Intraguild Predation

Intraguild predation (IGP) is a widespread ecological interaction combining interspecific resource competition and direct predator–prey consumption within the same foraging guild: an intraguild predator (IG predator) and an intraguild prey (IG prey) compete exploitatively for a shared limiting basal resource (R), while the IG predator simultaneously kills and consumes the IG prey (Polis, Myers, & Holt, 1989; Holt & Polis, 1997). Synthesized conceptually by American ecologists Gary A. Polis, Christopher A. Myers, and Robert D. Holt in a landmark 1989 review in the Annual Review of Ecology and Systematics and formalized mathematically by Robert D. Holt and Gary A. Polis in 1997, intraguild predation represents one of the four fundamental three-species community modules in food-web theory, alongside linear three-level food chains (trophic cascades), exploitative competition, and apparent competition.

Historically, classical food-web models assigned species to discrete integer trophic levels, assuming that consumers at the same trophic level only compete via Lotka–Volterra competition while predators at higher levels only eat herbivores below them. Polis, Myers, and Holt demonstrated that across terrestrial, freshwater, and marine ecosystems, omnivory within guilds is common: large carnivores (such as wolves Canis lupus and lions Panthera leo) kill smaller mesopredators (coyotes, foxes, and African wild dogs Lycaon pictus) with which they share ungulate prey; planktivorous copepods consume rotifers that graze the same phytoplankton; and generalist predatory insects consume specialist parasitoid larvae inside shared herbivorous hosts.

Symmetry and Ontogenetic Size Structure

Polis, Myers, and Holt (1989) classified intraguild predation systems along two structural axes—symmetry of attack and ontogenetic size structure:

1. Asymmetric (Unidirectional) Intraguild Predation: One species (the IG predator, P) always preys upon the second species (the IG prey, N) across all post-embryonic life stages, while N never consumes P. Examples include lynx (Lynx canadensis) killing red foxes (Vulpes vulpes) while both hunt snowshoe hares, or ladybird beetles (Coccinella septempunctata) consuming aphids that harbor developing endoparasitoid wasp larvae (Aphidius spp.).
2. Symmetric (Mutual / Reciprocal) Intraguild Predation: Both consumer species prey upon each other's vulnerable juveniles or molting adults while competing for the same prey guild. Mutual IGP is common between desert scorpions (Paruroctonus mesaensis and Hadrurus arizonensis), wolf spiders (Pardosa and Hogna), and larval amphibians (Ambystoma salamanders and Scaphiopus spadefoot toads) in ephemeral ponds, where arrival order frequently triggers strong priority effects.
3. Age- and Size-Structured (Ontogenetic) Intraguild Predation: In size-structured aquatic populations (such as largemouth bass Micropterus salmoides vs. bluegill sunfish Lepomis macrochirus, or Eurasian perch Perca fluviatilis vs. roach Rutilus rutilus), juvenile predators are small and must compete directly with adult and juvenile forage fish for benthic invertebrates and zooplankton (Daphnia). Only after surviving this competitive juvenile bottleneck do IG predators grow large enough to cross the gape-limitation threshold and switch to piscivory on their former competitors (ontogenetic niche shift, Mylius et al., 2001).

To determine when an IG predator (P), an IG prey (N), and a shared basal resource (R) can stably coexist, Robert D. Holt and Gary A. Polis (1997) analyzed the continuous-time dynamical system:

• Basal Resource (R): dR / dt = R [ r(1 − R / K) − a_N N − a_P P ]
• Intraguild Prey (N): dN / dt = N [ e_N a_N R − m_N − α P ]
• Intraguild Predator (P): dP / dt = P [ e_PR a_P R + e_PN α N − m_P ]

where r and K are the intrinsic growth rate and carrying capacity of the basal resource R; a_N and a_P are the attack rates (functional responses) of the IG prey and IG predator on the basal resource; α is the intraguild attack rate of P on N; e_N, e_PR, and e_PN are the trophic conversion efficiencies (numerical responses); and m_N, m_P are per-capita mortality rates.

In the absence of intraguild predation (α = 0), David Tilman's R^∗ rule of exploitative resource competition dictates that each consumer reduces the basal resource to its minimum equilibrium requirement:

• IG Prey Resource Requirement: R_N^∗ = m_N / (e_N a_N)
• IG Predator Resource Requirement: R_P^∗ = m_P / (e_PR a_P)

The Four Necessary Conditions for Three-Species Coexistence

Because the IG prey (N) suffers a double demographic penalty from the IG predator (P)—both exploitative resource depletion of R and direct lethal predation (αP)—Holt and Polis (1997) proved that stable three-species coexistence requires four conditions:

1. Condition 1 — The IG Prey Must Be the Superior Exploitative Competitor (R_N^∗ < R_P^∗): If the IG predator were both the superior resource competitor and the predator of N, then P would deterministically exclude N across all productivity levels. Thus, the IG prey must subsist on a lower equilibrium density of the shared resource R than the IG predator.
2. Condition 2 — The IG Predator Must Gain Net Energy from Consuming the IG Prey: The trophic benefit (e_PN α N) obtained by eating the IG prey must allow P to invade a system where N has depressed the basal resource down to R_N^∗.
3. Condition 3 — Intermediate Ecosystem Productivity (K): Community structure shifts predictably along a gradient of resource carrying capacity (K):
   • Very Low Productivity (K < R_N^∗): Neither consumer persists; only the basal resource R survives.
   • Low Productivity (R_N^∗ < K < K_inv,P): Only the superior competitor (the IG prey N) persists; resource productivity is insufficient to support the IG predator.
   • Intermediate Productivity (K_inv,P < K < K_excl,N): Three-species coexistence (R, N, and P persist together), provided mutual invasion criteria hold.
   • High Productivity / Nutrient Enrichment (K > K_excl,N): Paradoxically, enriching the basal resource subsidizes the IG predator (P) via direct consumption of abundant R, inflating P to a density where intraguild predation drives the superior competitor (IG prey N) to local extinction.
4. Condition 4 — Absence of Bistability: If the IG predator can persist on R alone and suppresses N below its invasion threshold while abundant N suppresses R below P's invasion threshold, the community exhibits priority effects determined by arrival order.

Disruption of Biological Control vs. Habitat Complexity Stabilization

In biological control of invasive species and agricultural pests, Jay A. Rosenheim and colleagues (1995) demonstrated that intraguild predation explains why releasing multiple natural enemy species can yield weaker pest suppression than releasing a single specialist parasitoid alone: when an IG predator (P) heavily consumes an efficient IG prey (N) while having a weak direct attack rate on the herbivore pest (R), adding P converts a two-level system (N → R) into a three-level trophic cascade (P → N → R) that releases the pest. Conversely, spatial habitat complexity and antipredator trait-mediated indirect interactions reduce the intraguild attack rate (α), stabilizing predator guild coexistence across the metacommunity.

1. Polis, G. A., Myers, C. A., & Holt, R. D. (1989). "The ecology and evolution of intraguild predation: potential competitors that eat each other." Annual Review of Ecology and Systematics, 20, 297–330. https://doi.org/10.1146/annurev.es.20.110189.001501
2. Holt, R. D., & Polis, G. A. (1997). "A theoretical framework for intraguild predation." The American Naturalist, 149(4), 745–764. https://doi.org/10.1086/286018
3. Rosenheim, J. A., Kaya, H. K., Ehler, L. E., Marois, J. J., & Jaffee, B. A. (1995). "Intraguild predation among biological-control agents: theory and evidence." Biological Control, 5(3), 303–335. https://doi.org/10.1006/bcon.1995.1038
4. Mylius, S. D., Klumpers, S., de Roos, A. M., & Persson, L. (2001). "Impact of intraguild predation and stage structure on simple communities along a productivity gradient." The American Naturalist, 158(3), 259–276. https://doi.org/10.1086/321321