Community Ecology and Disruptions in Ecosystems
In the 1990s, wolves were brought back to Yellowstone National Park after about 70 years without them. Within a few years, elk behaved differently, willows and aspens grew back along streams, and beavers returned. How can removing or adding a single species change an entire landscape? By the end of this page you will be able to explain it.
Food chains, food webs, and energy flow
Every community is a network of "who eats whom." A food chain shows one path of energy, with arrows pointing from the organism being eaten to the one eating it.
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Producers (autotrophs) make their own food, usually by photosynthesis.
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Consumers (heterotrophs) eat other organisms: primary consumers eat producers, secondary consumers eat primary consumers, and so on.
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Decomposers (bacteria and fungi) break down dead organisms and waste and return nutrients to the soil or water. Without them, nutrients would stay locked in dead material.
Real communities have many overlapping chains, called a food web.
The 10% rule
Each step up the chain is called a trophic level. Only about 10% of the energy at one level is stored in the next level. The rest is used for the organism's own metabolism (cellular respiration), lost as heat, or left in waste and uneaten parts. Energy flows in one direction and is lost as heat, but matter (carbon, nitrogen) is recycled.
Because energy shrinks at each step, food chains rarely have more than 4 or 5 levels, and top predators are few in number.
Metabolic rate matters too. Endotherms (birds, mammals) spend much energy keeping body temperature constant, so they convert a smaller share of their food into body mass than ectotherms (reptiles, fish, insects).
A food chain is grass → grasshopper → frog → snake → hawk. Identify the producer, the primary consumer, the secondary consumer, and the top predator. Then define…
A food chain is grass → grasshopper → frog → snake → hawk. Identify the producer, the primary consumer, the secondary consumer, and the top predator. Then define decomposer and state what role it plays.
In an ecosystem, producers capture 10,000 kJ of energy. Using the 10% rule, estimate the energy available to primary consumers and to secondary consumers. Briefly…
In an ecosystem, producers capture 10,000 kJ of energy. Using the 10% rule, estimate the energy available to primary consumers and to secondary consumers. Briefly explain what happens to the rest of the energy.
Interactions between species
Species in a community affect each other. These relationships are grouped by their effects (+ helps, − harms, 0 no effect).
| Relationship | Species 1 | Species 2 | Example |
|---|---|---|---|
| Mutualism | + | + | Bees and flowering plants |
| Commensalism | + | 0 | Cattle egrets feeding on insects stirred up by cattle |
| Parasitism | + | − | Tick on a deer |
| Predation / herbivory | + | − | Wolf and elk |
| Competition | − | − | Two plants sharing light and water |
Symbiosis means a close, long-term relationship between two species. Mutualism, commensalism, and parasitism are all types of symbiosis.
Competition and resource partitioning
The competitive exclusion principle says that two species competing for the same limited resource cannot coexist indefinitely: the slightly better competitor eventually drives the other to local extinction. In the classic Paramecium experiments, when two species shared one food supply, one population thrived and the other died out.
In nature, competing species often avoid exclusion by resource partitioning: using the resource in different ways, such as different tree heights, foods, or times of day. Several Anolis lizard species in Florida and the Caribbean perch at different heights on the same tree.
Classify each relationship as mutualism, commensalism, or parasitism: (a) a clownfish lives among the tentacles of a sea anemone, which protects the fish, and the fish…
Classify each relationship as mutualism, commensalism, or parasitism:
(a) a clownfish lives among the tentacles of a sea anemone, which protects the fish, and the fish drives away fish that eat the anemone;
(b) a tapeworm lives in a dog's intestine and absorbs digested food;
(c) barnacles attach to a whale, gaining transport, without harming or helping the whale;
(d) fungi in the roots of a tree absorb sugars from the tree and provide water and minerals.
Two species of Paramecium* were grown in separate tubes and then together, using the same food supply. Alone, species 1 reached a stable population of about 200 and…
Two species of Paramecium were grown in separate tubes and then together, using the same food supply. Alone, species 1 reached a stable population of about 200 and species 2 reached about 500. In the mixed culture, species 2 grew and species 1 declined to zero. (a) Explain the competitive exclusion principle and how these results illustrate it. (b) Explain how two competing species, such as several lizard species that perch at different heights in the same tree, can coexist in nature through resource partitioning. (c) Predict how the outcome might change if a predator preferred to eat species 2, and justify your prediction.
Keystone species, trophic cascades, and invasive species
A keystone species has an effect on its community that is much larger than its abundance would suggest. Removing it changes the whole community.
The classic case is the sea otter in kelp forests off California and Alaska. Otters eat sea urchins. Urchins eat kelp. When otters disappear, urchins boom and graze the kelp down to bare rock ("urchin barrens"). Fish and invertebrates that need kelp for food and shelter then decline.
A trophic cascade is a chain reaction of effects that passes down through trophic levels when a top predator is added or removed. Yellowstone's wolves are another example: wolves reduce and move elk, so streamside willows recover, which helps beavers and songbirds.
Invasive species
An invasive species is a non-native species that spreads and harms its new ecosystem. They often succeed because they have no natural predators, parasites, or diseases in the new habitat, and they may reproduce quickly or eat or outcompete native species. Examples: Burmese pythons in the Florida Everglades, kudzu in the Southeast, and zebra mussels in the Great Lakes.
When writing a CER (claim, evidence, reasoning) answer, state the claim, cite specific data, and link the data to a biological principle. Always consider alternative explanations (for example, disease, habitat loss, or hunting) and what data would rule them out.
In a coastal kelp forest, sea otters eat sea urchins, and sea urchins eat kelp. In a hypothetical survey, an area lost most of its otters. | Year | Otters (per km of…
In a coastal kelp forest, sea otters eat sea urchins, and sea urchins eat kelp. In a hypothetical survey, an area lost most of its otters.
| Year | Otters (per km of coast) | Urchins (per m²) | Kelp cover (%) |
|---|---|---|---|
| 1 | 12 | 2 | 80 |
| 5 | 3 | 15 | 40 |
| 10 | 1 | 40 | 5 |
(a) Describe the trends. (b) Explain this trophic cascade and why the sea otter is called a keystone species. (c) Predict what would happen to other species that depend on the kelp.
Burmese pythons were introduced into the Florida Everglades. In a hypothetical survey, raccoon sightings dropped from 20 per 100 hours of survey time to 1 per 100 hours…
Burmese pythons were introduced into the Florida Everglades. In a hypothetical survey, raccoon sightings dropped from 20 per 100 hours of survey time to 1 per 100 hours over the years after pythons became established. (a) Explain why invasive species often spread widely in a new habitat. (b) Write a claim, evidence, and reasoning (CER) statement connecting the pythons to the raccoon decline. (c) Suggest one alternative explanation that scientists would need to rule out.
Measuring biodiversity
Biodiversity includes species richness (how many species) and species evenness (how evenly individuals are spread among them). Simpson's diversity index uses both:
$$D = 1 - \sum \left(\frac{n}{N}\right)^2$$Here $n$ is the number of individuals of one species and $N$ is the total number of individuals of all species. $D$ ranges from 0 (no diversity) toward 1 (very high diversity).
A community with high diversity is usually more resilient: if a disease or disturbance eliminates one species, others can fill its role, so the ecosystem keeps functioning.
Simpson's diversity index is $D = 1 - \sum \left(\dfrac{n}{N}\right)^2$, where $n$ is the number of individuals of one species and $N$ is the total number of…
Simpson's diversity index is $D = 1 - \sum \left(\dfrac{n}{N}\right)^2$, where $n$ is the number of individuals of one species and $N$ is the total number of individuals. Community 1 has four species with 25 individuals each. Community 2 has four species with 85, 5, 5, and 5 individuals. (a) Calculate $D$ for each community. (b) Which community is more diverse, and why? (c) Argue which community would better recover from a disease that kills one species.
Disturbance and ecological succession
A disturbance (fire, flood, hurricane, volcanic eruption, human activity) changes a community. Ecological succession is the gradual change in the species of a community after a disturbance.
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Primary succession begins on bare rock or new land with no soil, such as after a lava flow or at Glacier Bay, Alaska, as glaciers retreat. Pioneer species like lichens and mosses break down rock and add organic matter, forming soil.
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Secondary succession begins where soil remains after a disturbance, such as after a wildfire or an abandoned farm field (old-field succession in the Piedmont). It is faster because seeds and nutrients are already present.
Pioneer species are the first to colonize. Over time, early species change conditions (facilitation), and later species replace them as they tolerate or outcompete the earlier ones, until a relatively stable climax community develops. Succession is not always predictable, and repeated disturbances can reset it.
After a wildfire in a forest in Yellowstone National Park, researchers recorded the percent cover of three plant groups at different times (hypothetical data). | Years…
After a wildfire in a forest in Yellowstone National Park, researchers recorded the percent cover of three plant groups at different times (hypothetical data).
| Years after fire | Grasses and forbs (%) | Shrubs (%) | Trees (%) |
|---|---|---|---|
| 1 | 60 | 5 | 0 |
| 10 | 40 | 25 | 5 |
| 50 | 10 | 20 | 50 |
(a) Explain why this is an example of secondary succession and identify the pioneer species. (b) Describe how the community changes over time and why the trees increase later. (c) Predict how a second severe fire only 5 years after the first could change the community, and justify your answer.
Back to the start: wolves in Yellowstone are a keystone predator. By limiting elk, they allowed streamside plants to recover, a trophic cascade that changed the whole community.
Summary
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Energy flows one way through food chains (producers → consumers), with about 10% passed up each trophic level; the rest is lost mostly as heat. Decomposers recycle nutrients.
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Species interact through competition, predation, and symbiosis (mutualism, commensalism, parasitism). Competitive exclusion is avoided by resource partitioning.
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Keystone species (sea otters, wolves) have outsized effects through trophic cascades. Invasive species often spread because they lack natural enemies.
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Simpson's index $D = 1 - \sum (n/N)^2$ measures diversity using richness and evenness; more diverse communities are usually more resilient.
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After a disturbance, communities change through primary (no soil) or secondary (soil present) succession, starting with pioneer species.