The Big Picture

Managing ecosystems and the populations that live in them requires that humans understand the complexities of those ecosystems, the ecological communities they contain, and interactions among the species in the community. This chapter will explain the concept of the ecosystem and the ecological community in detail. Whereas an ecosystem is composed of all the living and non-living components of the environment in a given area, an ecological community is composed of all the living species (or populations) in a given area that interact with one another. Note that this meaning of "community" is quite different that normally used by most people, which refers instead to all the people interacting in a social network. Humans attempt to manage species that are endangered or others that have grown so abundant that they have become pests, but without an understanding of how these species fit into their ecosystems and community food webs, humans cannot rationally manage them. Interactions among species in communities and ecosystems are often subtle and a change in one species’ abundance will influence many others, often in unpredictable ways. For example, in the Case Study (Figure 6.1), the species involved in a community food web in the eastern forest ecosystem interact with each other and changes in one species’ abundance influences the others. One of these species is the tick Scapularius (Figure 6.1c) that carries Lyme disease, a pest species that afflicts humans. The ticks are carried by and feed on mice and deer, and the mice and deer population levels are tied to fluctuations in the oak tree acorn production (they feed on acorns, especially in "mast" years of large acorn production) and the forest clearing patterns created by humans. Thus, the Lyme disease problem is changed by these natural and human-induced changes because of the interactions in a food web. Because the mice also feed on gypsy moth larvae, in years when mice populations decrease because of poor acorn production, there is an increase of gypsy moth larvae, which consume oak tree leaves and can cause the death of the oak forest. Thus, two pest species are involved, and thus forest and wildlife management policies that reduce mice and deer populations in order to control Lyme disease will also have the side effect (or interaction) of increasing gypsy moth larvae. In this chapter, the authors emphasize the complexity of ecosystem and community interactions such as these. In addition, the authors discuss how ecosystems and communities are structured, what trophic levels are and how they function in food chains and food webs, what indirect and direct interactions in ecological communities are, and why ecosystems are difficult to manage, restore and create.

Frequently Asked Questions

• An ecosystem is the minimum system of interacting species and their abiotic resources in an area necessary to sustain life. Normally, an ecosystem must have some kind of producers, consumers, and decomposers (see FAQ’s on each of these below).
• The boundaries of an ecosystem are often poorly defined or indistinct, because one ecosystem is really not independent of another. Thus, the Atlantic Ocean and the Hudson River are separate ecosystems, but because the Hudson flows into the Atlantic, and water, nutrients, and aquatic organisms are exchanged between them, they are not independent. Boundaries for ecosystem studies must be defined, and the amount of energy and material entering and leaving the defined area of an ecosystem can then be measured.

• An ecological community is composed of all the interacting species in an ecosystem, which are the populations of producers, consumers and decomposers. Because this definition excludes the abiotic environment, it is distinctive from an ecosystem.
• The organisms in a community must interact in some way to be considered part of the same community. The interactions can be direct (a predator consuming prey) or indirect (a predator prey’s is outcompeted by another animal, so that both the predator and prey populations are affected; See the FAQ’s on direct and indirect interactions below).
• Communities may be defined in such a way by ecologists that only a portion of the ecological community is examined (i.e., the bird community, the fish community, the plant community, etc.), but as with an ecosystem, the geographic area of the community must also be established.

• Trophic comes from the Greek word for nourishment or food. Trophic interactions are feeding interactions depicting which species consumes another.
• Trophic levels are the levels in a food chain where organisms typically feed.
• Tropho-dynamics are the changes in feeding relationships over space or time in ecosystems.

• A trophic level is a numerical level within a food chain or food web where an organism typically feeds or is consumed.
• Trophic level numbers are assigned starting with 1 at the base of the chain or web (the plants, primary producers, or autotrophs in the ecosystem), 2 for the herbivores (also known as the primary consumers), 3 for the carnivores (or secondary consumers), and continuing on until the decomposers are reached.

• An autotroph is an organism that produces its own food (auto = self, trophic = food in Greek). An equivalent term for an autotroph is producer.
• Typical autotrophs in ecosystems are photoautotrophs, such as plants, algae, and photosynthetic bacteria, all of which use the energy in sunlight to make the energy-rich molecule glucose, a sugar, from carbon dioxide and water.
• In some marine ecosystems where there is no light (like the deep sea rift vents), there are chemoautotrophs at the base of the food web. These chemoautotrophic bacteria make glucose from carbon dioxide and hydrogen sulfide in the water rather than the sun’s energy.

• A heterotroph is an organism that consumes other organisms for food (hetero = other in Greek). An equivalent term for a heterotrophic organism is consumer.
• All animals are heterotrophic, and many microscopic bacteria, fungi, and protozoans are also heterotrophic. Some algae, especially dinoflagellates, can switch between autotrophy and heterotrophy.
• Decomposers, scavengers, carnivores, herbivores, and omnivores are all different types of heterotrophs.

• A herbivore is a consumer that eats plants or autotrophs. Many insects, rabbits, deer, and cows are examples of herbivores.

• A carnivore is a consumer that kills and eats other consumers, but no producers. Wolves, lions, dogs, and cats, are all examples of carnivores.

• An omnivore is a consumer that eats other consumers and producers. Bears, chimpanzees, and humans are all omnivores.

• A scavenger is a non-fungal, non-bacterial consumer that eats recently dead organisms that have been killed by another organism or by an accident. Vultures, beetles, and crabs are scavengers.

• Bacterial and fungal consumers that consume dead organisms are decomposers. Typically, decomposers produce extracellular enzymes to consume their food, whereas scavengers consume the dead organisms and digest them internally.

• A primary consumer is a herbivore.

• A secondary consumer is a carnivore that consumes herbivores.

• A food chain is a linear arrangement of feeding relationships that trace energy flow. That is, a food web starts with an autotroph population (the producers), and then a heterotroph population (the primary consumers) consumes some of the autotrophs, and the sequence continues in a chain, with one or more additional heterotroph populations (secondary consumers, tertiary consumers, etc.) included.
• Decomposers are the last heterotrophic populations to consume the energy obtained by the autotrophs at the base of the chain.
• Although this food chain concept is central to understanding ecosystems, there are few isolated, linear food chains in nature. Food chains are actually part of larger, more complex food webs (see next FAQ).

• It is rare that a species is eaten by only one other species, or that a heterotroph eats only one species, as a food chain implies. A food web is a network of all the possible trophic interactions; thus it is an aggregation of the individual food chains in an ecosystem. Thus, a food web is a more realistic view than a food web of what happens during trophic interactions in an ecosystem.
• Food webs can be simple (see a Closer Look 6.1, "Yellowstone Hot Springs Food Chain", which is really a foodweb with 20 species interacting), and relatively complex [see Figure 6.3 (a), "A Terrestrial Food Web" and Figure 6. 3 (b) "An Oceanic Food Web"].
• All of the diagrams seen in this book and every other textbook are simplified versions of actual food webs, because at some level species are grouped together into large categories such as "decomposers" or "phytoplankton". Even the complex diagram shown in Figure 6.3 (c) ("The Food Web of the Harp Seal") is a simplified version of an actual food web. It would be desirable to specify the food web structure at the species level, but such a diagram would be an uninterpretable tangle of lines.
• Most ecologists are now using computer network analysis to depict the food web structure of an ecosystem at the species level. A computer model has no theoretical limit to the number of interactions that can be included, and more importantly, the trophic interaction strengths can be specified (that is, when a species eats more of one species than another, it has a potentially stronger interaction strength on the first species than the second).

• Just as a building has an underlying structure, ecosystems and communities have an underlying framework of interactions among living (or biotic) and non-living (abiotic) parts of the system that are depicted in the food web and the biogeochemical cycles.
• Recall that back in Chapter 4, we learned about the biogeochemical cycles in which interactions occur between abiotic and biotic parts of an ecosystem; these are part of the ecosystem structure, but not the ecological community structure.
• The community structure is restricted to the biotic interactions among species, including trophic interactions, competitive interactions, and symbiotic interactions.
• Ecologists measure ecosystem and community structure using mathematical indices that depend greatly on the number of species present in the community, their interaction strengths in the food web, the interspecific interactions and the amount of abiotic resources present in the ecosystem.

• Some food chains within an ecosystem food web can be shown to have as many as 6 or 7 linkages, so we can say that there may be an upper limit to the number of trophic levels at 7.
• The decomposers make it difficult to determine how many trophic levels are present, because they are poorly studied at the species level. Some of the decomposer’s energy is obtained at trophic level 2 (when a plant dies and is decomposed), but some is obtained after the top-most trophic level (such as when a human dies who has eaten sharks, which ate large fishes, which ate smaller fishes, which ate zooplankton, which ate phytoplankton). In this latter case, the decomposer is at trophic level 7.
• Typically it is rare for there to be more than 5 trophic levels in an ecosystem, because the energy required to support that many trophic transfers is too great (see Chapter 8). If humans were forced to consume nothing but sharks, for example, we would overfish the world’s shark populations within a single year. Some small amount of the energy obtained by producers makes it to trophic level 7, but most of it has gone to the decomposers by trophic level 4.

• When one species kills or consumes another in a community, it usually has a negative effect on the prey population. This is called a direct interaction.
• Much evidence has accumulated that predators can cause direct negative effects on their prey populations, locally causing the extinction of the prey. Unless there is a natural prey refuge or there is a limit by some other species to a predator population, the predator can eliminate the prey everywhere.
• Some species are currently listed as endangered or threatened because the community structure of the ecosystem where they live has been changed by humans and predators are directly causing their extinction. Human hunters are the best known example of this, having caused the extinction of woolly mammoths and saber tooth cats during pre-historic times, the great whales in the recent past, and are even today driving Bengal tigers and other wildlife to extinction (see Chapter 12).

• An indirect trophic interaction occurs when one species preys upon another and drives its population downward as in a direct effect, but a third species that interacts with the prey is caused to increase (positive indirect interaction) or decrease (negative indirect interaction) in abundance.
• These types of community interactions are very common, but their outcomes are difficult to predict because they require extensive knowledge of the food web structure and interaction strengths among the species.
• The best known example of the indirect community interaction effect occurs in the underwater kelp forests along the coast of Pacific Northwest of the USA as shown in Figure 6.4. In these communities, sea otters feed on sea urchins, which in turn consume the kelp. This simple food chain is really more complex, because humans prey on sea otters, and sea otters can also eat clams and other shellfish. In a typical situation, before humans came to prey heavily on the otters, the kelp forests were abundant. This was because the otters kept the urchin populations in check, and the urchins could only consume a limited amount of kelp. When humans began killing the otters (for fur, mainly) the urchins were no longer kept in check. There was an increase in sea urchin density (positive indirect effect of human predation on otters) around islands where humans lived and caused the local extinction of the sea otters (direct effect by humans preying on otters). However, humans created an additional negative indirect effect with their predation on sea otters: The kelp forests (as measured by percentage of the bottom covered by kelp forest) disappeared in areas without otters. This negative indirect effect occurred because the otters no longer held the urchins in check and they consumed the kelp. In addition, many other species were impacted by this human predation on sea otters, because the kelp forest supported a diversity of fishes and invertebrates that disappeared along with the kelp. It can be seen that changes in trophic interactions can cause far-reaching impacts in ecosystems, and hence much research effort is being devoted right now to understanding food webs and trophic interactions.

• Dominance is the degree to which one or two species is the most abundant in a community. This will be discussed in greater length in Chapter 7.

• The keystone species is one that, if it is removed or is caused to decrease in abundance, then its absence will cause a large change in community structure.
• The analogy is to the stone that is placed at the top of an arched doorway by masons. If the keystone is removed, the archway will collapse.
• A change in community structure can occur is certain species are removed. The sea otter in the example above is considered to be a keystone species.

• There is some controversy among ecologists over whether communities are holistic or individualistic in nature.
• Those that hold the holistic view suggest that communities act like superorganisms, that if any species are removed the community will no longer function as a whole. In support of this idea is the observation that many communities have autotroph, heterotroph, and decomposer species that occur in the same places together all the time. However, this view, while appealing intuitively, cannot be fully supported by experiment and observation. Removal experiments have shown that some species can be easily removed without a major impact on the others. Others species, like the sea otter or just about any dominant autotroph, if removed, can cause a major change in the ecosystem.
• The individualistic view suggests that all species occur together by chance and they just happen to survive where they are because they are lucky. Many species (like some trees and coral reef fishes) seem to have a random distribution pattern on a small time or spatial scale, so this lends support to this viewpoint. However, it may turn out that when these species are studied over a broader scale, there will be patterns of co-occurrence with other species that have not yet been detected.
• It is best to think of these viewpoints as representing extremes of a continuum. Some species are more central in the community structure than others and they are ones that give credence to the holistic view. Other species are rare or do well only when natural disturbances have eliminated their competitors, so they support the individualistic view. This is an important theoretical question in ecology, but also one that can have practical implications for saving endangered species and restoring ecosystems damaged by humans. For example, if species can be removed without impacting others greatly, humans may decide to allow species extinctions caused by humans to occur (although nearly every ecologist would oppose such a move no matter which view they hold, because of the danger of losing a species that might be needed later). Alternatively, if all species are needed to keep the Earth’s ecosystems alive and functioning, then we should strive at all costs to keep species from going extinct.

• Within a watershed, any drop of rain that falls to the ground flows out in the same stream.
• A watershed is a good way to define an ecosystem occurring on land, because much of the nutrients, soils, and water are forced to enter via rainfall and exit that stream or river. The biotic community is less constrained by the watershed boundaries, but often they seem to reside within a single watershed their whole lives.

• By controlling the inputs and output of certain abiotic factors and removing or planting certain species, humans have learned to manage ecosystems for their benefit.
• Agriculture can be viewed as a form of ecosystem management, because species of crops are planted, fertilizer and water are added, and competitor species (weeds), pest species (herbivorous insects) and predators (like coyotes and wolves) are removed.

Ecology In Your Backyard

• Can you develop a food web for your own feeding interactions in your ecological community? Try to do this by writing down everything you consume for a week, noting what kind of organism it was made from. Many of the things we eat are made from plants as well as animals and everything we consume comes from an ecosystem somewhere. Take a hamburger, for instance: the meat comes from cattle in the Midwestern US or it is imported from South America, the bun comes from wheat flour, the yeast used to leaven the bun’s dough is actually a live fungus and thus a decomposer, the tomatoes and lettuce are grown in California on a farm. Assign a trophic level to each component of the food you consume and you can begin to draw a food web of your diet. Here are some examples of common foods and their trophic levels (remember you would be feeding one trophic level higher) :

• At what trophic level are you feeding most of the time? To determine this, you can count the number of trophic interactions with each of the trophic levels you’ve recorded above (50 links to trophic level 1, 20 links to trophic level 2, 10 links to trophic level 3, etc.). The one that has the greatest number of links is where you feed most of the time.

• If you were to weigh your food at each meal, then assign that weight to each trophic level, at which trophic level would you consume most of your food on a weight basis?

• Please respond to these questions or send your thoughtful examples and comments to:

Hardcopy Links In The Library

• The journals Ecology, Ecological Monographs, and Ecological Applications are published by the Ecological Society of America. Each month, they contain many new scientific articles on food webs, trophic interactions, direct and indirect effects, and community and ecosystem studies.
• Krebs, C. J. 1985. Ecology. Harper Collins, New York. This is an excellent introductory text on Ecology for the interested student. More in-depth discussions of the topics in this chapter are available here.
• Perry, D.A. 1994. Forest Ecosystems. John Hopkins University Press. Baltimore. 649 pp.
• Smith, R.L. 1996. Ecology and Field Biology. HarperCollins College Publishers. New York. An introductory ecology textbook. 740+ pp.
• Teal, J. and M. Teal. 1969. Life and Death of the Salt Marsh. Ballantine Books, New York.

Ecolinks On The Web

• http://www.sdsc.edu/~ESA/ESA.html - The home page of the Ecological Society of America, which is the foremost professional ecological science organization in the USA. Members are typically scientists from academia, government, and private industry. On their home page are information about membership and upcoming ESA meetings, links to other ESA pages on ecological issues, ecosystem management, updates on the endangered species act, environmental policy issues and ecological success stories. In order to publicize the benefits of the science of ecology, the Ecological Society of America has begun a series of "Ecological Success Stories." Each issue will highlight how ecology and ecologists have had a direct impact on human health, the economy, or quality of life. One of the first concerns Lyme Disease, which is featured in this chapter’s case study.
• http://lternet.edu - The Long-term Ecological Research (LTER) site homepage and links to other LTER sites. LTER is sponsored by the National Science Foundation, which provides funding for regular sampling of ecological data at several sites around the nation.
• http://lternet.edu/about/sites/08_hbr.htm - Learn about the Hubbard Brook LTER (Long-term Ecological Research site) under the gopher selection LTER Main Menu selection.
• http://sparc.ecology.uga.edu/ - Access the Coweeta LTER website. Coweeta is a Long Term Ecological Research site located in the mountains of North Carolina.
•  http://sflwww.er.usgs.gov/ - The USGA - South Florida Ecosystem Program is described at this website.

Note: If any of these links are not working, please see if alternative links are available at the Ecolink Update Site.

Ecotest Online

1. An ________________ is the minimum system of interacting species and their abiotic resources in an area necessary to sustain life.

a. ecological community

b. ecosystem

c. ecological food web

d. ecological food chain

2. A species interaction that causes a decline in the abundance of a species one trophic level below its predator is called ____________________________________.

a. an indirect negative interaction

b. an indirect positive interaction

c. a direct negative interaction

d. a direct positive interaction

3. The prefix _____________ comes from the Greek word for food.

a. tropho

b. auto

c. photo

d. chemo

e. hetero

4. Which of the following groups in a food web would be most likely to provide the sugar (which is fructose and sucrose, but these are similar to glucose) for a cola soft drink?

a. Autotrophs

b. Primary consumers

c. Secondary consumers

d. Decomposers

5. A manatee that feeds on seagrasses is a ____________________.

a. plant eater.

b. herbivore

c. primary consumer.

d. All of these choices are correct.

6. A wolf that feeds on a rabbit (which eats only grass) is a ________________.

a. primary consumer.

b. secondary consumer.

c. tertiary consumer.

d. herbivore

e. All of these choices are correct.

7. Carefully examine the diagram in Figure 6.3 (c) "The Food Web of the Harp Seal". How many trophic levels are involved in the longest food chain that you can locate in this food web? Be sure to start with phytoplankton and end with the harp seal, using the arrows to represent a one-way link between each species (do not consider any decomposer links).

a. 3

b. 4

c. 5

d. 6

e. 7

8. The mushroom on your pizza is an example of a ____________.

a. producer

b. herbivore

c. decomposer

d. carnivore

e. None of these are correct

9. The structure of an ecological community is determined by:

a. The interactions among the biotic parts of the community.

b. The interactions among the abiotic and the biotic parts of the community.

c. The interactions among the abiotic parts of the community.

d. None of these are correct.

e. All of these are correct.

10. A keystone species is one that:

a. causes a direct trophic interaction within a community.

b. causes an indirect trophic interaction within a community.

c. causes a major change in the community structure if it is removed.

d. is the same as the dominant species in a community.

e. None of these are correct.