Community Ecology

Core Concepts

The niche: who does what where

A species' niche is its full role in the community — the resources it uses, the conditions it tolerates, and its interactions. Ecologists distinguish the fundamental niche (the full range of conditions and resources a species could use with no competitors or predators) from the realized niche (the part it actually occupies, usually narrowed by competition and predation). The distinction explains why a species is often found in only a slice of the habitat it could tolerate.

Competition and the competitive exclusion principle

When two species use the same limiting resource, they compete (−/−). The competitive exclusion principle states that two species cannot occupy the same niche indefinitely — one will outcompete the other locally, or the two will diverge. In nature, coexistence is common because species partition resources: they divide the resource by space, time, or food type (e.g., warblers feeding in different parts of the same tree). Over evolutionary time, competition can drive character displacement — competing species evolve different body or beak sizes, as in Darwin's finches, reducing overlap.

Predation, herbivory, and the arms race

Predation (+/−) is one species consuming another; herbivory is the special case of animals eating plants. Both drive coevolution: prey evolve defenses and predators evolve countermeasures. Commonly taught defenses include:

• Camouflage — blending with the background.
• Warning coloration (aposematism) — bright colors advertising that an animal is toxic or dangerous; predators learn to avoid it.
• Batesian mimicry — a harmless species resembles a dangerous one (a harmless fly that looks like a stinging bee).
• Müllerian mimicry — two or more genuinely dangerous species evolve to look alike, so predators learn the shared signal faster.

Predator–prey dynamics also link back to the population cycles of the previous topic: predator and prey numbers chase each other, often with time lags.

Symbiosis: living together

Symbiosis means species living in close association; the outcomes vary:

• Mutualism (+/+): both benefit. Pollinators and flowers, nitrogen-fixing bacteria and legumes, mycorrhizal fungi and plant roots, corals and their photosynthetic zooxanthellae.
• Commensalism (+/0): one benefits, the other is unaffected. Barnacles attached to a whale gain transport and feeding opportunities while the whale is unharmed.
• Parasitism (+/−): the parasite benefits at the host's expense — tapeworms, ticks, mistletoe. Parasites rarely kill their host quickly, because a dead host is a dead food supply; they are a major selective force on host populations.

Keystone and foundation species

A keystone species has a disproportionately large effect on community structure relative to its abundance or biomass. Classic, commonly taught examples: the intertidal sea star Pisaster (its removal lets mussels dominate and eliminates other species), sea otters (they control sea urchins, protecting kelp forests), and wolves in Yellowstone (they reshape elk behavior and, indirectly, vegetation and streamside habitat). A foundation species physically creates or modifies habitat that others depend on — corals, kelp, and dominant forest trees. The difference: keystone species hold the community together through their interactions; foundation species build the structure.

Invasive species

An invasive species is a non-native species that spreads aggressively and disrupts the community. Introduced to a new region, it often leaves behind its natural predators, parasites, and competitors, so its population grows with few checks. Kudzu, zebra mussels, and cane toads are frequently cited examples. Invasives can outcompete natives, alter nutrient cycles, and drive local extinctions — one of the leading human-caused threats to biodiversity.

Succession: communities rebuilding

Succession is the predictable sequence of species change after a disturbance:

• Primary succession starts on bare substrate with no soil — cooled lava, bare rock, a retreated glacier. Pioneer species (lichens, mosses) break down rock and build the first soil, enabling later plants.
• Secondary succession starts where soil remains — abandoned farmland, a forest after fire or logging. Because soil and often seed banks persist, recovery is faster.

Succession proceeds through stages toward a relatively stable community. The classic "climax community" concept — a single final, stable state — is a simplification: disturbances, chance, and changing conditions mean communities are often mosaics rather than one fixed endpoint. This is a model limitation worth remembering.

Common Confusions