Biology · Unit 8 Ecology · 14 min read · Updated 2026-05-10
AP Biology Community Ecology — AP Biology
AP Biology · Unit 8 Ecology · 14 min read
1. Interspecific Interactions & Competitive Exclusion★★☆☆☆⏱ 3 min
Interspecific interactions are classified by their net effect on the fitness of each interacting individual. There are four core interaction types tested on the AP exam:
**Competition (-/-)**: Both species have reduced fitness because they share a limiting resource
**Predation/Parasitism (+/-)**: One species gains fitness at the expense of the other
**Mutualism (+/+)**: Both species gain increased fitness from the interaction
**Commensalism (+/0)**: One species gains fitness, the other is entirely unaffected
Exam tip: On FRQs asking to distinguish fundamental and realized niche, always explicitly mention the role of competition to earn full points.
2. Community Diversity: Simpson's Diversity Index★★★☆☆⏱ 4 min
Community diversity has two core components: species richness (the total number of different species in the community) and relative abundance (the proportion of the total community that each species makes up). Simpson's Diversity Index accounts for both richness and evenness (equality of relative abundance) to give a more holistic measure of diversity than richness alone.
D = 1 - \left( \frac{\sum n(n-1)}{N(N-1)} \right)
Where $n$ = number of individuals of a single species, $N$ = total number of individuals across all species, and $D$ ranges from 0 to 1. Values closer to 1 indicate higher diversity, while values closer to 0 indicate lower diversity.
Exam tip: Always double-check that you subtract the fraction from 1. AP question writers frequently use the unsubtracted dominance value as a distractor for MCQs.
3. Ecological Succession & Keystone Species★★☆☆☆⏱ 3 min
Ecological succession is the predictable, sequential change in community structure over time following a disturbance that alters the existing community. It is divided into two main types based on starting conditions after disturbance:
**Primary succession**: Occurs when all soil and nearly all living organisms are removed, for example after a volcanic eruption creates new rock or a glacier retreats leaving bare bedrock. Pioneer species (lichens, mosses) break down rock to build soil over centuries, eventually leading to a stable climax community.
**Secondary succession**: Occurs when a disturbance removes most above-ground vegetation but leaves the soil profile intact, for example after a wildfire or clear-cutting. Secondary succession proceeds much faster than primary succession because soil and native seeds are already present.
Exam tip: Always explicitly mention the presence or absence of intact soil in your FRQ justification for succession type—this is almost always a required scoring point.
4. AP Style Practice Check★★★★☆⏱ 4 min
Common Pitfalls
Why: Students confuse competition with predation, or assume only the losing species is harmed.
Why: Some textbooks use this alternative value to measure species dominance, not diversity.
Why: Students forget Simpson's D accounts for relative abundance as well as richness.
Why: Students misinterpret the prefixes 'primary' and 'secondary' as order of events, not starting conditions.
Why: Students confuse keystone species with dominant species, which are abundant and have large effects.
Why: Older textbooks describe climax communities as stable, but AP CED emphasizes that disturbance is a natural part of all ecosystems.