few autotrophs make food from inorganic chemicals in anaerobic (without oxygen) environments, through the process of chemosynthesis. Chemosynthesis is only carried out by a few specialized bacteria, called chemotrophs, some of which are found in hydrothermal vents deep in the ocean. At this point, let’s discuss a few other environmental science terms that you’ll be required to know for the exam. The Net Primary Productivity (NPP) is the amount of energy that plants pass on to the community of herbivores in an ecosystem. It is calculated by taking the Gross Primary Productivity, which is the amount of sugar that the plants produce in photosynthesis, and subtracting from it the amount of energy the plants need for growth, maintenance, repair, and reproduction. NPP is measured in kilocalories per square meter per year (kcal/m2/y). In other words, the Gross Primary Productivity of an ecosystem is the rate at which the producers are converting solar energy to chemical energy (or, in a hydrothermal ecosystem, the rate of productivity of the chemotrophs). Perhaps not surprisingly, the net productivity of an ecosystem
is a limiting factor for its number of consumers. A limiting factor is a factor that controls a population’s growth. It can be many things: space, available food, water, nutrients, and as we just mentioned, the net productivity of an ecosystem.
Consumers
Consumers are organisms that must obtain food energy from secondary sources, for example, by eating plant or animal matter. There are a number of different types of consumers, all of which you should commit to memory!
Primary consumers: This category includes the herbivores, which consume only producers (plants and algae).
Secondary consumers: An organism that consumes a primary consumer is a secondary consumer.
Tertiary consumers: An organism that consumes a secondary consumer is a tertiary consumer.
Detritivores: The organisms in this group derive energy from consuming nonliving organic matter such as dead animals or fallen leaves. They include termites, earthworms, and crabs.
Decomposers: These are organisms that consume dead plant and animal material. The process of decomposition returns nutrients to the environment.
Saprotrophs: These are decomposers that use enzymes to break down dead organisms and absorb the nutrients; they include bacteria and fungi.
Note that one organism may occupy multiple levels of a food chain. When eating a hamburger with toppings, you are a primary consumer because you are eating tomatoes and lettuce, and a secondary consumer by eating the beef. Let’s move on and talk about how energy flows through all of these different types of organisms in ecosystems.
Food Chains
As you probably recall, energy flows in one direction through ecosystems: from the sun to producers, to primary consumers, to secondary consumers, to tertiary consumers. In an ecosystem, each of these feeding levels is referred to as a trophic level. With each successive trophic level, the amount of energy that’s available to the next level decreases. In fact, the laws of thermodynamics dictate that only about 10 percent of the energy from one trophic level is passed to the next; most is lost as heat, and some is used for metabolism and anabolism. Interestingly enough, this is why food chains rarely have more than four trophic levels. Food chains are usually represented as a series of steps, in which the bottom step is the producer and the top step is a secondary or tertiary consumer. In food chains, the arrows depict the transfer of energy through the levels, and in fancier food chains, the relative biomass (the dry weight of the group of organisms) of each trophic level will often be represented.
One final note about food chains: in a food chain, only about 10% of the energy is transferred from one level to the next. The other 90% is used for things like respiration, digestion, running away from predators—that is, it’s used to power the organism doing the eating! This is known as the 10% Rule. In other words, the producers have the most energy in an ecosystem; the primary consumers have less energy than producers; the secondary consumers have less energy than the primary consumers; and the tertiary consumers will have the least energy of all. The amount of energy (in kilocalories) available at each trophic level organized from greatest to least is an energy pyramid.
Food Webs—Tangled Food Chains
As you’re probably already aware, food chains are an oversimplified way of demonstrating the myriad feeding relationships that exist in ecosystems. Because there are so many different types of species of plants and animals in ecosystems, their relationships in real-world ecosystems are much more complicated than can be depicted in a single food chain. Therefore, we use a food web in order to represent feeding relationships in ecosystems more realistically.
When something changes in an ecosystem, the effects of that change can quickly spread, partly because food webs link species together. Food webs contain positive and negative feedback loops, so that when one species is added or removed, the rest of the food web is affected, sometimes drastically.
BIODIVERSITY
The term biodiversity is used to describe the number and variety of organisms found within a specified geographic region, or ecosystem. It also refers to the variability among living organisms, including the variability within and between species and within and between ecosystems. Therefore, when we talk about the biodiversity of an area, we must specify the aspect of biodiversity that we’re describing, or else the term is too vague to be comprehensible. Species richness refers to the number of different species found in an ecosystem. In general, however, biodiversity in an ecosystem is a good thing. The more biodiversity in a certain species within an ecosystem, the larger and more diverse the species’ gene pool, and the greater its chance of adaptation and thus survival.
What happens when an ecosystem is threatened or habitat is lost? This habitat loss tends to lead to a loss of specialist species, which then can lead to a loss of generalist species. Additionally, species that require large territories tend to suffer, and their numbers are reduced. When a large proportion of a population is lost, this leads to a bottleneck, which can reduce genetic diversity within the species. A disturbance in an ecosystem will affect the total biomass, species richness, and net productivity over time. Ecosystems with larger numbers of species (species richness) tend to recover more easily from disruptions. This is a reflection of the fact that the more genetically diverse a given population is, the better it can cope with an
environmental disturbance. So you can see that ecosystems and biodiversity are inextricably linked. One important law to be familiar with for this test is the Law of Tolerance. The Law of Tolerance describes the degree to which living organisms are capable of tolerating changes in their environment. Living organisms exhibit a range of tolerance, and even individuals within a population tolerate changes to their environment differently. This concept is the basis for natural selection, which drives evolution. Another important law for you to know is the Law of the Minimum, which states that living organisms will continue to live, consuming available materials until the supply of these materials is exhausted.
One special case of the interrelatedness of habitat, biodiversity, and adaptation is the theory of island biogeography, which is a field that studies species richness and diversification in isolated communities: oceanic islands, and also other isolated ecosystems, such as mountain peaks, oases, seamounts, and fragments of habitat separated by human development. The number of species found on an island or in an isolated area is determined by two factors: immigration and extinction. Since islands are often colonized by new species arriving from elsewhere, immigration is a main factor. Once established on an island or in an isolated ecosystem, many species evolve to become specialists
as an adaptation to the limited resources available. Immigration becomes a factor again when invasive species arrive, since the typically-generalist invasives may outcompete the native specialists and threaten their long-term survival. In addition to understanding how ecosystems and biodiversity function together and how disturbances affect them, you should also have an
understanding of how the complex systems at play in the natural world benefit humans.
Ecosystem Services
Ecosystem services are benefits that humans receive from the ecosystems in nature when they function properly. There are four categories: provisioning services: providing humans with water, food, medicinal resources, raw materials, energy, and ornaments; regulating services: waste decomposition and detoxification, purification of water and air, pest and disease control and regulation of prey populations through predation, and carbon sequestration; cultural services: use of nature for science and education, therapeutic and recreational uses, and spiritual and cultural uses; and supporting services (the ones that make other services possible): primary production, nutrient recycling, soil formation, and pollination. Human disruptions to ecosystem services can detrimentally affect our ability to benefit from them, resulting in ecological and economic consequences for us.
How Ecosystems Change
Believe it or not, oftentimes the biotic balance in a community is maintained by a single species, known as the keystone species. The name keystone comes from the last stone placed in an arch bridge, which is the key stone. A keystone species is a species whose very presence contributes to an ecosystem’s diversity and whose extinction would consequently lead to the extinction of other forms of life. For example, fig trees are the keystone species in a tropical forest; likewise, wolves were introduced back into Yellowstone Park because without wolves to control the number of herbivores, the ecosystem had drastically changed. As a general rule, if the keystone species is removed from an ecosystem, then the ecosystem completely changes. Indicator species are species that are used as a standard to evaluate the health of an ecosystem. They are more sensitive to biological changes within their ecosystems than are other species, so they can be used as an early warning system to detect dangerous changes to a community. Trout are a common indicator species, because they are particularly sensitive to pollutants in water. The disappearance of trout from a particular habitat is a warning that that habitat is becoming polluted. Indigenous species are those that originate and live or occur naturally in an area or environment. With increasing frequency, however, new species are being introduced into ecosystems by chance, by accident, or with intention. While some introduced species cannot find a niche and die out, many others are quite happy in their new environment, and compete successfully with the indigenous species. One example of this is grey squirrels, which were introduced to England in 1876. The grey squirrel competed with England’s native species of squirrel, the red squirrel, and today there are fewer than 30,000 red squirrels alive in England. Another example of the harm that introduced species can do was seen when, in 1904, a fungus was introduced accidentally into the deciduous forests of the eastern United States. This fungus caused a blight that killed nearly all of the chestnut trees by the early 1950s. Although some people don’t like to use the term invasive species because they feel that it’s derogatory, it is often used to describe introduced species. Two other examples of invasive species are zebra mussels, which were introduced into the Great Lakes when ships dumped ballast water into the lakes, and the quickly growing vine kudzu, which was originally introduced in the southeastern United States in order to control the problem of erosion.
Ecological Succession
Communities are not static; they are constantly changing. Species of plants and animals are continually coming and going, evolving and dying out. Some of the changes that take place in a geographic area are predictable ones that can be described as ecological succession. If ecological succession begins in a virtually lifeless area, such as the area below a retreating glacier, it is called primary succession. Secondary succession is ecological succession that takes place where an existing community has been cleared (by disturbance events such as fire, tornado, or human impact), but the soil has been left intact. Succession in a disturbed ecosystem will affect the total biomass, species richness, and net productivity over time. The organisms in the first stages
of either type of succession are referred to as pioneer species, and typically have wide ranges of environmental tolerance. These pioneers, over time, usually adapt to the particular conditions of the habitat. This may result in the origin of new species. The communities in each stage of succession facilitate the environmental changes that will allow the next stage to take over. The final stage of succession, in which there is a dynamic balance between the abiotic and biotic components of the community, is referred to as the climax community.
How does a new habitat full of bare rocks eventually turn into a forest? The first stage of the job usually falls to a community of lichens. Lichens are hardy organisms. They can invade an area, land on bare rocks and erode the rock surface, and over time turn them into soil. Lichens are pioneer organisms. Once lichens have made an area more habitable, other organisms can settle in. Lichens are replaced (out-competed!) by mosses and ferns, which in turn are replaced by tough grasses, then low shrubs, then conifers, then short-lived hardwood trees such as dogwood and red maple trees, and finally long-lived hardwood trees. Refer to the flowchart of ecological succession for a deciduous forest. Note that the stages are classified by the major new plant group, but remember that with the introduction of each new plant species comes an array of different animal species that exploit it. When the size of an organism’s natural habitat is reduced, or when, for example, development occurs that isolates the habitat, this process is called habitat fragmentation. Habitat fragmentation can be quite damaging. As you know, ecosystems are not isolated; they abut each other and meet at wide and overlapping boundaries, called ecotones. At these boundaries, there is greater species diversity and biological density than there is greater species diversity and biological density than there is in the heart of ecological communities, and this is called the edge effect. Some species can only live on the edge of certain habitats, and if the boundaries of a habitat are changed, a new edge is created, damaging both the edge and interior habitats. When the changes taking place in a geographic area result from less-predictable events and have more drastic consequences for an ecosystem, they may be considered disruptions. Human-made disruptions such as pollution, habitat destruction, and depletion of natural resources will be a major focus in the coming chapters. But what about natural disruptions? Some natural disruptions to ecosystems have environmental consequences that can exceed those caused by humans. The Earth’s climate has changed, over the course of geological time, many
times and for varied reasons, including internal causes (changes in the type and distribution of species and the effects they produce on climate and changes in ocean-atmosphere circulations) and external ones (changes in the Earth’s orbit, solar output, volcanism, plate tectonics and the resulting size and configuration of continents, and asteroid impacts). Some of these factors cause periodic or episodic change, while others cause change at random times, and the timescales involved vary greatly.
When global climate change occurs, the effects are far-reaching. For example, changes in the amount of glacial ice on Earth have caused sea levels to vary quite significantly over geological history, which in turn changes the size and shapes of landmasses. When big changes occur in the environment, habitats can change on enormous scales, which in turn can cause extinctions, bottlenecks, and short- and long-term migrations among the species inhabiting the affected ecosystems.
Whew. You’re done with this chapter! Before moving on to the next chapter (Populations), answer all of the questions in the drill following the key terms list—and don’t forget to use the techniques you learned in Part III.