In general, when we compare defense costs to defense benefits, we learn whether a defense is beneficial. In other words, cost-benefit analyses allow us to address questions such as ''When should a permanent defense evolve?'' or ''When should an inducible defense be expressed?''
Compared to their benefits, the costs of defenses are relatively independent of prey density, so in a cost-benefit analysis, it might be reasonable to make the simplifying assumption that defense costs are density independent. Given the density-dependent benefits of step 1-5 defenses, we can deduce that the benefits of step 1 defenses will exceed their costs at intermediate prey densities: they are not beneficial at low or high densities. By contrast, the benefits of step 2-5 defenses exceed their costs at low but not at high prey densities. It has been shown that these considerations are supported by empirical evidence.
See also: Adaptation; Antipredation Behavior; Chemical Communication; Evolution of 'Prey-Predator' Systems; Hunting; Optimal Foraging; Optimal Foraging Theory; Parasites; Predation; Prey-Predator Models.
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