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Species interact in ways classified by their effects on each participant: competition (−/−) reduces fitness of both; predation and parasitism (+/−) benefit one and harm the other; mutualism (+/+) benefits both; commensalism (+/0) benefits one with no effect on the other. Competitive exclusion (Gause's principle) states that two species competing for identical resources cannot coexist indefinitely. Resource partitioning and character displacement allow ecologically similar species to coexist by specializing on different niches.
Use Lotka-Volterra competition equations to predict competitive exclusion vs. coexistence outcomes based on interspecific and intraspecific competition coefficients. Study empirical examples of character displacement (e.g., Darwin's finch beak size) as evidence for competition structuring communities.
Communities are not just collections of species — they are networks of interactions that shape population sizes, evolutionary trajectories, and ecosystem structure. Building on what you know about natural selection and population dynamics, this topic classifies species interactions by their fitness consequences and explores how they structure ecological communities.
The standard classification uses a two-symbol notation for each interacting species: (+) if the interaction increases fitness, (−) if it decreases it, and (0) if it has no effect. Competition (−/−) reduces both species' fitness through shared resource depletion or interference. Predation and parasitism (+/−) benefit one organism and harm the other. Mutualism (+/+) benefits both partners. Commensalism (+/0) benefits one species with no measurable effect on the other. These categories are useful but simplified — the sign of an interaction can shift with environmental context, population density, and evolutionary history.
Competition is particularly important for community structure. Gause's competitive exclusion principle predicts that two species competing for completely identical resources cannot coexist indefinitely in a stable, uniform environment — one will have even a slight advantage and drive the other to local extinction. In practice, complete niche overlap is rare. Resource partitioning — the division of resources along some dimension — reduces overlap and enables coexistence. A striking evolutionary outcome is character displacement: when two ecologically similar species co-occur, selection favors individuals that differ more from the competitor, gradually pushing the species' niches further apart. Darwin's finches in the Galápagos show this pattern in beak morphology, with species that co-occur on islands showing greater beak divergence than those found in isolation.
Predation and parasitism drive coevolutionary arms races: prey evolve defenses (camouflage, toxins, warning coloration), while predators evolve counter-adaptations (better detection, venom, cooperative hunting). An important community-level insight is that predators and parasites are not simply negative forces — they can be biodiversity engineers. By preferentially targeting dominant competitor species, they free resources for subordinate species that would otherwise be excluded. Remove a keystone predator and communities often simplify dramatically, as seen with the reintroduction of wolves to Yellowstone.
Mutualism is often presented as straightforward cooperation, but the evolutionary logic is subtler. A mutualistic interaction is maintained because each partner currently gains more from the interaction than it costs. When that cost-benefit balance shifts — for example, if one partner becomes so common that the other can get the benefit without reciprocating — the mutualism can degrade into commensalism or even parasitism. Many mutualisms are therefore conditional and facultative rather than obligate partnerships, and understanding them requires thinking about the economics of cooperation under varying ecological conditions.