We introduced the effect of evolutionary change in the value of either predator or prey traits on prey-predator dynamics. It is definitely possible that trait values for both prey and predators will change. With a combination of prey and predator evolution as explained above, the prey-predator dynamics may cycle, whereas all four variables (population densities and mean trait values for prey and predator) may not change temporally. In some cases, either trait values or predator density do not change. As mentioned above, predator evolution often stabilizes the prey-predator system.
Coevolutionary cycles in prey and predator trait values may occur in models with fixed population sizes. Another theoretical model suggests that either one or two alternative stable equilibria are possible. In a theoretical study that included speciation in quantitative genetic traits, prey speciation likely occurred because of a reduction in the risk of predation. This occurs because disruptive selection by the predators on the prey results in prey speciation. Another theoretical study suggested that pairwise speciation of prey and predators may evolve because of prey-predator coevolution, which concurs with several empirical examples of pairwise coevolution.
Prey-predator coevolution sometimes results in the escalation of both prey and predator phenotypes. As predator capture ability increases, prey-antipredator traits all improve, producing an 'arms race'. If these traits are costly, the arms race often decreases the fitness of both prey and predators. Unlike the predictions of some theoretical works, arms races rarely result in infinitely increasing traits. Regardless, these escalations often increase the risk of extinction for both the prey and predator populations.
It is widely considered that 'adaptive' evolution of a particular trait may result in a reduction in the population size of that species. Some theoretical studies have suggested that increased capture abilities by predators can lead to a decreased predator population because of a reduction in prey density. Consider a three-trophic-level system that includes a top predator, a consumer, and a prey species. If the abundance of the top predator increases, the optimal foraging time of the consumer decreases because the benefit of food intake reaches a point of saturation and the risk of being killed by the top predator increases proportionally with foraging time. Decreased foraging time results in increased prey abundance, which consequently increases the consumer population. The top predator eventually prevents the overexploitation of the consumer.
Some evidence exists that the responses by prey to predators are often larger than the responses by predators to prey. This is referred to as the 'life-dinner principle', although some biologists have criticized this speculative expression. The principle is simply a consideration that the prey loses its life while the predator simply obtains one meal during a single act of predation.
In the case of prey-predator coevolution, prey vulnerability does not always monotonically increase with antipredator traits or foraging effort. The capture rate of prey by a predator may increase by matching the prey's phenotype. As a simple example, prey may have a bidirectional axis of vulnerability to a predator with a particular foraging behavior. The risk of predation is reduced for prey whose phenotype values are either larger or smaller than those targeted by the predator's phenotype. The 'worst' prey phenotype depends on the predator's phenotype. In the bidirectional axis of vulnerability, prey-predator coevolution may show cyclical changes in values. Prey-antipredator efforts do not always impose a cost.
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