AP Biology

## Learning Outcomes

- Define a keystone species and distinguish it from a dominant or abundant species.
- Explain how removing or adding a keystone species can change community structure.
- Describe trophic cascades using predator-prey examples such as wolves in Yellowstone.
- Predict how changes in keystone species populations affect biodiversity.
- Connect keystone species to food webs, carrying capacity, competition, and habitat change.
- Use evidence from graphs, data tables, or experimental designs to support claims about keystone species.
- Avoid common AP Biology errors when interpreting ecological interactions.

## AP Biology Syllabus

Keystone species fit within AP Biology Unit 8 because they show how interactions among organisms can produce large effects at the community level. Exam questions may ask you to interpret food webs, predict results after a species is removed or reintroduced, evaluate changes in biodiversity, or justify a claim using population data.

- Community ecology examines interactions among populations living in the same area.
- Keystone species have effects on community structure that are large relative to their population size or biomass.
- Predators can act as keystone species by limiting herbivore populations and protecting plant diversity.
- Keystone species may influence competition, resource availability, habitat structure, and trophic relationships.
- Removal of a keystone species can decrease biodiversity and may cause major changes in food webs.
- Reintroduction of a keystone species can restore some community interactions if suitable conditions remain.
- AP questions often require predictions, graph interpretation, experimental design, and evidence-based reasoning.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. What makes a keystone species different from a species that is simply very common?
2. How can a predator increase plant biodiversity in a community?
3. Why did wolf removal from Yellowstone lead to changes in plant and animal populations?
4. What is a trophic cascade, and how could it appear in a food web question?
5. If a nonnative species reduces the population of a keystone predator, what changes might occur in the community?

## Introduction

Communities are shaped by many interactions: predation, competition, mutualism, herbivory, disease, and resource use. Some species have unusually large effects on these interactions. These are called keystone species. Their presence can help maintain community structure, while their removal can cause major shifts in population sizes, species diversity, and food web relationships.

In AP Biology, keystone species questions are often less about memorizing examples and more about applying cause-and-effect reasoning. You may be given a graph showing changes in predator and prey abundance, a food web with several trophic levels, or a scenario involving species removal. Your task is to identify the likely relationships, predict changes, and support your claim with evidence.

> **Key Term: keystone species**
> A species that has a disproportionately large effect on community structure compared with its abundance or biomass.

### How keystone species shape community structure

A keystone species is not necessarily the most numerous species in a community. Instead, its role is unusually influential. Removing it changes the interactions among many other species. This can reduce biodiversity, alter population sizes, and change the physical structure of the habitat.

A classic AP-style example is the wolf in Yellowstone National Park. When wolves were hunted to very low numbers, populations of large herbivores increased. More herbivores fed on young plants, which reduced the diversity and abundance of plant species. Animals that depended on those plants for food or shelter also declined. When wolves were reintroduced in the 1990s, they limited herbivore populations and changed herbivore behavior. Plant populations increased in some areas, and other animal populations also benefited from the greater plant diversity.

This example shows that a keystone predator can affect organisms it does not directly eat. Wolves prey on herbivores, but their effects can extend to plants, birds, small mammals, insects, and streamside habitats. On the AP exam, this type of question often asks you to follow the chain of interactions through multiple trophic levels.

> **Key Term: community**
> All the populations of different species living and interacting in a particular area.
>
> **Key Term: biodiversity**
> The variety of life in a biological community, often considered in terms of species richness and relative abundance.

A keystone species can maintain biodiversity by preventing one species from becoming too abundant and excluding others. For example, if a predator limits a dominant herbivore, more plant species may survive. If more plant species survive, more niches are available for insects, birds, and other consumers. In this way, a predator can indirectly increase biodiversity.

Keystone species may also be habitat modifiers. Some organisms create or maintain physical conditions that other species require. For instance, beavers build dams that alter water flow and create ponds and wetlands. These changes can provide habitat for fish, amphibians, insects, birds, and plants. In such cases, the keystone effect is not only about eating or being eaten; it is also about changing resource availability and habitat structure.

> **Test Tip:**
> When asked whether a species is a keystone species, focus on effect size, not population size. A rare species can be keystone if removing it causes large community changes.

### Trophic cascades and food web effects

Many keystone species produce their effects through trophic cascades. A trophic cascade occurs when a change at one trophic level causes linked changes at other trophic levels. In a simplified chain, predators reduce herbivores, herbivores consume plants, and plants support many other organisms. Changing the predator population can therefore affect plant abundance and the species that depend on plants.

> **Key Term: trophic cascade**
> A series of indirect effects across trophic levels caused by a change in the population of one species or group.

Consider this simplified food chain:

- Wolves eat large herbivores.
- Large herbivores eat young trees and shrubs.
- Trees and shrubs provide food and nesting sites for other animals.

If wolves are removed, herbivore populations may increase because predation pressure decreases. Increased herbivory can reduce young trees and shrubs. This may lower plant biodiversity and reduce habitat or food for other animals. If wolves are reintroduced, herbivore populations may decrease or change their feeding behavior, allowing some plant populations to recover.

AP questions may present this as a food web instead of a simple chain. In a food web, one species often has several food sources, and one food source may support several consumers. Still, the same logic applies: trace direct effects first, then indirect effects.

For example, suppose a graph shows that after a predator is removed, herbivore abundance rises and plant cover falls. A strong AP response would say:

- The predator likely limited the herbivore population.
- Removing the predator reduced predation pressure.
- Herbivores increased and consumed more plants.
- Plant cover decreased because herbivory increased.
- Species dependent on the plants may decline, reducing biodiversity.

This type of reasoning is stronger than simply saying “the food web changed.” AP Biology rewards specific cause-and-effect explanations.

> **Exam Warning:**
> Do not assume every predator is a keystone species. A predator is keystone only if evidence shows that its effect on the community is unusually large relative to its abundance.

Another well-known example involves sea otters, sea urchins, and kelp forests. Sea otters eat sea urchins. Sea urchins eat kelp. When sea otter populations decline, sea urchin populations can increase, and kelp abundance can fall. When otters are present at effective population sizes, they can limit urchins and help maintain kelp habitat. This example is useful because it has a clear three-level trophic cascade: predator, herbivore, producer.

Another classic example involves a predatory sea star that eats mussels. If the sea star is removed, mussels may become dominant and occupy space that many other species need. Species richness can decrease. This shows how a keystone predator can maintain diversity by preventing competitive exclusion.

> **Key Term: competitive exclusion**
> The principle that two species competing for the same limiting resource cannot coexist indefinitely if their niches are too similar.

### Keystone species, carrying capacity, and population change

Keystone species often affect the carrying capacity of other populations. Carrying capacity is the maximum population size that an environment can support over time, based on resources such as food, space, water, and shelter. When a keystone predator is removed, prey populations may rise temporarily because fewer individuals are being eaten. However, if the prey population exceeds available resources, the community may experience overgrazing, starvation, disease, or habitat damage.

> **Key Term: carrying capacity**
> The maximum population size that can be supported by available resources in a particular environment over time.

In the Yellowstone example, wolf removal allowed herbivore populations to increase. More herbivores placed greater pressure on plant resources. This reduced plant biodiversity and affected animals dependent on those plants. The herbivore population was not simply “better off” forever because resource depletion can later limit survival and reproduction.

AP Biology may connect this subtopic to population growth models. If limiting factors are absent or reduced, a population may show rapid growth. Predation is a density-dependent limiting factor because its effect often depends on population density. When predators are removed, prey may increase until other limiting factors become stronger. These factors can include food availability, disease, competition, and space.

A common exam scenario might state that a predator was removed from an island community. The herbivore population initially increased, then declined. You might be asked to explain the pattern. A good answer would mention that predator removal allowed herbivore numbers to rise, but increased herbivore density caused stronger competition for food, reducing birth rates or increasing death rates.

> **Test Tip:**
> In graph questions, describe both the pattern and the biological reason. For example: “Herbivore abundance increased after predator removal because predation pressure decreased,” not just “the line went up.”

Keystone effects are also relevant to invasive species. If a nonnative species enters a community and has no natural predators, it may outcompete native species. It may also reduce or eliminate a keystone species. Either route can cause large changes in biodiversity.

> **Key Term: invasive species**
> A nonnative species that spreads in a new area and harms native species, community structure, or resources.

Cane toads in Australia are an example of a harmful invasive species. They were introduced to control cane beetles but lacked natural predators in their new environment. Their populations grew, they competed with native species for food and habitat, and their toxins killed some animals that attempted to eat them. This type of disruption can reduce native biodiversity and alter food web relationships.

If an invasive species harms a keystone predator, herbivores may increase and plant communities may decline. If it outcompetes a keystone habitat-former, many dependent species may lose shelter or breeding sites. AP questions may ask you to predict these effects from a food web or data table rather than from a memorized example.

### Designing and interpreting keystone species investigations

AP Biology frequently assesses science practices: forming hypotheses, identifying variables, choosing controls, graphing data, and supporting claims with evidence. Keystone species are well suited to experimental and observational studies because their effects can be measured through changes in population size, species richness, percent cover, biomass, or diversity indexes.

A possible investigation might test whether a predator is a keystone species in an intertidal community. Researchers could remove the predator from several plots and leave it present in other similar plots. Over time, they could measure the number of species in each plot and the abundance of dominant competitors.

A clear experimental design would include:

- An independent variable: predator presence or absence.
- A dependent variable: species richness, percent cover, biomass, or abundance of other species.
- A control group: plots where the predator remains present.
- Constants: similar plot size, location, sampling time, and measurement method.
- Replication: multiple plots in each treatment group.
- Data analysis: comparing means, error bars, or confidence intervals.

> **Key Term: control group**
> A comparison group that does not receive the experimental treatment and is used to evaluate the effect of the independent variable.
>
> **Key Term: independent variable**
> The factor deliberately changed or compared in an experiment.
>
> **Key Term: dependent variable**
> The measured response that may change because of the independent variable.

A strong null hypothesis might be: removing the predator will have no effect on species richness compared with plots where the predator remains. The alternative hypothesis might be: removing the predator will decrease species richness because the predator normally prevents one competitor from becoming dominant.

If the data show that predator-removal plots have fewer species and greater abundance of one competitor, the evidence supports the claim that the predator helps maintain biodiversity. If the difference between groups is small or error bars overlap greatly, the conclusion should be cautious. AP Biology expects students to connect statistical evidence to biological claims rather than overstate results.

> **Exam Warning:**
> Avoid writing that an experiment “proves” a species is keystone. Use phrasing such as “supports the claim” or “provides evidence,” especially when interpreting limited data.

Free-response questions may ask for a prediction after a keystone species is removed. A high-scoring answer should name the affected populations and explain the mechanism. For example:

> **Question:**
> Predict one effect of removing wolves from a community with wolves, herbivores, shrubs, and nesting birds.

A strong response would be: herbivore populations would likely increase because predation by wolves would decrease. Increased herbivory would reduce shrub abundance, which could decrease nesting bird populations if the birds depend on shrubs for nesting habitat.

This answer earns credit because it includes a prediction and a biological justification. It also traces the effect through more than one trophic level.

Keystone species can also be linked to evolution. When community conditions change, selection pressures may change. For instance, if predator removal causes increased herbivory, plant traits that reduce herbivory may become more favorable. Mutations occur randomly, but natural selection is not random: individuals with traits that improve survival and reproduction in the changed environment are more likely to pass those traits to offspring. AP Biology may connect Unit 8 interactions to Unit 7 natural selection in this way.

## Key Point Checklist

This article has covered the following key knowledge points:

- A keystone species has a large effect on community structure relative to its abundance or biomass.
- Keystone species are not always the most common or largest organisms in a community.
- Predators can be keystone species when they limit prey populations and indirectly protect other species.
- Wolf removal from Yellowstone increased herbivore pressure on plants and reduced biodiversity.
- Wolf reintroduction helped limit herbivore populations and supported recovery of some plant and animal populations.
- A trophic cascade occurs when changes at one trophic level cause effects across other trophic levels.
- Keystone species can maintain biodiversity by preventing competitive exclusion.
- Some keystone species modify habitat, changing resource availability for many other organisms.
- Invasive species can disrupt communities by outcompeting native species or harming keystone species.
- AP questions may require predictions, graph interpretation, experimental design, and evidence-based claims.
- Strong responses identify mechanisms, not just outcomes.
- Experimental claims should be supported with controls, variables, replication, and data.

## Key Terms and Concepts

- keystone species
- community
- biodiversity
- trophic cascade
- competitive exclusion
- carrying capacity
- invasive species
- control group
- independent variable
- dependent variable


Community ecology - Keystone species