Competition, Niches and What Allows Coexistence

The standard two-species Lotka-Volterra competition model describes populations whose growth is reduced by their own density and by the density of a competitor. Its conclusions depend on the model's assumptions.

For stable coexistence, each species must grow when rare while the other is at its single-species equilibrium. Test both directions. Success by only one rare species does not establish stable coexistence. If each species declines against an established rival, either can resist invasion once established. The winner can then depend on starting numbers, a priority effect.

The phrase stronger self-limitation needs care. A per-individual effect must be considered together with the population density producing it. Comparing two raw coefficients without their density scales is not enough. In field communities, resource variation, enemies and additional species can change outcomes, so a two-species result should not be presented as a universal rule.

Species can differ in resource type, feeding place or feeding time. Such partitioning can reduce competition, but it need not remove every overlap. Two birds can both eat insects while taking different proportions from upper and lower branches. Two seed-eating rodents can prefer different seed sizes while still sharing some foods.

Name and measure the resource. Counts of feeding events describe use; they do not automatically describe the amount of food available. Different preferences also do not prove that both populations will persist.

A removal experiment can test whether a competitor restricts resource use. Randomly assign comparable plots to removal or sham handling, then compare responses. Diet expansion only after removal supports release from competition. Check whether removal also changed habitat, predators or other species. A treatment response can support a hypothesis without identifying every causal link.

R* is the concentration of a limiting resource at which a species replaces its losses. It is not the amount that one individual consumes and is not a growth rate.

Consider species that can each persist alone in a constant, well-mixed system limited by one resource, with no other limits, enemies or immigration. If replacement thresholds differ, the species with the lower R* can reduce the resource below its rival's threshold. At equilibrium the rival cannot replace its losses, so the model predicts exclusion.

This prediction needs its assumptions. Multiple limiting resources, environmental fluctuations, predators and partners can alter the result. Observing two species together at one moment is also not proof of stable coexistence. Their numbers may be changing toward exclusion. Longer records and growth tests help distinguish persistence from temporary overlap.

A predator can affect two competitors differently. If it suppresses the dominant competitor, the weaker one may gain access to more of their shared resource. That pathway can permit persistence, but the predator's presence alone does not establish the outcome.

Partners can also change resource access. Nitrogen-fixing microbes supply a plant with usable nitrogen derived from atmospheric nitrogen, while receiving resources such as carbon compounds. The partnership can improve growth under nitrogen shortage without removing limits from water, light or other nutrients. Symbiosis means a close, long-term association; only associations benefiting both partners are mutualistic.

A fundamental niche describes conditions allowing persistence under specified assumptions about other species. A realized niche describes actual occupancy with interactions and limits on access. Competition can restrict occupancy, but facilitators and mutualists can change conditions or resource access. A realized niche is therefore not always a smaller range merely because other species are present.