Species Interactions: Competition and Predation

When two species share resources or one consumes the other, their population dynamics become coupled, producing competitive exclusion, coexistence, or the linked oscillations of predators and their prey.

Definition

Species interactions in this topic are the demographic effects that competing species and consumer-resource pairs have on one another's population growth, analysed through coupled models of competition and predation.

Scope

This topic covers the two-species interactions that most strongly shape population dynamics: interspecific competition for shared resources, and consumer-resource interactions including predation, herbivory, and parasitism. It treats the Lotka-Volterra competition and predation equations, the competitive exclusion principle, resource and apparent competition, functional and numerical responses of predators, and the conditions for stable coexistence or cyclic dynamics.

Core questions

• When does competition lead to exclusion versus coexistence of species?
• How do the Lotka-Volterra equations describe competition and predation?
• How does a predator's functional response shape predator-prey dynamics?
• What stabilises or destabilises consumer-resource cycles?

Key theories

Competitive exclusion principle

Two species competing for the same single limiting resource cannot coexist indefinitely; the more efficient competitor displaces the other unless niche differences or other mechanisms relieve the competition.

Lotka-Volterra dynamics and functional responses

Coupled equations link predator and prey densities through attack and conversion rates, and a predator's functional response—how its consumption rate saturates with prey density—determines whether the interaction yields stable equilibria or oscillations.

Mechanisms

In competition, each species depresses the shared resource and thereby the other's per-capita growth; the outcome depends on competition coefficients and carrying capacities, with coexistence requiring that each species limits itself more than its competitor. In predation, prey growth is reduced in proportion to encounters with predators while predator growth depends on prey consumed; Holling's functional responses describe how per-predator intake rises and saturates with prey density, and handling time and prey refuges modulate stability.

Clinical relevance

Understanding competition and predation informs biological pest control, the management of invasive species, fisheries and predator conservation, and the prediction of how removing or adding a species reshapes a community. This is educational context, not management prescription.

History

Lotka and Volterra independently derived competition and predator-prey equations in the 1920s, and Gause tested them experimentally with protists in the 1930s, formulating the competitive exclusion principle. Holling described predator functional responses in 1959, and Tilman's resource-competition theory of the 1980s grounded coexistence in resource use.

Debates

How important is competition in structuring nature?

The relative importance of competition versus predation, disturbance, and chance in determining community membership was vigorously debated, with field experiments showing competition is strong in some systems but weak or intermittent in others.

Key figures

• Alfred Lotka
• Vito Volterra
• Georgy Gause
• C. S. Holling
• David Tilman

Seminal works

• begon2006
• gause1934
• holling1959

Frequently asked questions

What is the competitive exclusion principle?

It states that two species competing for exactly the same limiting resource cannot coexist indefinitely; one will outcompete and eliminate the other unless they differ in resource use or other factors intervene.

What is a predator's functional response?

A functional response describes how the number of prey eaten per predator changes with prey density; it typically rises and then saturates because handling each prey item takes time.