Exercises: Community Ecology and Disruptions in Ecosystems

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.

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.

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.

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.

(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.

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.

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.

After a wildfire in a forest in Yellowstone National Park, researchers recorded the percent cover of three plant groups at different times (hypothetical data).

(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.

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.