Community Ecology

1. Interspecific interactions

Interspecific interactions are relationships between individuals of different species, summarized by their effect on each participant (+ benefit, − harm, 0 neutral):

• Competition (−/−) — two species use the same limited resource, harming both.
• Predation (+/−) — one organism kills and eats another.
• Herbivory (+/−) — an animal eats part of a plant or alga.
• Parasitism (+/−) — one organism lives on or in a host, harming it without usually killing it immediately.
• Mutualism (+/+) — both species benefit.
• Commensalism (+/0) — one benefits, the other is unaffected.

The competitive exclusion principle states that two species with identical niches cannot coexist indefinitely — one will be driven out. Coexistence is possible when species reduce overlap through resource partitioning. A species' fundamental niche is the full range of conditions it could use without competitors; its realized niche is the narrower set it actually occupies when competitors are present. Prey evolve defenses: camouflage (cryptic coloration), aposematic (warning) coloration, and mimicry — Batesian (a harmless species copies a harmful one) or Müllerian (two harmful species resemble each other).

2. Trophic structure and keystone species

Trophic structure describes feeding relationships as food chains (a single linear path) and food webs (interconnected chains). Trophic levels proceed from producers (autotrophs) to primary consumers (herbivores), secondary consumers, and tertiary consumers, with decomposers recycling dead material. Energy is lost at each transfer, so food chains are usually short.

A keystone species has an effect far out of proportion to its abundance (the sea otter eating sea urchins, protecting kelp forests). A dominant species is the most abundant or has the most biomass — but abundance alone does not make a species a keystone. Species diversity has two parts: species richness (how many species) and relative abundance (how evenly individuals are spread among species). Diverse communities tend to be more productive and resilient to disturbance.

3. Community change and succession

Ecological succession is the gradual, directional change in community composition after a disturbance or on new substrate. Primary succession begins on lifeless substrate with no soil (bare rock after a volcanic eruption or glacial retreat), starting with lichens and mosses that build soil, then grasses, shrubs, and trees. Secondary succession begins where soil remains (an abandoned farm field or after a fire) and proceeds faster because soil and seeds are already present. Disturbance — fire, storms, floods, human activity — is a natural part of community dynamics and can maintain diversity by preventing any one species from dominating.

How it works

Secondary succession, step by step:

1. A disturbance (fire, farming, flood) removes vegetation but leaves soil and a seed bank intact.
2. Fast-growing pioneer species — grasses, herbs, and light-loving plants — colonize first.
3. These species alter the environment (adding organic matter, shade, and nitrogen), making it suitable for the next group.
4. Shrubs, then fast-growing trees, arrive and shade out the pioneers.
5. Slower-growing, shade-tolerant species establish, and the community reaches a relatively stable state (which modern ecology views as dynamic, not a fixed endpoint).

Common confusions

Memory aids

"C-P-P-H-M-C" for interactions — Competition, Predation, Parasitism, Herbivory, Mutualism, Commensalism. For succession, remember "primary = pioneer on rock; secondary = speedy with soil." And for keystone species: "small part, big role" — abundance is not influence.

Quick review

Topic Recap

• Communities are shaped by competition, predation, herbivory, parasitism, mutualism, and commensalism.
• The competitive exclusion principle and niche concepts explain how competing species coexist.
• Food chains and webs organize trophic structure; keystone species exert disproportionate influence.
• Primary succession builds communities from bare rock; secondary succession rebuilds them from existing soil.
• Disturbance is a normal driver of community change and can maintain diversity.

Knowledge Check

1. What does the competitive exclusion principle predict, and how do species avoid it?
2. How does Batesian mimicry differ from Müllerian mimicry?
3. What is the difference between a keystone species and a dominant species?
4. Which proceeds faster, primary or secondary succession, and why?
5. Why are most food chains limited to about four or five trophic levels?

Answers and Rationales

1. Answer: Two species with identical niches cannot coexist indefinitely because one outcompetes the other; species coexist by partitioning resources and differentiating their niches. Why: Competition is strongest when niches overlap completely.
2. Answer: Batesian mimicry is a harmless species resembling a harmful one; Müllerian mimicry is two harmful species resembling each other. Why: Batesian is deceptive; Müllerian reinforces a shared warning signal.
3. Answer: A keystone species has an influence far greater than its abundance suggests; a dominant species is simply the most abundant or has the greatest biomass. Why: Influence and abundance are separate community roles.
4. Answer: Secondary succession, because soil and a seed bank already remain, whereas primary succession must first build soil from bare rock. Why: Soil and existing propagules speed recolonization.
5. Answer: Energy is lost as heat at each transfer (roughly 90% per level), so too little energy remains to support many additional levels. Why: Energy, not biomass alone, limits chain length.