Time in patch

Figure 13.9. Patch use strategy of a predator that depresses patch quality by frightening its prey. Prey catchability is highest upon the entry of the predator into the area. The longerthe predator remains in the area, the less catchable the prey become as they become aware ofthe predator's presence. Once prey catchability declines to a threshold, the predator should abandon the area and seek another. The predator's threshold of acceptability should be higher in an environment rich in prey than in one poor in prey. Furthermore, prey catchability declines fasterwith time when the prey begin with a high baseline level of apprehension. A predatorwill spend less time in an area where prey have a high baseline level of vigilance, and will spend more time in patches where prey have a low baseline level of apprehension. (After Brown et al. 1999.)

involves oscillatory dynamics around the lower, unstable interior equilibrium point.

When the deer have imperfect information, the deer-lion foraging game produces very different isoclines. The cost of fear responses can offset the safety in numbers that deer gain from the dilution effect. Increasing the number of deer makes them more catchable as they lower their baseline vigilance level. The prey isocline still follows a humped shape, but deer fear responses shift the peak to the left. Increasing the number of lions makes the deer less catchable because the deer increase their baseline vigilance level. This produces a predator isocline with a positive slope. The resulting stable equilibrium point occurs in a region where both deer and lions have negative intraspecific direct effects (fig. 13.10). When the deer have imperfect information, the deer-lion model (Brown et al. 1999) produces isoclines and population dynamics similar to the gerbil-owl foraging game model (Brown et al. 2001). In all cases to date, predator-prey foraging games produce isoclines and dynamics that would be hard or impossible to predict without some understanding of fear-stealth games.

Figure 13.10. The isoclines of the deer-mountain lion foraging game when the deer have imperfect information on the lions' whereabouts. The hump in the prey isocline occurs at low prey densities. The predator isocline has a positive slope. The positive slope occurs because the presence of more predators renders the prey less catchable. Hence, with more predators, the predators require a higher prey density to subsist. The resulting equilibrium point is stable. (After Brown et al. 1999.)

Prey density

Figure 13.10. The isoclines of the deer-mountain lion foraging game when the deer have imperfect information on the lions' whereabouts. The hump in the prey isocline occurs at low prey densities. The predator isocline has a positive slope. The positive slope occurs because the presence of more predators renders the prey less catchable. Hence, with more predators, the predators require a higher prey density to subsist. The resulting equilibrium point is stable. (After Brown et al. 1999.)

13.8 Summary

Predation creates a special foraging cost because it poses a catastrophic risk: the forager risks losing everything through injury or death. Foraging animals should and do respond strongly to this risk via vigilance behaviors, habitat selection, and group size. These nonlethal effects of predators have implications for the ecology of prey species, of their predators, and of their resources. The ecology of fear examines the ecological and evolutionary implications of foragers responding behaviorally to their predators and vice versa.

In most situations, the richest foraging opportunities also carry the greatest predation risk. Because predation risk represents a cost of foraging, foraging animals should titrate food and safety as they seek and deplete foraging opportunities. Foragers should deplete safe patches more thoroughly than risky patches, thus creating a correlation between predation risk and foraging opportunities. Ecologically, this relationship between predation risk and foraging opportunities means that a forager can improve its energy state if it is willing to risk the possibility of predation, or it can purchase some safety by forgoing the best foraging opportunities.

Feeding animals should distribute their foraging efforts in response to both resource productivity and the landscape of fear. When predation risk varies in space (the landscape of fear), a population's distribution will not match resource productivities, as simple resource-matching models predict. In general, foragers will underutilize risky areas and overutilize safe areas.

From the perspective of population dynamics, prey can cope with predators in two ways. They can increase fecundity to offset losses to predators (N-driven systems), or they can forgo fecundity and increase their survivorship (|l-driven systems). In N-driven systems, the foragers harvest more food, turn over faster, and feed the predators. The foragers pay the predation cost of foraging directly to the predators, and forager mortality feeds and supports the predators. In | -driven systems, the prey forgo foraging opportunities, forgo fecundity, and deny food to the predators. N-driven systems enhance the transfer of energy up the food chain, while |-driven ones stifle this transfer ofenergy.

Trade-offs in fear responses that affect habitat selection or vulnerability to predators create nonlethal effects that increase opportunities for species coexistence. In conjunction with habitat selection, each prey species will thrive in the habitat within which it feels safest. Alternatively, one prey species may be able to exploit resources more competitively under safe conditions while the other does so more competitively in the face of predation risk. Predator-induced shifts in foraging behavior and predator-induced mortality can influence community dynamics in similar ways.

When both prey and predator alter their behavior in response to each other, the predator-prey interaction becomes a foraging game. In a foraging game, the prey can alter their vigilance, habitat selection, activity patterns, activity times, and group size in response to predator numbers and behavior. The predators can adjust their habitat selection, stealthiness, and boldness in response to prey abundance and behavior. These responses can stabilize or destabilize predator-prey dynamics. Regardless of its effects on the dynamics, this game increases the number and intensity of direct and indirect effects relating predators to their prey. For instance, if predators frighten their prey, then the mere presence of an individual predator can reduce the success of other predators. When the prey have excellent information about predator numbers and whereabouts, the predator-prey foraging game tends to destabilize predator-prey dynamics. When the prey have imperfect or poor information, it tends to stabilize population dynamics.

The ways in which predators frighten their prey may be more important than the number of prey they kill (Kotler and Holt 1989; Schmitz, Beckerman, and O'Brien 1997). Foraging theory in the guise of the ecology of fear permits an integration and appreciation of the full impact of predators on ecological systems. In fact, in the absence of foraging theory, it may be impossible to predict and understand most predator-prey systems.

13.9 Suggested Readings

Rosenzweig and MacArthur (1963) underpin current textbook models of predator-prey dynamics, and they anticipate in advance the significance and consequences of predator-prey foraging games. Lima (2002), using part review and part synthesis, makes the case for incorporating fear responses into classic approaches to predator-prey interactions. Holt (1983) and Abrams (2000) provide inspiration and some clear paths to incorporating optimal foraging behaviors into models of population interactions, as well as discussing the consequences ofadaptive behaviors for population dynamics and stability. Sih (1998) reviews and contributes novel ideas for placing predation in the context of game theory. Grand (2002) provides an introduction to and formalisms for how predation risk contributes to the coexistence of habitat selectors. Hugie and Dill's model (1994) deserves careful study as the first explicitly game theoretical model of a predator-prey foraging game. Laundre et al. (2001) for elk and wolves, Schmitz, Beckerman, and O'Brien (1997) for grasshoppers and spiders, and Abramsky et al. (1998, 2002a) for gerbils and owls provide experimental and empirical demonstrations ofthe crucial roles that fear plays in the ecologies ofpredator and prey.

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  • antonietta
    How numbers are likely to change over time from a predator lion and prey deer?
    7 years ago

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