AP Biology

## Learning Outcomes

- Define predation and competition as community interactions and distinguish their effects on population size.
- Explain how predation and competition act as density-dependent limiting factors.
- Describe predator and prey adaptations, including camouflage, defenses, mimicry, and herbivore deterrents.
- Predict how changes in predator abundance or resource availability affect community structure.
- Explain competitive exclusion, niches, and resource partitioning using AP-level examples.
- Interpret graphs or data from ecological experiments involving predators, prey, or competing species.
- Connect predation and competition to natural selection, biodiversity, and stability in biological communities.

## AP Biology Syllabus

In AP Biology Unit 8, predation and competition are tested as interactions that affect population growth, community structure, and biodiversity. Questions often place these ideas in unfamiliar experimental settings, so you should be ready to apply ecological principles to graphs, data tables, and scenarios rather than only recall definitions.

- Predation, herbivory, and parasitism can reduce prey or host survival and reproduction.
- Competition occurs when organisms use the same limited resource, such as food, space, light, water, or mates.
- Predation and competition are density-dependent factors because their effects often increase as population density rises.
- Species interactions can alter carrying capacity, population growth, and community composition.
- Natural selection favors predator and prey traits that improve survival or reproductive success.
- Competitive exclusion can occur when two species occupy very similar niches and one outcompetes the other.
- Resource partitioning can reduce direct competition and allow similar species to coexist.
- AP questions may require you to identify controls, variables, error bars, confidence intervals, or supported claims from ecological data.

## 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. Why are predation and competition classified as density-dependent limiting factors?
2. How can camouflage increase the fitness of a prey population in one environment but not another?
3. What is the difference between a fundamental niche and a realized niche?
4. What does the competitive exclusion principle predict when two species require the same limiting resource?
5. How could removing a top predator change the abundance of several other species in a community?

## Introduction

Community ecology studies how populations of different species interact in the same area. Two of the most frequently tested interactions are predation and competition. Predation includes one organism consuming another, while competition occurs when organisms require the same limited resource. Both can affect population size, natural selection, and biodiversity.

On the AP Biology exam, these topics often appear in scenarios involving data. You might be asked to interpret a predator-prey graph, explain why one prey color becomes more common after environmental change, predict how adding a competitor changes population growth, or design an experiment with controls and dependent variables. The key is to connect the interaction to survival, reproduction, population size, and resource availability.

> **Key Term: Predation**
> An interaction in which one organism, the predator, kills and consumes another organism, the prey.

### Predation as a density-dependent interaction

Predation affects both predator and prey populations. When prey are abundant, predators may have more food, survive at higher rates, and reproduce more successfully. As predator numbers rise, prey mortality may increase. If prey become scarce, predator survival and reproduction may fall. This can create linked changes in population size over time.

Predation is a density-dependent limiting factor because its effect can depend on population density. For example, when prey organisms are crowded into a small area, predators may find them more easily. In contrast, a low-density prey population may be harder to locate. Density-dependent factors also include disease and competition for food, habitat, or mates.

Population growth can be described using changes in population size over time:

$$\frac{dN}{dt}=B-D$$

Here, $$N$$ is population size, $$B$$ is birth rate, and $$D$$ is death rate. Predation can increase $$D$$ in a prey population. If predation causes deaths to exceed births, the prey population decreases. Predators can also affect carrying capacity indirectly by changing how much food, space, or shelter is available to other species.

> **Key Term: Carrying capacity**
> The maximum population size that an environment can support over time based on limiting resources and conditions.

Predator-prey relationships are not always simple. A prey population may decline after predator abundance increases, but the timing can lag. For example, if hare numbers increase, lynx populations may later rise because more food is available. As lynx become more abundant, hare mortality may increase. Later, if hare become scarce, lynx numbers may drop. AP Biology questions may give a graph and ask which claim is best supported by the data.

> **Test Tip:**
> When reading a predator-prey graph, identify the axes and units before interpreting the trend. Predator peaks often occur after prey peaks, not necessarily at the same time.

Predation can also change community structure. In some communities, a predator prevents one prey species from becoming so abundant that it excludes others. If that predator is removed, the dominant prey species may increase sharply and reduce biodiversity. A classic example involves sea stars feeding on mussels in rocky shore communities. When sea stars are removed, mussels can occupy more space and reduce the number of other species able to persist.

> **Key Term: Keystone species**
> A species whose effect on community structure is much larger than would be expected from its abundance alone.

Not every predator is a keystone species. A predator earns that label only if removing it causes a large change in community composition. On the exam, avoid assuming that the largest or most abundant organism is always the keystone species. Instead, use evidence: what changes when the species is added, removed, or reduced?

### Adaptations in predators, prey, and herbivores

Predation creates selection pressure. Traits that help predators capture prey can increase predator fitness, while traits that help prey avoid being eaten can increase prey fitness. Fitness means reproductive success, not physical strength alone. A trait is favored by natural selection if individuals with that trait survive and reproduce more than individuals without it.

Predator adaptations include speed, sharp claws, venom, hunting behavior, sensory ability, and cooperative hunting. Prey adaptations include camouflage, warning coloration, mimicry, chemical defenses, shells, spines, herding, alarm calls, and rapid escape behaviors. Plants also have defenses against herbivores, including thorns, toxins, and physical barriers.

> **Key Term: Herbivory**
> An interaction in which an animal consumes plant material, often reducing plant growth, survival, or reproduction.

A useful AP-level example is variation in trichomes on plant leaves. Trichomes are short, hairlike structures on leaf surfaces or edges. They can deter herbivores by making leaves harder or less attractive to eat. If herbivores are present, plants with more trichomes may experience less damage and leave more offspring. If students artificially select plants with many trichomes, the next generation may show a higher average trichome number, provided the trait has a genetic basis.

Camouflage is another common example. Suppose green and beige moths live in grass. In wet years, green grass may hide green moths from birds, so green moths survive at higher rates. During drought, grass may turn brown, making beige moths harder to detect. Birds then eat more green moths, and beige moths become a larger percentage of the population. This is natural selection because the environment affects which heritable traits improve survival and reproduction.

> **Exam Warning:**
> Do not write that individual organisms “evolve” because they need to survive. Populations evolve when allele frequencies change across generations because individuals with certain inherited traits leave more offspring.

Some prey use mimicry. In Batesian mimicry, a harmless species resembles a harmful or unpalatable species. Predators that have learned to avoid the harmful species may also avoid the mimic. In Müllerian mimicry, two or more harmful species resemble each other, reinforcing predator avoidance. AP Biology usually tests the logic: if a predator avoids a pattern, prey with that pattern may have higher survival.

Plant-herbivore interactions are also part of predation in a broad ecological sense, although the herbivore usually does not kill the whole plant immediately. Herbivory can reduce photosynthetic surface area, growth rate, or reproductive output. In response, plant populations may show more defensive structures or chemicals over generations if those traits are heritable and improve reproductive success.

Predation can also produce trade-offs. A prey species with heavy armor may be safer but slower. A plant that produces toxic chemicals may lose energy that could otherwise support growth. In AP free-response answers, mentioning trade-offs can strengthen an explanation if the prompt asks why a trait is not always present at the highest possible level.

### Competition, niches, and limiting resources

Competition occurs when organisms use the same limited resource. This can happen within a species or between species. Intraspecific competition occurs among members of the same species. Interspecific competition occurs between different species. Both can reduce growth, survival, or reproduction because organisms spend energy obtaining resources and may receive less of what they need.

> **Key Term: Competition**
> An interaction in which organisms require the same limited resource, reducing access to that resource for one or more organisms.

Resources can include food, water, light, mineral nutrients, nesting sites, territory, pollinators, or mates. In plants, competition for light can cause taller individuals to shade shorter ones. In animals, competition for food or territory can reduce reproductive success. In microbes, competition may involve nutrients or space on a surface.

Competition is density-dependent because it becomes stronger as more individuals require the same limited resource. If a population grows rapidly with abundant food and habitat, it may eventually reach limits. Logistic growth describes population growth that slows as population size approaches carrying capacity:

$$\frac{dN}{dt}=r_{\max}N\left(\frac{K-N}{K}\right)$$

In this equation, $$r_{\max}$$ is the maximum per capita growth rate, $$N$$ is population size, and $$K$$ is carrying capacity. Competition is one reason growth slows as $$N$$ gets closer to $$K$$.

> **Key Term: Niche**
> The role and requirements of a species, including its resource use, habitat, interactions, and environmental tolerances.

A species’ fundamental niche is the full range of conditions and resources it could use if no competitors or predators restricted it. Its realized niche is the range it actually uses in nature after interactions with other species. For example, a barnacle species might be able to survive lower on a rocky shore, but competition from another barnacle species may restrict it to higher areas. Predation can also restrict a realized niche if prey avoid areas with high predator risk.

When two species have nearly identical niches and the same limiting resource, one may outcompete the other. This idea is called competitive exclusion.

> **Key Term: Competitive exclusion**
> The principle that two species cannot coexist indefinitely in the same area if they require the same limiting resource in the same way.

Competitive exclusion does not mean that every pair of similar species must lead to extinction. It predicts exclusion when niches are too similar and a resource is limiting. If species differ in feeding time, food size, habitat zone, root depth, or behavior, they may coexist because direct competition is reduced.

Resource partitioning is one way similar species coexist. Different warbler species may feed in different parts of the same tree. Different lizard species may perch at different heights or use different microhabitats. Plants may reduce competition by rooting at different depths, with shallow roots absorbing surface water and deep roots accessing lower soil water.

> **Key Term: Resource partitioning**
> The division of limited resources by species in different ways, reducing direct competition and allowing coexistence.

Competition can be tested experimentally. Suppose researchers grow species A and species B separately and together. If each grows well alone but species B declines when grown with species A, the data support the claim that species A is the stronger competitor under those conditions. A well-designed experiment should include a control group, a clear independent variable, and a measurable dependent variable.

> **Test Tip:**
> For competition experiments, the independent variable is often species combination or resource level. The dependent variable is often population size, growth rate, biomass, survival, or reproductive output.

### Community effects and AP-style data interpretation

Predation and competition rarely act alone. In real communities, a species may be both predator and prey, or both competitor and food source. A small fish may prey on zooplankton while being eaten by larger fish. A plant may compete with neighboring plants for light while being consumed by insects. AP Biology prompts often ask you to reason through these linked interactions.

A trophic cascade occurs when a change at one feeding level affects other levels. For example, if wolves reduce deer abundance or change deer feeding behavior, plants eaten by deer may increase. More plant growth may provide habitat for birds or insects. The exam may not require the phrase “trophic cascade,” but you should be able to explain the chain of cause and effect.

Predation can increase, decrease, or maintain biodiversity depending on the situation. If a predator reduces a competitively dominant prey species, other species may persist, increasing diversity. If an introduced predator consumes native prey that lack defenses, diversity may decline. Competition can also reduce diversity if one species excludes others, but resource partitioning can allow multiple species to coexist.

Data analysis is central to AP Biology. You may be asked whether error bars or confidence intervals support a difference between treatments. If 95% confidence intervals do not overlap, that often supports a statistically meaningful difference. If they overlap strongly, be cautious about claiming a clear difference unless the prompt provides a statistical test.

A strong free-response explanation should link data to a biological mechanism. For example:

> **Question:**
> A graph shows that plants exposed to herbivores have higher average trichome numbers after three generations than plants not exposed to herbivores. Explain the likely cause.

A high-scoring answer might state that herbivores acted as a selection pressure. Plants with more trichomes were less likely to be eaten or damaged, so they survived and reproduced more. If trichome number is heritable, alleles associated with higher trichome production became more common, increasing the average trichome number in later generations.

Notice that the answer identifies the selective agent, explains differential survival or reproduction, and connects the trait to inheritance. That structure is useful for many predation and competition prompts.

For competition, a sample explanation might be:

> **Question:**
> Two species of freshwater protists are grown separately and together. Both increase when grown alone. When grown together, species X remains abundant while species Y declines to near zero. Explain the result.

A strong answer would say that species X and species Y likely require the same limiting resource. Species X has a competitive advantage under the experimental conditions, so it obtains more of the resource and reduces species Y’s survival or reproduction. This supports competitive exclusion in that environment.

Be careful with wording. “Species X wanted the resource more” is not scientific. Instead, write about measurable differences: faster resource uptake, higher reproductive rate, greater tolerance, or more efficient feeding.

Experimental design questions may ask for controls. If testing whether predator odor changes prey behavior, the experimental group may receive water containing predator chemical cues, while the control group receives the same water without predator cues. Constants could include tank size, light level, temperature, prey number, and observation time. The dependent variable might be time spent hiding, feeding rate, movement rate, or survival.

> **Exam Warning:**
> Do not claim causation from a correlation unless the experiment controls other variables. If predator abundance and prey decline occur together, predation may be the cause, but other factors such as disease, weather, or food shortage could also contribute unless controlled or ruled out.

AP multiple-choice questions often include unfamiliar organisms. Do not panic if the species names are new. The prompt will provide enough context. Focus on the interaction: Who eats whom? What resource is limiting? Which population increases or decreases? Which trait improves survival or reproduction? Which claim is directly supported by the data?

A final point: predation and competition connect Unit 8 to evolution. Ecological interactions can change which traits are favored. Over generations, predator and prey populations may show reciprocal selection, where each species influences selection on the other. However, avoid assuming perfection. Natural selection works on existing variation and is limited by trade-offs, mutation, gene flow, and environmental change.

## Key Point Checklist

This article has covered the following key knowledge points:

- Predation occurs when a predator kills and consumes prey.
- Herbivory is a form of consumption that can reduce plant growth, survival, or reproduction.
- Predation and competition are density-dependent limiting factors.
- Predation can increase the death rate of prey and affect predator reproduction through food availability.
- Predator-prey population cycles may show time lags between prey peaks and predator peaks.
- Natural selection can favor prey defenses such as camouflage, mimicry, trichomes, toxins, shells, and escape behavior.
- Competition occurs when organisms require the same limited resource.
- Intraspecific competition occurs within one species; interspecific competition occurs between different species.
- A niche includes a species’ resource use, habitat, interactions, and environmental tolerances.
- Competitive exclusion predicts that species with highly similar niches cannot coexist indefinitely if the same resource is limiting.
- Resource partitioning reduces direct competition and can support coexistence.
- AP Biology answers should use data, identify variables, and link ecological interactions to survival, reproduction, and population change.

## Key Terms and Concepts

- Predation
- Carrying capacity
- Keystone species
- Herbivory
- Competition
- Niche
- Competitive exclusion
- Resource partitioning


Community ecology - Predation and competition