Apparent Competition

Apparent competition is an indirect negative interaction (−/−) between two or more species at the same trophic level that arises not because they compete for shared limiting resources (such as food, water, light, or nesting space), but because they are attacked by a shared natural enemy—a generalist predator, parasitoid, herbivore, ectoparasitic vector (such as ixodid ticks including the American dog tick), or infectious pathogen (Holt, 1977). Coined and formalized mathematically by American theoretical ecologist Robert D. Holt in 1977, the term reflects a fundamental observational trap in field ecology: when an increase in the abundance of Prey Species 1 causes a decline in the equilibrium abundance (or local extinction) of Prey Species 2, the empirical population pattern looks identical to classic Lotka–Volterra resource competition, even when the two prey species have zero dietary or habitat overlap.

To isolate indirect enemy-mediated interactions from resource competition, Holt (1977) analyzed a three-species community consisting of two non-competing prey species (N₁ and N₂, with zero interspecific resource competition α₁₂ = α₂₁ = 0) and a single shared food-limited predator (P):

• dN₁ / dt = r₁ N₁ (1 − N₁ / K₁) − a₁ N₁ P
• dN₂ / dt = r₂ N₂ (1 − N₂ / K₂) − a₂ N₂ P
• dP / dt = P ( b₁ a₁ N₁ + b₂ a₂ N₂ − m )

where r_i is the intrinsic per-capita growth rate of prey i, K_i is its resource-determined carrying capacity, a_i is the predator's per-capita attack rate on prey i, b_i is the predator's assimilation efficiency converting consumed prey i into new predators, and m is the predator's per-capita mortality rate.

1. Single-Prey Baseline (N₁ Alone with Predator P): When only Prey 1 is present, the predator reaches equilibrium (dP/dt = 0) at prey density N₁^* = m / (b₁ a₁), sustaining a predator density of P₁^* = (r₁ / a₁) (1 − N₁^* / K₁).
2. Addition of Prey 2 and the P*** Rule:** When productive Prey 2 (K₂ > m / [b₂ a₂]) is added to the ecosystem, it subsidizes the shared predator, elevating the total equilibrium predator density to P > P₁. Because each individual of Prey 1 now faces a higher density of searching predators (a₁ P^*), the equilibrium abundance of Prey 1 is depressed below its single-species level (∂N₁^* / ∂K₂ < 0).
3. Asymmetrical Exclusion ("The P* Rule" of Apparent Competition): Analogous to David Tilman's R* rule for resource competition (where the species that survives at the lowest resource level R^* wins), in a purely predator-limited system where prey lack density-dependent refuges (K_i → ∞), each prey species has a maximum tolerable predator density P_i^* = r_i / a_i (its ratio of intrinsic growth rate to vulnerability). The prey species with the higher r_i / a_i ratio drives the shared predator up to a density P^* = r₁ / a₁ > r₂ / a₂, driving the more vulnerable or slower-reproducing Prey 2 (N₂ → 0) to complete local extinction (Holt, 1977; Holt & Lawton, 1994).

Apparent competition operates across two distinct timescales:

• Long-Term Numerical Response (Consumer Biomass Subsidy): A high-productivity primary prey species increases the birth rate and survival of the shared predator or parasitoid over generations, inflating predator abundance (P).
• Short-Term Aggregative / Functional Response (Within-Generation Spillover): Even before predator reproduction occurs, mobile vertebrate predators (such as raptors, foxes, or wolves) or foraging parasitoid wasps actively congregate in patches containing a high-density prey species (aggregative numerical response), increasing incidental encounter rates with rare secondary prey in the same patch (Holt & Kotler, 1987). Conversely, if a predator becomes satiated on a sudden pulse of Prey 1 (Type II functional response) or develops a specialized search image for Prey 1 (Type III switching), it can temporarily relieve predation on Prey 2 (apparent mutualism), before the subsequent numerical surge in predator offspring triggers severe delayed apparent competition.

In conservation biology, ignoring apparent competition frequently causes single-species management interventions to backfire:

• Hyperpredation on Oceanic Islands (Courchamp, Langlais & Sugihara, 2000): When humans introduce a fast-reproducing invasive mammal—such as the European rabbit (Oryctolagus cuniculus), black rat (Rattus rattus), or feral pig—onto an oceanic island alongside an invasive or colonizing apex predator (feral cat Felis catus, red fox, or golden eagle), the r-selected invasive prey sustains predator densities far above what native island endemics could ever support. Even as the endemic bird, reptile, or marsupial species declines toward zero, the predator suffers no food shortage because it subsists on the invasive rabbit or rodent—driving the native endemic to deterministic extinction.
• Danger of Eradicating Exotic Prey Alone: If conservation managers abruptly eradicate an abundant invasive prey species (such as island rabbits) without simultaneously controlling the inflated population of invasive predators (such as feral cats), starving predators switch 100% of their foraging effort onto the remaining threatened native seabirds or reptiles, triggering a catastrophic transient predation pulse (Zavaleta et al., 2001). Successful island and boreal restorations (such as the Channel Islands fox recovery and British Columbia maternal-penning + moose/wolf management trials) demonstrate that simultaneous control of both the subsidizing prey and the shared predator is required to break an apparent-competition extinction vortex.

1. Holt, R. D. (1977). "Predation, apparent competition, and the structure of prey communities." Theoretical Population Biology, 12(2), 197–229. https://doi.org/10.1016/0040-5809(77)90042-990042-9)
2. Holt, R. D., & Lawton, J. H. (1994). "The ecological consequences of shared natural enemies." Annual Review of Ecology and Systematics, 25, 495–520. https://doi.org/10.1146/annurev.es.25.110194.002431
3. Müller, C. B., & Godfray, H. C. J. (1997). "Apparent competition between two aphid species." Journal of Animal Ecology, 66(1), 57–64. https://doi.org/10.2307/5964
4. Roemer, G. W., Donlan, C. J., & Courchamp, F. (2002). "Golden eagles, feral pigs, and insular carnivores: How exotic species turn native predators into prey." Proceedings of the National Academy of Sciences, 99(2), 791–796. https://doi.org/10.1073/pnas.012422499
5. Serrouya, R., Seip, D. R., Hervieux, D., McLellan, B. N., McNay, R. S., Steenweg, R., et al. (2019). "Saving endangered species using adaptive management." Proceedings of the National Academy of Sciences, 116(13), 6181–6186. https://doi.org/10.1073/pnas.1816923116