Predators, Prey, and Keystone Species

Essential Question

How can one species change an entire community?

Eating and being eaten link species together, so a change in one population can spread through a whole community. In this lesson, you’ll learn how predator and prey populations affect each other, what a keystone species is, and how data show the effects of removing or adding a predator.

Predators and Prey

• Predator: an organism that eats other organisms
• Prey: the organisms that are eaten
• More prey can support more predators
• Predation can limit prey populations

A predator kills and eats other organisms, called prey. Predator and prey populations affect each other. When prey are abundant, predators have more food and their numbers can rise. As predators increase, more prey are eaten and the prey population can fall, which then leaves the predators with less food. In some systems this produces repeating cycles, with the predator peak following the prey peak. The graph is an illustrative model. Real populations are also affected by weather, disease, food supply, and other species, so their patterns are usually messier.

Keystone Species

• Keystone species: has a large effect compared with its numbers
• Often a predator, but not always
• Removing it can change the whole community
• Paine’s sea star experiment (1960s)

A keystone species has an effect on its community that is much larger than its abundance would suggest. In the 1960s, ecologist Robert Paine removed the predatory sea star Pisaster from sections of a rocky shore in Washington State. Without the sea stars, mussels, which the sea stars eat, spread and crowded out many other species, and the number of species in the community dropped to about half. Where sea stars were left in place, the community stayed diverse. Later work has shown that the strength of a keystone effect can depend on the place and the conditions.

Trophic Cascades

• Trophic cascade: effects pass down through food chain levels
• Predator affects prey, which affects producers
• Example: sea otters, sea urchins, and kelp
• Effects can be large but vary by place

A trophic cascade happens when a change at one level of a food chain spreads to other levels. Along parts of the Pacific coast, sea otters eat sea urchins, which graze on kelp. Where otters are present, urchin numbers tend to stay lower and kelp forests can thrive. Where otters have been absent, urchins can become abundant and graze kelp down to bare areas called urchin barrens. Because kelp forests shelter many other species, a change in one predator can reach many others, although the size of the effect varies from place to place.

Invasive Predators

• Introduced predators may find prey with few defenses
• Native prey can decline quickly
• Example: Burmese pythons in the Everglades
• Effects can spread through the food web

Burmese pythons, native to Asia, are now established in southern Florida. In road surveys in Everglades National Park, researchers found that observations of raccoons, opossums, and bobcats fell sharply between the 1990s and 2003 to 2011, while pythons became common. The declines were greatest in the areas where pythons had been present the longest. This pattern suggests that python predation is a major cause, although a survey of sightings is not a direct count of every animal. Declines in prey species can also affect the species that those prey eat or compete with.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Removing the Sea Stars

Use the graph of Paine’s experiment. Write a claim, evidence, and reasoning to answer: is the sea star a keystone species in this community?

Show the solution

1. Read the graph: 15 species where sea stars were left in place, and 8 where they were removed.
2. Claim: yes, the sea star acts as a keystone species in this community.
3. Evidence: removing the sea stars lowered the number of species from 15 to 8, a loss of 7 species (46.7%), even though the sea stars were only one species.
4. Reasoning: sea stars eat mussels. Without them, mussels spread and crowded out other species for space. A predator that keeps a dominant species in check can protect diversity.
5. Answer: Yes. Removing one species cut the number of species from 15 to 8, a loss of 46.7%.

Example 2: Sea Otters and Kelp

Use the trophic cascade model. A disease greatly reduces the number of sea otters along a coast that has a kelp forest. Predict what might happen to sea urchins and to kelp.

Show the solution

1. Otters: with fewer otters, fewer sea urchins are eaten.
2. Urchins: their numbers are likely to rise, since predation on them drops.
3. Kelp: more urchins graze more kelp, so kelp is likely to decline and may be overgrazed.
4. Community: animals that use kelp forests for food or shelter may also decline. Other factors, such as food for urchins and other predators, also matter.
5. Answer: Urchins are likely to increase and kelp is likely to decline, which can change the whole community.

Example 3: Pythons and Raccoons

Raccoon observations in Everglades National Park fell by 99.3%. Suppose a survey route once recorded 300 raccoon observations. How many would you expect after a 99.3% decrease, and what percent of the original frequency remains?

Show the solution

1. Percent remaining: 100% − 99.3% = 0.7%.
2. Expected observations: 300 × (1 − 0.993) = 300 × 0.007 = 2.1 observations.
3. Answer: About 2.1 observations, which is 0.7% of the original frequency.

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Practice

Describe predator-prey relationships, use experiment data to identify a keystone species, trace a trophic cascade, and calculate a percent change. Try it on your own first, then check your work with the answer key.

• Predators, Prey, and Keystone Species Practice Worksheet (PDF)
• Predators, Prey, and Keystone Species Practice Answer Key (PDF)

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Lesson Slides

• 3.5 Predators, Prey, and Keystone Species Slides (PDF)

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Big Ideas

• Predator and prey populations affect each other, and can rise and fall in cycles
• A keystone species has a large effect on its community compared with its numbers
• A trophic cascade passes the effect of a predator down through the food chain
• An introduced predator can cause rapid declines in native prey
• Removing or adding one species can change a whole community

Back to the essential question: one species can change an entire community when its feeding keeps other populations in check. Remove a keystone predator, or add an invasive one, and the effects can spread through the food web. Next, you’ll look at close relationships between species, called symbiosis.

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