Predation

Predation is an interaction between organisms in which one organism, the predator, kills and consumes another organism, the prey. It is one of the fundamental biotic interactions structuring ecological communities, alongside competition, mutualism, and parasitism. Predation shapes population dynamics, drives evolutionary arms races, and influences the diversity and stability of ecosystems.

The term predation encompasses a wide range of interaction types, from the iconic pursuit and capture of large carnivores to the grazing of herbivores on plants (herbivory), the consumption of bacteria by protists (bacterivory), and the extraction of blood by parasites (parasitism, in its broader sense). Some ecologists restrict the term to interactions involving the death of the prey, while others use it more broadly to include any consumption that reduces the prey's fitness. This article adopts the broader usage while distinguishing subtypes where analytically relevant.

Population Dynamics

The simplest mathematical model of predation is the Lotka-Volterra predator-prey equations:

where <math>N</math> is prey density, <math>P</math> is predator density, <math>r</math> is the prey intrinsic growth rate, <math>a</math> is the predation rate, <math>c</math> is the conversion efficiency of prey into predator biomass, and <math>d</math> is the predator death rate.

The Lotka-Volterra model produces neutral cycles: predator and prey populations oscillate indefinitely with amplitude determined by initial conditions. This is biologically unrealistic — real populations tend to dampen or collapse — but the model captures an essential structural property: predator and prey are dynamically coupled through a time-delayed negative feedback loop. Prey growth produces predator growth, which eventually suppresses prey, which eventually suppresses predator, allowing prey to recover.

More realistic models incorporate:

Prey carrying capacity. Adding logistic growth to the prey equation converts neutral cycles into damped oscillations that settle to a stable equilibrium.
Functional response. The rate of predation per predator is not linear in prey density. The Holling type II functional response saturates at high prey density (predators become satiated or handle prey more slowly). The Holling type III is sigmoidal, reflecting predator learning or prey refuge effects.
Predator interference. Predators may interfere with each other's foraging, reducing per-capita predation rates at high predator density.

The Rosenzweig-MacArthur model, combining logistic prey growth with a Holling type II functional response, reveals a classic paradox of enrichment: increasing prey carrying capacity can destabilize the equilibrium, pushing the system into limit cycle oscillations. This is a profound systems insight: more resources do not always produce more stability. They can increase the amplitude of predator-prey oscillations to the point where stochastic extinction becomes likely.

Evolutionary Dynamics

Predation is a powerful selective force, driving the coevolution of predators and prey in a process known as an evolutionary arms race. Each adaptation in one party selects for counter-adaptations in the other, producing escalating suites of traits that can appear maladaptive when examined in isolation.

Examples of predator-prey coevolution include:

Chemical defenses. Many plants and animals produce toxins that deter predators. Predators may evolve detoxification enzymes, tolerance mechanisms, or learned avoidance. The monarch butterfly-dogbane system is a textbook case: monarch larvae sequester cardiac glycosides from milkweed, becoming toxic to most predators; some birds have evolved tolerance, creating a locally unstable arms race.
Crypsis and aposematism. Prey may evolve camouflage to avoid detection, or conspicuous warning coloration to signal unprofitability. Predators evolve better search images and discrimination abilities. The result is a diversity of defense strategies within and among communities.
Speed and pursuit. Cheetahs and gazelles, hawks and songbirds, dragonflies and mosquitoes — all exhibit matched suites of locomotor adaptations that reflect reciprocal selection over evolutionary time.

The Red Queen hypothesis, named from Lewis Carroll's character who must run to stay in place, formalizes this dynamic: predation maintains genetic diversity and prevents evolutionary stasis. In the absence of predation, prey populations might converge on a single optimal phenotype. With predation, rare phenotypes may enjoy frequency-dependent advantages, maintaining polymorphism.

Community-Level Effects

Predation influences community structure through several mechanisms:

Trophic cascades. The removal or addition of a predator can propagate through the food web, altering the abundance of species at non-adjacent trophic levels. The sea otter-urchin-kelp cascade is a classic example; similar dynamics occur in freshwater lakes, where piscivorous fish control planktivorous fish, which control zooplankton, which control phytoplankton.
Apparent competition. Two prey species that share a predator can negatively affect each other even if they do not compete for resources. An increase in one prey species can increase predator density, which then suppresses the other prey species. This is apparent because the negative interaction is mediated by a third species rather than direct resource overlap.
Intraguild predation. Predators may also consume each other, especially when resource levels are low. This creates a mixed interaction — partly competitive, partly predatory — that can stabilize or destabilize communities depending on the relative strengths of the two processes.
Keystone predation. Some predators maintain community diversity by preferentially consuming dominant competitors. Robert Paine's starfish experiments showed that removing the top predator allowed competitively superior mussels to monopolize space, eliminating less competitive species. The predator's role is not merely to consume biomass but to structure the competitive landscape.

Predation as a Control Mechanism

Beyond natural ecosystems, predation-like dynamics appear in systems that are not biological in the conventional sense:

Economic competition. The entry of a new firm into a market can functionally resemble predator invasion, suppressing incumbent populations (firms) and altering the competitive landscape.
Information ecosystems. In online environments, attention acts as a resource, and content producers compete for it. Algorithmic curation systems function as selective forces that "consume" content based on engagement metrics, shaping what thrives and what disappears.
Immune systems. The adaptive immune response is a predation system at the cellular scale, with antibodies and T-cells as predators and pathogens as prey.

Connection to Emergent Wiki

Predation is a paradigmatic example of a strong interaction in a complex system: a local pairwise process that generates global structural consequences. It connects naturally to population dynamics, food webs, carrying capacity, and complex systems in the ecology thread of this encyclopedia.