Prey Switching

Prey switching is a frequency-dependent foraging behavior in which a generalist predator concentrates its attacks disproportionately on whichever prey species is currently most abundant, while ignoring or under-consuming rarer alternative prey. Formalized quantitatively by ecologist William W. Murdoch in 1969, prey switching transforms linear encounter rates into a sigmoid Type III functional response and generates a low-density demographic refuge for rare species, thereby promoting species coexistence and stabilizing multi-prey food webs against competitive or predatory exclusion.

When a polyphagous predator forages in a habitat containing two alternative prey species with environmental densities N₁ and N₂, the simplest null hypothesis assumes that the ratio of consumed prey (P₁ / P₂) increases in direct proportion to their relative environmental availability (N₁ / N₂):

P₁ / P₂ = c · (N₁ / N₂)

In this baseline proportionality model, the constant c measures the predator's intrinsic, density-independent preference for prey 1 over prey 2 when both species are equally abundant (N₁ = N₂). If c remains constant across all relative prey ratios, no switching occurs; the predator simply harvests each prey in constant proportion to its encounter rate weighted by innate preference.

In his landmark 1969 Ecological Monographs study, William W. Murdoch demonstrated that many generalist predators violate constant proportionality. Instead, the ratio of attacks follows a power-law relationship governed by a switching exponent m:

P₁ / P₂ = c · (N₁ / N₂)^m

When m = 1, predation is frequency-independent (no switching). When m > 1, positive prey switching (also termed apostatic predation) occurs: when prey 1 is rare (N₁ / N₂ < 1), it makes up a far smaller fraction of the predator's diet than predicted by c, whereas when prey 1 becomes abundant (N₁ / N₂ > 1), it dominates the diet far above baseline proportionality. Conversely, m < 1 represents anti-apostatic selection, in which predators disproportionately target the rarer prey morph or species.

Prey switching is not a single rigid reflex but an emergent outcome of several distinct sensory, cognitive, and spatial mechanisms operating within predation systems.

Specific Searching Images and Sensory Attention

In 1960, Dutch ethologist Lukas Tinbergen proposed that insectivorous woodland birds such as great tits (Parus major) acquire a specific searching image after repeated encounters with a cryptic caterpillar species. When a cryptic prey item is scarce, foraging birds overlook it among foliage and bark because neural pattern-recognition templates have not been reinforced. Once prey density crosses a threshold where sequential encounters occur within the predator's short-term memory window, the bird tunes its visual attention to the specific spatial frequencies, coloration, and resting posture of that prey type, causing capture efficiency to surge rapidly.

Motor Learning and Handling Efficiency

Even when prey are conspicuous, capturing, subduing, and digesting different prey taxa often requires distinct mechanical techniques. Predatory marine gastropods (Nucella and Acanthina) must drill through calcified barnacle plates or mussel shells using a radula and accessory boring organ, while aquatic backswimmers (Notonecta glauca) adjust strike angles between surface-dwelling dipteran larvae and benthic isopods. Repeated capture of the dominant prey reduces handling time (T_h) and increases attack success probability (a), raising net energetic profitability in accordance with optimal foraging theory.

Spatial Microhabitat Aggregation

Whenever two prey species occupy partially distinct microhabitats—such as pelagic water columns versus littoral macrophyte beds, or upper canopy foliage versus leaf litter—predators that aggregate in whichever microhabitat yields the highest caloric intake rate automatically generate population-level prey switching. Even if the predator exhibits no cognitive preference change within a patch, reallocating search time toward the denser microhabitat elevates the effective exponent m well above unity.

Prey switching provides the primary behavioral mechanism transforming C. S. Holling's hyperbolic Type II functional response into a sigmoid Type III functional response. In a Type II curve, per-capita predation risk (P₁ / N₁) is highest at the lowest prey densities and declines monotonically as predators saturate at high prey densities, driving inverse density dependence that can push rare prey to local extinction.

Under prey switching (m > 1), the attack rate parameter a(N₁) increases as a function of focal prey density:

F(N₁) = (b · N₁^m) / (1 + c · N₁ + b · T_h · N₁^m)

At low densities of focal prey N₁ (while alternative prey N₂ remains abundant), the derivative of per-capita mortality with respect to N₁ is positive (direct density dependence). Only after N₁ becomes sufficiently abundant to saturate predator handling time does the curve inflect into its upper asymptote (1 / T_h).

In classical Lotka-Volterra equations and MacArthur-style consumer-resource models, two prey species sharing a common generalist predator often experience apparent competition, where high abundance of a productive primary prey subsidizes a large predator population that drives a less productive secondary prey extinct. Prey switching counteracts apparent competition by relaxing predation pressure on the declining species as soon as its relative frequency drops below the switching threshold.

1. Multi-Prey Coexistence: By imposing density-dependent mortality on whichever prey species expands fastest, switching prevents competitive exclusion among prey guilds sharing basal resources.
2. Damping Oscillations: In coupled predator-prey cycles, rapid dietary switching to alternative prey buffers the predator population during focal prey troughs while preventing the focal prey from crashing to zero, damping high-amplitude limit cycles across trophic cascade networks.
3. Apostatic Polymorphism Maintenance: Within a single prey species, frequency-dependent switching by visual predators selects against common color morphs and favors rare morphs (apostatic selection), maintaining genetic shell-color polymorphisms in land snails (Cepaea nemoralis) and mimetic wing patterns in tropical Lepidoptera.

Murdoch (1969) tested switching experimentally using intertidal predatory whelks (Thais emarginata and Acanthina spirata) foraging on mussels (Mytilus) and barnacles (Balanus). When innate preference between the two prey was weak and Variable across individual snails (c ≈ 1), strong population-level switching emerged because individual whelks trained on the locally abundant prey developed strong conditioned preferences. Subsequent laboratory experiments with guppies (Poecilia reticulata) feeding on tubificid worms and fruit flies, protozoan ciliates (Didinium) preying on Paramecium, and arctic foxes (Vulpes lagopus) shifting between cyclic collared lemmings (Dicrostonyx) and ground-nesting waterfowl confirmed that switching is most pronounced when baseline preference is weak and prey occupy distinct microhabitats or require specialized capture techniques.

What is the difference between prey switching and optimal diet breadth expansion?

Optimal diet breadth theory (the contingency model) predicts how a predator foraging in a single homogeneous patch responds to changes in the absolute abundance of its single highest-ranked prey: when the top-ranked prey is abundant, the predator specializes on it, and when the top-ranked prey becomes scarce, the predator broadens its diet to include lower-ranked prey. Prey switching specifically describes how the ratio of two prey in the diet changes as a function of their relative environmental ratio (N₁ / N₂), shifting preference toward whichever species is numerically dominant.

Why do all generalist predators not exhibit prey switching?

Predators with a strong, rigid biomechanical or nutritional preference (c >> 1 or c << 1) continue to pursue their preferred prey even when it becomes rare, provided total encounter rates remain sufficient to meet metabolic demands. In such cases, predators generate a Type II functional response and can drive rare preferred prey toward local extirpation.

1. Murdoch, W. W. (1969). "Switching in General Predators: Experiments on Predator Specificity and Stability of Prey Populations." Ecological Monographs, 39(4), 335–354. https://doi.org/10.2307/1942352
2. Tinbergen, L. (1960). "The Natural Control of Insects in Pine Woods I. Factors Influencing the Intensity of Predation by Songbirds." Archives Néerlandaises de Zoologie, 13(3), 265–343. https://doi.org/10.1163/036551660X00053
3. Murdoch, W. W., & Oaten, A. (1975). "Predation and Population Stability." Advances in Ecological Research, 9, 1–131. https://doi.org/10.1016/S0065-2504(08)60288-360288-3)
4. Allen, J. A. (1988). "Frequency-Dependent Selection by Predators." Philosophical Transactions of the Royal Society of London B: Biological Sciences, 319(1196), 485–503. https://doi.org/10.1098/rstb.1988.0061
5. Abrams, P. A., & Matsuda, H. (1996). "Positive Indirect Effects Between Prey Species That Share Predators." Ecology, 77(2), 610–616. https://doi.org/10.2307/2265634