8.5 Community Ecology

What is community ecology in AP Biology?

A community is all the different populations living and interacting in one area, and community ecology studies how those species are arranged and how they affect each other. You describe a community using species composition and species diversity (richness plus evenness), and you can measure diversity with Simpson's Diversity Index. Interactions like competition, predation, and symbiosis (parasitism, mutualism, commensalism) shape how species access energy and matter and how the whole community changes over time.

Why This Matters for the AP Biology Exam

Community ecology shows up when you describe community structure, calculate diversity, and explain how species interactions drive population dynamics. On the exam you may be asked to calculate and interpret Simpson's Diversity Index, compare two communities, or predict how a change in one population affects others through interactions like predator-prey relationships, trophic cascades, and niche partitioning. This topic also connects to food webs and energy flow from earlier in Unit 8, so being able to explain how interactions determine access to energy and matter helps you support claims with evidence in free-response questions.

Key Takeaways

• Community structure is described two ways: species composition (which species are present) and species diversity (richness and evenness combined).
• Simpson's Diversity Index = 1 - Σ(n/N)² ranges from 0 to 1; values closer to 1 mean higher diversity.
• The main species interactions are competition (-/-), predation (+/-), mutualism (+/+), commensalism (+/0), and parasitism (+/-).
• Interactions determine how populations access energy and matter, and they can be modeled as positive or negative effects.
• Examples you can model include predator-prey dynamics, cooperation, trophic cascades, and niche partitioning.
• A change in one population can ripple through the community because populations interact and change over time.

What Is a Community?

A community is the set of different populations living and interacting in the same area. Communities exist at every scale, from the bacteria in your gut to the plants and animals across a forest. The way these species interact shapes the structure and function of the whole community.

When ecologists study a community, they look at both who lives there (species composition) and how many of each species are present (which feeds into species diversity). These measurements let scientists describe a community and compare it to others.

Community Structure

Community structure is the organization of species within a community. You measure it using species composition and species diversity. These measurements help track changes over time and compare different communities.

Species Composition

Species composition is the list of all species present in a community. It tells you which organisms are there but not how abundant each one is. Two communities can have the same species list but function very differently if the relative abundances differ. A forest with many oaks and few maples works differently than one with many maples and few oaks.

Species Diversity

Species diversity combines two factors:

• Species richness: the total number of different species present
• Species evenness: how equally individuals are distributed among those species

A highly diverse community has many species (high richness) with relatively similar population sizes (high evenness). Low-diversity communities have fewer species or are heavily shaped by just a few.

Simpson's Diversity Index

Simpson's Diversity Index measures diversity using both richness and evenness. The formula is:

Diversity Index = 1 - Σ(n/N)²

Where:

• n = number of organisms of a particular species
• N = total number of organisms of all species
• Σ = sum of the calculations for all species

The index ranges from 0 to 1:

• Values closer to 0 indicate low diversity
• Values closer to 1 indicate high diversity

Example Calculation

Diversity Index = 1 - 0.3862 = 0.6138

This moderate-to-high value indicates decent diversity in this sample community.

Community Interactions

Communities are groups of interacting populations of different species that change over time based on those interactions. These relationships determine how species access energy and matter within the community. Some species cooperate, others compete, and others prey on one another. Together, these interactions shape community structure and influence its stability over time.

Types of Species Interactions

Predator-Prey Relationships (+/-)

Predator-prey relationships involve one organism (the predator) consuming another (the prey). This transfers energy up the food chain and helps regulate population sizes. A predator in one relationship may be prey in another, creating interconnected food webs. A snake might eat a mouse but then be eaten by a hawk.

Predators can create "top-down control" in a community. A trophic cascade is a community-level effect in which a change at one trophic level, especially among predators, indirectly alters populations at other trophic levels. Trophic cascades are an important model showing how species interactions can restructure an entire community.

As an applied example, when wolves were reintroduced to Yellowstone, they reduced elk numbers, which let willow trees recover, which supported beavers, which built dams that changed stream flow. This illustrates the trophic cascade concept but is not required AP content for this topic.

Competition (-/-)

Competition happens when species use the same limited resources, such as food, water, shelter, or territory. When resources are scarce, competition intensifies and can reduce populations of less competitive species. Hawks and owls might compete for the same rodent prey, especially in years when rodent numbers are low.

To reduce competition, species often evolve differences in how they use resources, a process called niche partitioning. Warblers feeding in the same tree might specialize on different parts of it, with some feeding at the top and others in the middle or lower branches. This specialization lets more species coexist by reducing direct competition.

Cooperation

Cooperation is an interaction in which individuals or populations work together in ways that improve access to resources, energy, or survival. In community ecology, cooperation can change feeding success, resource use, and species persistence, all of which influence community structure. Examples include pack hunting in wolves, where coordinated effort lets the pack take larger prey than one animal could alone, and cross-feeding among microbes, where one species' waste products become nutrients for another.

Mutualism (+/+)

Mutualism describes relationships where both species benefit. These partnerships often evolve over long periods and can become so interdependent that neither species can survive without the other. Mutualism helps species access resources they could not obtain alone.

A classic example is the relationship between acacia trees and acacia ants. The tree provides shelter and nectar-filled structures as food for the ants. In return, the ants defend the tree against herbivores and trim competing plants around its base. This partnership improves both species' chances of survival.

Commensalism (+/0)

Commensalism occurs when one species benefits while the other is neither helped nor harmed. These relationships are common but can be hard to confirm, since subtle effects may exist that are not obvious.

Barnacles attached to whales are a common example. The barnacles gain mobility and access to food-rich waters as the whale swims, while the whale seems unaffected. Birds nesting in trees similarly benefit from shelter without significantly affecting the tree.

Parasitism (+/-)

Parasitism involves one organism (the parasite) living on or in another organism (the host) and gaining benefits at the host's expense. Unlike predators, parasites typically keep the host alive for longer and feed on them over time.

Parasites differ from predators in a few ways:

• Parasites are usually much smaller than their hosts
• Parasites typically feed on parts of the host rather than consuming it entirely
• The relationship is often long-term

For example, ticks attach to a dog to feed on its blood. The dog suffers blood loss and possible disease, while the tick gains nourishment. Both parasitism and predation involve one organism benefiting at another's expense, but these differences separate them.

Energy and Matter Access in Communities

Interactions among populations determine how species access energy and matter within a community. Predation, competition, symbiosis, cooperation, trophic cascades, and niche partitioning all influence resource access and community structure.

When organisms cooperate or form symbiotic relationships, they can often access energy more effectively than they could alone. Coral polyps host algae that photosynthesize and share nutrients with the coral, a mutualism that lets corals thrive in nutrient-poor tropical waters. Mycorrhizal fungi partner with plant roots, improving the plant's access to water and nutrients while receiving sugars in return.

These interactions help explain why communities develop the way they do and how a shift in one population can ripple through the rest of the community.

How to Use This on the AP Biology Exam

Problem Solving

When you see a data table of species counts, calculate Simpson's Diversity Index by finding n/N for each species, squaring each value, summing the squares, and subtracting from 1. Keep your work organized in a table like the example above so you do not lose track of a species. Remember the result is between 0 and 1, and a higher value means greater diversity.

Free Response

If a question asks you to compare two communities, do not just state which index value is higher. Connect the number to richness and evenness, and explain what that means for the community. When a question describes a change in one population, predict the effect on others by naming the specific interaction involved (for example, predation or competition) and tracing the positive or negative effect through the connected populations.

Common Trap

When describing interactions, name the effect on each species clearly using +, -, or 0. Mixing up commensalism (+/0) and mutualism (+/+), or parasitism (+/-) and predation (+/-), is a frequent mistake. For parasitism versus predation, point to the differences in size, how the host or prey is used, and the length of the relationship.

Common Misconceptions

• Species composition is not the same as diversity. Composition is just the list of species. Diversity adds in how many of each species are present (evenness) along with richness.
• A higher Simpson's value means more diversity, not less. Because the formula is 1 minus the sum of squared proportions, values near 1 mean high diversity and values near 0 mean low diversity.
• Parasitism and predation are not identical even though both are +/-. Parasites are usually smaller, feed on part of the host, and maintain a long-term relationship instead of consuming the whole organism right away.
• Commensalism is +/0, not +/+. Only one species benefits; the other is unaffected. If both benefit, it is mutualism.
• Trophic cascades start with changes at one trophic level affecting others indirectly. A predator change does not just affect its prey; it can ripple to plants and other species several levels away.
• More diversity is not automatically about more individuals. A community can have many individuals but low diversity if one species fills, because evenness would be low.

Related AP Biology Guides

• Unit 8 Overview: Ecology
• 8.1 Responses to the Environment
• 8.2 Energy Flow Through Ecosystems
• 8.3 Population Ecology
• 8.4 Effect of Density of Populations
• 8.6 Biodiversity