Community Ecology, Succession & Biodiversity

You’ll be able to

Apply the competitive exclusion principle and niche partitioning to predict whether species coexist
Distinguish keystone from foundation species and predict the consequences of removing each
Calculate and interpret Simpson’s Diversity Index for a community

Competitive exclusion and niche partitioning

Species in a community interact in ways summarized by their effect on each partner: competition (−/−), predation/herbivory/parasitism (+/−), mutualism (+/+), and commensalism (+/0). Competition drives one of the most tested ideas in ecology: the competitive exclusion principle — two species that use a limited resource in the exact same way cannot coexist indefinitely; the slightly better competitor drives the other to local extinction. Gause showed this with Paramecium: grown separately both thrived, but on one shared food source one species always eliminated the other. Coexistence is rescued by niche partitioning (resource partitioning): species divide the resource so their realized niches overlap less. MacArthur’s warblers feed in the same spruce trees but at different heights, so each has a slice to itself. Partitioning can be spatial, temporal, or dietary — reduce the overlap enough and the exclusion principle no longer forces one species out.

Interaction outcomes

competition −/− · predation +/− · mutualism +/+ · commensalism +/0

Read each pair as the effect on species 1 / species 2. Competitive exclusion applies specifically to the −/− case when two species share one limiting resource with fully overlapping niches.

Keystone vs. foundation species

Both have outsized effects, but through opposite mechanisms. A keystone species exerts its influence through interactions — usually predation — that hold the community’s structure together, and it is typically not abundant. Remove Pisaster sea stars and mussels overrun the rock, crushing diversity. A foundation species exerts its influence through sheer biomass and physical structure — it builds the environment. Kelp, reef-building corals, and forest canopy trees are foundation species: they are highly abundant and their removal collapses the habitat everything else depends on. Keystone = disproportionate impact for its low abundance; foundation = habitat-forming and abundant.

Succession and the recovery of diversity

Disturbance resets a community, and ecological succession rebuilds it. Primary succession begins on lifeless substrate with no soil — bare rock from a retreating glacier or cooled lava — so pioneer species (lichens, mosses) must first weather the rock into soil; it is extremely slow. Secondary succession begins where a disturbance (fire, flood, abandoned field) removed the community but left the soil, seeds, and roots intact, so it proceeds far faster. Diversity typically climbs through succession, often peaking at an intermediate stage of disturbance rather than at the final climax community — a pattern worth being able to name.

Simpson’s Diversity Index

D = 1 − Σ(n/N)²

n = number of individuals of one species; N = total individuals of all species; Σ sums the squared proportions across every species. D ranges from 0 (one species dominates, no diversity) to nearly 1 (many equally abundant species). Higher D = greater diversity.

Worked example

A survey plot holds 4 species with individual counts of 40, 30, 20, and 10 (N = 100 total). Calculate Simpson’s Diversity Index, D = 1 − Σ(n/N)².

1.Convert each count to a proportion n/N: 40/100 = 0.40, 30/100 = 0.30, 20/100 = 0.20, 10/100 = 0.10.
2.Square each proportion: 0.40² = 0.16, 0.30² = 0.09, 0.20² = 0.04, 0.10² = 0.01.
3.Sum the squares: Σ(n/N)² = 0.16 + 0.09 + 0.04 + 0.01 = 0.30.
4.Subtract from 1: D = 1 − 0.30 = 0.70.

Answer: D = 0.70. On the 0-to-1 scale this is a fairly diverse community; a plot where one species held 85 of the 100 individuals would give a much lower D (about 0.27).

Checkpoint

A quadrat contains 3 plant species with 60, 30, and 10 individuals (N = 100). Using D = 1 − Σ(n/N)², what is Simpson’s Diversity Index for this community?

Watch out

Do not stop at Σ(n/N)². Simpson’s index is D = 1 − Σ(n/N)² — the sum of squared proportions is the probability that two random individuals are the same species (dominance), so you subtract it from 1 to get diversity. Forgetting the "1 −" step inverts your answer, and higher squared-sum actually means lower diversity.

Checkpoint

Two warbler species feed in the same spruce trees, but one forages mainly in the top canopy while the other stays on the lower inner branches, and both species persist for decades. This coexistence is best explained by:

On the exam

Nail the two "big impact" categories: a keystone species shapes the community through interactions despite low abundance (remove it → diversity collapses); a foundation species shapes it by building habitat and is highly abundant. And on any diversity calculation, show the full path — proportions → squared → summed → subtracted from 1 — because partial credit tracks each step.

Answer the 2 checkpoints as you read.