Skills you'll gain in this topic:
- Describe the structure of a community according to its species composition and diversity
- Calculate and interpret Simpson's Diversity Index to compare different communities
- Explain how species interactions like predation, competition, and symbiosis influence community structure
- Analyze how energy flows through communities based on species relationships
- Predict community changes resulting from shifts in species interactions and environmental factors
What is a Community?
A community consists of different populations living and interacting in the same area. Communities can be found in all environments, from the bacteria in your gut to the animals in a forest. They vary greatly in size, from microscopic communities in a drop of pond water to vast ecosystems spanning entire continents. The way these species interact shapes the structure and function of the entire community.
When ecologists study communities, they look at both who lives there (species composition) and how many of each species are present (species diversity). These measurements help scientists understand ecosystem health, stability, and resilience to environmental changes.
Community Structure
Community structure refers to the organization of species within an ecological community. Ecologists measure community structure by examining species composition and diversity. These measurements help scientists track changes over time and compare different communities. Understanding community structure is essential for conservation efforts and predicting how ecosystems might respond to disturbances.
Species Composition
Species composition is simply the list of all species present in a community. This inventory tells us which organisms are present but doesn't account for their abundance. Two communities might have identical species lists but function very differently if the relative abundances of those species differ. For example, a forest with many oak trees and few maples functions 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 the individuals are distributed among species
A highly diverse community has many different species (high richness) with relatively similar population sizes (high evenness). Low diversity communities have fewer species or are dominated by just a few species.
Simpson's Diversity Index
Simpson's Diversity Index is a mathematical measure of diversity that accounts for 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:
| Species | Number of individuals (n) | n/N | (n/N)² |
|---|---|---|---|
| Rabbits | 10 | 10/40 = 0.25 | 0.0625 |
| Foxes | 5 | 5/40 = 0.125 | 0.0156 |
| Hawks | 3 | 3/40 = 0.075 | 0.0056 |
| Mice | 22 | 22/40 = 0.55 | 0.3025 |
| Total | N = 40 | Σ(n/N)² = 0.3862 |
Diversity Index = 1 - 0.3862 = 0.6138
This moderate-high value indicates decent diversity in this sample community.
Community Interactions
Communities change over time based on the interactions between populations. These relationships determine how species access energy and matter within the community. Some species form cooperative relationships, while others compete or prey upon each other. These interactions shape the entire community structure and influence its stability over time.
Types of Species Interactions
| Interaction Type | Species 1 Effect | Species 2 Effect | Example |
|---|---|---|---|
| Predation | + | - | Wolf eating a rabbit |
| Competition | - | - | Two bird species competing for the same nest sites |
| Mutualism | + | + | Bees pollinating flowers while collecting nectar |
| Commensalism | + | 0 | Barnacles attaching to whales for transport |
| Parasitism | + | - | Ticks feeding on deer |
Predator-Prey Relationships (+/-)
Predator-prey relationships involve one organism (the predator) consuming another (the prey). This relationship transfers energy up the food chain and helps regulate population sizes. The predator in one relationship may be prey in another, creating interconnected food webs throughout the community. For instance, a snake might eat a mouse but then be eaten by a hawk.
Predators create "top-down control" in ecosystems, often triggering trophic cascades that affect many other species. When wolves were reintroduced to Yellowstone, they reduced elk populations, which allowed willow trees to recover, which provided habitat for beavers, which created dams that changed stream flow. This cascade demonstrates how predators can reshape entire communities.
Competition (-/-)
Competition occurs when species vie for the same limited resources such as food, water, shelter, or territory. When resources are scarce, competition intensifies and can lead to population declines in less competitive species. For example, hawks and owls might compete for the same rodent prey, especially during years when rodent populations are low.
To reduce competition, species often evolve differences in how they use resources—a process called niche partitioning. For instance, warblers feeding in the same tree might specialize on different parts of the tree—some feeding at the top, others in the middle or bottom branches. This specialization allows more species to coexist by reducing direct competition.
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. Mutualistic relationships help species access resources they couldn't obtain alone.
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A classic example is the relationship between acacia trees and acacia ants. The tree provides shelter and special nectar-filled structures as food for the ants. In return, the ants aggressively defend the tree against herbivores and even trim competing plants around the tree's base. This partnership enhances both species' chances of survival in challenging environments.
Commensalism (+/0)
Commensalism occurs when one species benefits while the other is neither helped nor harmed. These relationships are common but sometimes difficult to identify because subtle negative or positive effects might exist that aren't immediately obvious.
Barnacles attached to whales exemplify commensalism. The barnacles gain mobility and access to more food-rich waters as the whale swims, while the whale seems unaffected by the barnacles' presence. Similarly, birds nesting in trees benefit from the shelter without significantly impacting the tree.
Parasitism (+/-)
Parasitism involves one organism (the parasite) living on or in another organism (the host) and deriving benefits at the host's expense. Unlike predators, parasites typically don't kill their hosts immediately, instead feeding on them over extended periods.
Parasites differ from predators in several 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 dogs to feed on their blood. The dog suffers from blood loss and potential disease transmission, while the tick receives nourishment. Though both parasitism and predation involve one organism benefiting at another's expense, these key differences separate the two interactions.
Energy Flow in Communities
Communities are structured around the availability and transfer of energy. Species interactions determine how efficiently energy moves through the community. When organisms cooperate or develop symbiotic relationships, they can often access energy sources more effectively than they could alone. Understanding these energy dynamics helps explain why certain communities develop and how they might respond to changes.
Cooperation and Energy Access
Coordination between organisms can enhance access to energy and matter within a community. Some examples include:
- Symbiotic relationships: Coral polyps host algae that photosynthesize and share nutrients with their hosts
- Pack hunting: Wolves hunting in groups can take down larger prey than individuals
- Farming: Leaf-cutter ants cultivate fungus gardens that process plant material they couldn't digest themselves
- Microbial consortia: Different bacterial species working together to break down complex compounds
These cooperative strategies allow species to access energy sources that would otherwise be unavailable to them, creating more complex and often more stable community structures.
Community ecology reveals how species interactions shape the structure and function of ecosystems. From predation and competition to various forms of symbiosis, these relationships determine how energy flows through communities and influence their diversity and stability over time. By measuring species composition and diversity, ecologists can assess ecosystem health and predict responses to environmental changes. Understanding these complex interactions is essential for conservation efforts, especially as human activities continue to alter natural communities worldwide. The insights from community ecology help us recognize that no species exists in isolation—each plays a role in the intricate web of life that sustains our planet.
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.
| Term | Definition |
|---|---|
| commensalism | A symbiotic relationship where one organism benefits while the other organism is neither helped nor harmed. |
| community | A group of interacting populations of different species that live in the same area and change over time based on interactions between those populations. |
| community structure | The composition and organization of a community, determined by the types and relative abundances of populations and their interactions. |
| competition | An interaction between populations where organisms compete for the same limited resources, negatively affecting both populations. |
| mutualism | A symbiotic relationship where both organisms benefit from the interaction. |
| niche partitioning | The division of resources among species that allows multiple populations to coexist by utilizing different aspects of their environment. |
| parasitism | A symbiotic relationship where one organism (parasite) benefits while the other organism (host) is harmed. |
| population | A group of organisms of the same species living in the same geographic area. |
| population dynamics | Changes in population size and structure over time, influenced by interactions with other populations and environmental factors. |
| predation | An interaction where one organism (predator) hunts and consumes another organism (prey). |
| predator/prey interactions | Relationships between populations where one organism (predator) hunts and consumes another (prey), influencing population dynamics and energy flow. |
| Simpson's Diversity Index | A quantitative measure of species diversity that accounts for both the number of species and the evenness of their abundance in a community. |
| species composition | The identity and relative abundance of different species present in a community. |
| species diversity | A measure of the variety of species in a community, accounting for both the number of species and their relative abundance. |
| symbiosis | A close, long-term relationship between two different species living together. |
| trophic cascades | Ecological changes triggered by the addition or removal of top predators, affecting multiple levels of the food chain. |
Frequently Asked Questions
What is community ecology and how is it different from population ecology?
Community ecology studies whole assemblages of interacting populations of different species—their species composition (which species are present) and species diversity (richness + evenness; you can quantify it with Simpson’s Diversity Index: 1 − Σ(n/N)²). Population ecology focuses on a single species: its size, growth, and intraspecific factors (births, deaths, carrying capacity). So community ecology asks how interspecific interactions (predation, competition, mutualism, commensalism, parasitism), keystone species, trophic cascades, niche partitioning, and character displacement shape the whole community’s structure and energy/matter flow (LO 8.5.A and LO 8.5.B). Population ecology might predict a logistic growth curve for one species; community ecology explains how that species’ fate changes when predators, competitors, or mutualists are present. For AP prep, make sure you can: define species composition vs. diversity, calculate Simpson’s index, and explain how interactions (positive/negative) alter community structure (see the Topic 8.5 study guide: https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc). For more practice, try the unit problems (https://library.fiveable.me/practice/ap-biology).
How do you calculate Simpson's Diversity Index and what does it actually tell us?
Simpson’s Diversity Index measures species diversity by combining species richness (how many species) and evenness (how evenly individuals are spread). Use the CED formula: Diversity Index = 1 − Σ(n/N)², where n = number of individuals of one species and N = total individuals of all species. Steps: for each species compute (n/N), square it, sum those squared values across all species, then subtract that sum from 1. Interpretation: D ranges from 0 to 1. Values near 0 = low diversity (one or few species dominate); values near 1 = high diversity (many species with similar abundances). On the AP exam you may be asked to calculate D from a table or explain what a change in D implies about community structure (richness vs. evenness). For more Topic 8.5 review see the Community Ecology study guide (https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc). For extra practice questions, visit (https://library.fiveable.me/practice/ap-biology).
I'm confused about the difference between species composition and species diversity - can someone explain?
Species composition = which species are present in a community (the actual list). Species diversity = a measure that combines species composition with how many individuals of each species there are. AP wording: composition = who’s there; diversity = richness + evenness (LO 8.5.A). - Species richness = number of different species. - Species evenness = how equal the abundances are across those species. Two communities can have the same richness but different diversity if one is dominated by one species (low evenness) and the other has similar abundances (high evenness). To quantify diversity use indices like Simpson’s Diversity Index: Diversity = 1 − Σ(n/N)² (n = individuals of one species, N = total individuals). Higher values = more diverse (more species + more even). For AP review, see the Topic 8.5 study guide (https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc), the Unit 8 overview (https://library.fiveable.me/ap-biology/unit-8), and practice problems (https://library.fiveable.me/practice/ap-biology).
What's the difference between mutualism, commensalism, and parasitism with examples?
Mutualism, commensalism, and parasitism are all types of species interactions (symbioses) that shape community structure (LO 8.5.B, EK 8.5.B.4). - Mutualism: both species benefit (+/+). Example: bees pollinating flowering plants—bees get nectar; plants get pollen transfer. Mycorrhizal fungi and plant roots are another key example that increases nutrient uptake for plants and carbon for fungi. Mutualisms can boost population growth and influence energy flow and niche partitioning. - Commensalism: one benefits, the other is unaffected (+/0). Example: barnacles on a whale gain transport and food particles, while the whale is essentially unaffected; epiphytic plants (orchids) growing on tree branches get light without harming the tree. These interactions change species composition without strong negative effects. - Parasitism: one benefits, the host is harmed (+/–). Example: ticks on mammals or tapeworms in intestines. Parasites often reduce host fitness and can drive population dynamics, trophic cascades, and disease ecology. These are tested on the AP exam as part of community interactions and population effects (see the Topic 8.5 study guide: https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc). For more practice, try the AP Bio question bank (https://library.fiveable.me/practice/ap-biology).
Why do predator-prey interactions create cycles in population sizes?
Predator–prey interactions create cycles because the sizes of the two populations are tightly linked with time lags. When prey are abundant, predators have more food, so predator birth rates rise and predator numbers increase. That increased predation then reduces the prey population; because prey decline first, predators soon face food shortage and their numbers fall. With fewer predators, prey rebound, and the cycle repeats. This is modeled by Lotka–Volterra-style dynamics and is driven by density-dependent effects (birth/death rates change with population size), functional responses of predators to prey, and time delays in reproduction. These oscillations shape community structure, can cause trophic cascades, and are a core example of how interspecific interactions drive population dynamics (EK 8.5.B.3–4). For a concise review tied to the CED, see the Topic 8.5 study guide (https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc). For broader unit review and practice, check Unit 8 (https://library.fiveable.me/ap-biology/unit-8) and the practice problems (https://library.fiveable.me/practice/ap-biology).
How does competition between species affect community structure over time?
When two species compete for the same limited resources, community structure shifts over time through a few predictable paths. If competition is strong and niches overlap completely, the competitive exclusion principle says one species will decline or go locally extinct, reducing species richness and changing species composition (you’d see Simpson’s Diversity Index drop). More often, populations respond by niche partitioning or character displacement: they evolve or shift behaviorally/temporally to use different resources, which preserves diversity but changes relative abundances and evenness. Competition can also cascade through trophic links (changing who’s abundant at each trophic level). On the AP exam, be ready to explain these outcomes with CED terms (interspecific competition, niche partitioning, competitive exclusion, character displacement) and link them to diversity metrics (EK 8.5.A.1, EK 8.5.B.4). For a concise review and practice questions on this topic, check the Topic 8.5 study guide (https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc) and the unit overview (https://library.fiveable.me/ap-biology/unit-8).
What are trophic cascades and how do they work in real ecosystems?
Trophic cascades are indirect effects that flow through a food web when a top-level change (usually a predator) alters the abundance or behavior of species at lower trophic levels. In a classic top-down cascade, removing a top predator increases herbivore numbers, which then reduces producer (plant/algae) biomass—changing species composition and diversity. Keystone species often drive strong cascades because their presence disproportionately shapes community structure. Real examples: wolf reintroduction in Yellowstone reduced elk browsing, allowing willow and aspen to recover; sea otter declines let sea urchins boom and kelp forests collapse. Trophic cascades illustrate EK 8.5.B.3 (predator/prey interactions, trophic cascades) and connect to species composition/diversity (LO 8.5.A). On the exam, expect questions asking you to predict effects of adding/removing a species and justify ecosystem-level outcomes using food-web logic. Review more examples and practice problems in the Topic 8.5 study guide (https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc) and practice sets (https://library.fiveable.me/practice/ap-biology).
I don't understand niche partitioning - how do species avoid competing for the same resources?
Niche partitioning is how species avoid direct competition by using different parts of the same “resource pie.” The competitive exclusion principle says two species can’t occupy the exact same niche long-term, so they reduce overlap by partitioning resources in space (use different microhabitats), time (feed at different times), behavior (different foraging methods), or morphology (different bill sizes). Over generations this can lead to character displacement—traits diverge where species coexist (think warblers feeding on different parts of the same tree or Darwin’s finches with different beak sizes). On the AP exam this fits LO 8.5.B and EK 8.5.B.3 (niche partitioning, interspecific competition, character displacement) and is often tested with community-structure or species-interaction questions. For a quick refresher and examples, check the Topic 8.5 study guide (https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc). Want more practice? Try AP-style questions at (https://library.fiveable.me/practice/ap-biology).
Can someone explain how to interpret Simpson's Diversity Index values - is higher or lower better?
Simpson’s Diversity Index measures species diversity (richness + evenness). The formula in the CED is D = 1 − Σ(n/N)², where n = individuals of one species and N = total individuals. Interpretation: higher D = more diverse. Why? If one species dominates, (n/N)² is large so Σ is large and D is small (near 0). If many species are present and fairly even, each (n/N)² is small so Σ is small and D is larger (approaching 1). Example: one-species community → D = 0. Four species with equal abundance (25,25,25,25 of 100): Σ = 4*(0.25²)=0.25 so D = 0.75 (more diverse). This index ties directly to LO 8.5.A (community structure). For practice applying it to data, see the Topic 8.5 study guide (https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc) and try practice problems (https://library.fiveable.me/practice/ap-biology).
What happens to a community when a keystone species is removed?
If you remove a keystone species, the community often shifts dramatically because that species has a disproportionate effect on species composition and diversity. Common outcomes: a trophic cascade (e.g., predator gone → prey explodes → primary producers decline), loss of species richness or evenness, and rearranged food-web connections. Overall species diversity (measured by Simpson’s index, 1 − Σ(n/N)²) usually falls because some populations boom and others crash. In extreme cases habitats can change (like kelp forests collapsing when sea otters are removed because urchins overgraze kelp). On the AP exam this is a classic LO 8.5.B scenario—expect to predict population changes, explain energy/matter flow shifts, and link interactions (predation, competition) to community structure. For a quick review, see the Topic 8.5 study guide (https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc) and practice related questions at (https://library.fiveable.me/practice/ap-biology).
How do symbiotic relationships actually drive population dynamics in communities?
Symbiotic relationships change population dynamics by altering birth rates, death rates, and how species access energy and matter (EK 8.5.B.2–B.4). Mutualism (both benefit) can increase population growth or carrying capacity for partners (e.g., pollinators + plants), raising species abundance and changing species composition. Parasitism adds mortality or reduces fecundity in hosts, lowering host population size and potentially causing cycles or local extinctions. Commensalism shifts resource use without obvious harm, which can change local abundance or allow niche partitioning and coexistence. These interactions can cascade through food webs (trophic cascades) or create keystone effects where one symbiont disproportionately shapes community diversity (affecting Simpson’s index). On the AP exam you may be asked to explain or predict these effects (LO 8.5.B), so link the interaction type to changes in birth/death rates, resource access, and resulting shifts in species composition/diversity. For a focused review, see the Topic 8.5 study guide (https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc) or the Unit 8 overview (https://library.fiveable.me/ap-biology/unit-8). For practice, try problems at (https://library.fiveable.me/practice/ap-biology).
Why do we need to know about positive and negative interactions between populations?
You need to know positive and negative interactions because they’re the rules that shape a community’s species composition and diversity (LO 8.5.A & LO 8.5.B). Positive interactions (mutualism, commensalism) and negative ones (competition, predation, parasitism, herbivory) change who survives, how energy and matter move through food webs, and which species become common or rare. Those interactions also create bigger effects like trophic cascades and determine roles of keystone species and niche partitioning—all things you’ll be asked to explain or predict on the exam (CED EK 8.5.B.2–B.4). Practically, understanding these interactions helps you interpret graphs, justify predictions in FRQs, and link changes in one population to shifts in community diversity (use Simpson’s Index when needed). Review examples and practice Qs in the Topic 8.5 study guide (https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc) and try practice problems (https://library.fiveable.me/practice/ap-biology) to build exam-ready reasoning.
What's the difference between interspecific and intraspecific competition in community ecology?
Interspecific competition is competition between different species for the same limited resources (like food, light, or space). It can change community structure by reducing one species’ abundance, causing competitive exclusion (one species locally disappears) or driving niche partitioning/character displacement so species use different resources or times. Intraspecific competition is competition among individuals of the same species; it’s the main driver of density-dependent effects on population size (limits growth as you approach carrying capacity) and can regulate population dynamics. On the AP Bio CED, these fit LO 8.5.B and EK 8.5.B.3–B.4: inter- and intraspecific interactions affect how populations access energy/matter and shape community composition and diversity. For extra review and practice on these ideas, see the Topic 8.5 study guide (https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc), the full unit page (https://library.fiveable.me/ap-biology/unit-8), and practice problems (https://library.fiveable.me/practice/ap-biology).
I missed the lab on community structure - how do you measure species diversity in the field?
In the field you measure community structure by recording species composition (which species are present) and species diversity (richness + evenness). Common methods: quadrats (count all individuals in repeated plots), line or belt transects (record organisms along a line), and timed/standardized searches or pitfall traps for mobile animals. Count individuals per species (n) and total N, then calculate Simpson’s Diversity Index: Diversity = 1 − Σ(n/N)². The index ranges 0–1; higher values = more diversity (more species and/or more evenness). Also report species richness (total species) and describe evenness (are counts similar across species?). Make sure sampling is randomized, replicate plots, and effort is reported—AP free-response often asks for methods, calculations, and interpretation. Review examples and practice calculating/applying Simpson’s index in the Topic 8.5 study guide (https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc) and more unit practice (https://library.fiveable.me/ap-biology/unit-8).
How do invasive species change the structure and diversity of existing communities?
Invasive species change community structure by altering species composition and species diversity (LO 8.5.A). They often outcompete natives for resources (interspecific competition), displace or reduce populations (lower species richness) and unbalance species evenness—all of which lower diversity (you could show this with Simpson’s Diversity Index: D = 1 − Σ(n/N)²). Invasives can introduce new predator–prey links or remove predators/prey, causing trophic cascades that shift energy flow (EK 8.5.B.2–B.4). They also force niche shifts (niche partitioning or character displacement) and can remove or substitute keystone species, producing large community changes. For AP exam practice, be ready to explain mechanisms (competition, predation, mutualism changes) and predict effects on diversity metrics and food webs. Review Topic 8.5 study guide for quick examples and practice (https://library.fiveable.me/ap-biology/unit-8/community-ecology/study-guide/GhiVt7Egu8crmrHtQXXc). For extra practice questions, check Fiveable’s AP practice bank (https://library.fiveable.me/practice/ap-biology).