AP Biology Community Ecology

AP Biology· AP Biology CED — 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

📘 Definition

Competitive Exclusion Principle

Two species cannot occupy identical fundamental niches in the same environment indefinitely. The more efficient competitor will exclude the weaker competitor locally.

Example:

First demonstrated by Gause's experiments with two Paramecium species.

📘 Definition

Niche Types

Fundamental vs Realized

A niche is the full set of biotic and abiotic resources a species needs to survive and reproduce. The fundamental niche is the full range of resources a species can use without competition; the realized niche is the smaller range it actually uses when competition is present.

📐 Worked Example

A researcher studies two species of wild sunflower that grow in the same prairie habitat. Species A can grow in soil pH ranging from 5.5 to 7.0 when grown alone, but when Species B is present, Species A only grows in pH 5.5 to 6.2. Identify the realized niche of Species A, and explain whether this supports competitive exclusion or resource partitioning.

1
Recall that the realized niche is the actual range of resources an organism uses in the presence of competition, while the fundamental niche is the range without competition.
2
Match to the given data: When alone, Species A occupies a fundamental niche of 5.5–7.0 pH. With competition, it is restricted to 5.5–6.2 pH, so this range is its realized niche.
3
Evaluate the outcome: Both species coexist, each restricted to a subset of their fundamental niche. Competitive exclusion would result in one species being eliminated entirely, which does not occur here.
4
Conclusion: The realized niche of Species A is 5.5–6.2 pH, and this observation supports resource partitioning.

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.

📐 Worked Example

A researcher samples a 10m x 10m plot of a regenerating forest and counts 50 total individual trees, with the following species counts: Species 1 = 20, Species 2 = 18, Species 3 = 12. Calculate Simpson’s Diversity Index for this plot.

1
Confirm total number of individuals: $N = 20 + 18 + 12 = 50$ , which matches the given total.
2
$N(N-1) = 50 \times 49 = 2450$
3
Calculate $n (n - 1)$ for each species:
4
$\text{Species 1}: 20 \times 19 = 380 \\ \text{Species 2}: 18 \times 17 = 306 \\ \text{Species 3}: 12 \times 11 = 132$
5
Sum the $n (n - 1)$ values: $380 + 306 + 132 = 818$
6
Plug into the formula:
7
$D = 1 - \left(\frac{818}{2450}\right) = 1 - 0.334 = 0.666$

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.

📘 Definition

Keystone Species

A species that has a disproportionately large effect on community structure relative to its biomass or abundance. Removing a keystone species causes dramatic collapse of community diversity, often via a trophic cascade, where indirect effects ripple through all lower trophic levels.

📐 Worked Example

A wildfire burns through a Rocky Mountain conifer forest, removing 90% of above-ground vegetation but leaving the majority of the soil profile intact. Researchers measure tree community diversity 10, 50, and 100 years after the fire, leading up to the climax community. Is this primary or secondary succession? Justify your answer and describe the trend in Simpson’s Diversity Index over this period.

1
Recall the key distinction between succession types: primary succession requires complete removal of soil, while secondary succession leaves intact soil after disturbance.
2
Justify the classification: The scenario explicitly states soil remains intact, so this is secondary succession.
3
Describe the diversity trend: 10 years after the fire, only a few fast-growing pioneer species are abundant, so diversity is low. As time passes, slower-growing climax species colonize and coexist with remaining pioneer species, increasing diversity.
4
Conclusion: This is secondary succession, and Simpson’s D will increase from 10 to 100 years after the fire before stabilizing at a high level in the climax community.

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

✓ Quick check

Test your understanding with this AP-style multiple choice question:

A marine ecologist compares two intertidal communities. Community 1 has 4 species with the following counts: 25, 25, 25, 25. Community 2 has 5 species with the following counts: 91, 2, 2, 2, 3. Which of the following correctly compares the species richness and Simpson's Diversity Index of the two communities?
- A) Community 2 has higher species richness and higher Simpson's D than Community 1
- B) Community 2 has higher species richness, but Community 1 has higher Simpson's D than Community 2
- C) Community 1 has higher species richness and higher Simpson's D than Community 2
- D) Species richness is the same for both communities, but Community 1 has higher Simpson's D
Reveal answer
B —
Correct. Species richness is simply the number of species: Community 2 has 5 species, so higher richness than Community 1's 4. Simpson's D accounts for evenness: Community 1 is perfectly even, so its D (~0.758) is much higher than Community 2's D (~0.171), where one species dominates.

📐 Worked Example

Ecologists study the effect of invasive garlic mustard on understory plant communities in Midwestern deciduous forests. They sample two 100m² plots: one uninvaded, and one invaded by garlic mustard, with both plots having 120 total individuals. Counts: Uninvaded: Trillium (25), Trout lily (35), Wild geranium (20), Spring beauty (30), Garlic mustard (10). Invaded: Garlic mustard (85), Trillium (10), Trout lily (15), Wild geranium (5), Spring beauty (5). Calculate Simpson's Diversity Index for both plots, and interpret your result in the context of invasive species impact.

1
Uninvaded plot calculation: $N = 120$ , so:
2
$N(N-1) = 120 \times 119 = 14280$
3
Sum of $n (n - 1)$ across all species:
4
$(25 \times 24) + (35 \times 34) + (20 \times 19) + (30 \times 29) + (10 \times 9) = 3130$
5
$D_{uninvaded} = 1 - \frac{3130}{14280} \approx 0.781$
6
Invaded plot calculation: $N = 120$ , same $N (N - 1) = 14280$ :
7
$\sum n(n-1) = (85 \times 84) + (10 \times 9) + (15 \times 14) + (5 \times 4) + (5 \times 4) = 7480$
8
$D_{invaded} = 1 - \frac{7480}{14280} \approx 0.476$
9
Interpretation: Invasion by garlic mustard reduces overall understory plant diversity. The uninvaded community has a much higher Simpson's D than the invaded community. Garlic mustard outcompetes native species, reducing native richness and evenness to lower overall community diversity.

5. Common Pitfalls

Wrong move:

Calling interspecific competition a +/- interaction.

Why:

Students confuse competition with predation, or assume only the losing species is harmed.

Correct move:

Always classify competition as -/- because both species have reduced access to resources and lower fitness than they would without competition.

Wrong move:

Reporting Simpson's Diversity Index as $\frac{\sum n ( n - 1 )}{N ( N - 1 )}$ instead of 1 minus that value.

Why:

Some textbooks use this alternative value to measure species dominance, not diversity.

Correct move:

Remember AP uses the diversity version where higher values equal higher diversity, so always subtract the ratio from 1 before reporting.

Wrong move:

Assuming higher species richness always means higher Simpson's diversity.

Why:

Students forget Simpson's D accounts for relative abundance as well as richness.

Correct move:

When comparing diversity, always check evenness: a community with 10 species dominated by one species has lower D than a community with 5 evenly distributed species.

Wrong move:

Claiming primary succession happens 'first' before secondary succession.

Why:

Students misinterpret the prefixes 'primary' and 'secondary' as order of events, not starting conditions.

Correct move:

Always distinguish the two based on whether intact soil remains after disturbance (secondary) or is completely removed (primary).

Wrong move:

Labeling the most abundant species in a community as a keystone species.

Why:

Students confuse keystone species with dominant species, which are abundant and have large effects.

Correct move:

Remember keystone species are defined by their disproportionate effect relative to their abundance; rare top predators are keystone, abundant trees are dominant.

Wrong move:

Stating that climax communities are permanent and unchanging.

Why:

Older textbooks describe climax communities as stable, but AP CED emphasizes that disturbance is a natural part of all ecosystems.

Correct move:

Describe climax communities as relatively stable over long periods, but still subject to future disturbances that restart succession.

6. Quick Reference Cheatsheet

Category	Formula / Rule	Notes
Interspecific Interaction Classification	Competition = (-/-); Predation/Parasitism = (+/-); Mutualism = (+/+); Commensalism = (+/0)	Classified by net fitness impact on each species
Competitive Exclusion Principle	No two species with identical niches coexist permanently	One species outcompetes the other for local extinction
Niche Definitions	Fundamental = no competition; Realized = with competition	AP tests this distinction more than other concepts here
Simpson's Diversity Index	D = 1 - \left( \frac{\sum n(n-1)}{N(N-1)} \right)	Higher $D$ = higher diversity; $n$ = per species count, $N$ = total individuals
Succession Types	Primary = no intact soil; Secondary = intact soil remains	Distinguished by starting conditions, not order of events
Keystone Species	Disproportionate effect relative to abundance/biomass	Removal causes major diversity loss via trophic cascades

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · MCQ
Compare community diversity values
2022 · FRQ
Analyze keystone species role
2021 · MCQ
Classify interspecific interactions

What's Next

Mastering community ecology provides the foundation for understanding broader ecosystem dynamics and global change impacts on biodiversity, which are common high-weight topics in AP Biology FRQ sections. The concepts you learned here, particularly diversity measurement and species interactions, connect directly to topics like conservation biology, invasive species management, and the impact of climate change on ecological communities. You will also reuse the niche concept when exploring evolutionary adaptations and species distribution patterns in other units. Building on this knowledge will help you connect individual concepts to the big picture of ecology that is frequently tested on the AP exam.