While evolution favors defenses with a high level of protection at relatively little costs, some costs cannot be avoided. Many defenses require material or energy either for building or maintaining the defense. Other costs are created by lost opportunities. For instance, a hiding organism cannot usually forage for food or defend a territory. Recent studies emphasize the role of environmental costs, that is, costs originating from changes in environmental conditions. A change in the predation regime to predators with different selectivities may create a mismatch of defenses and thus high costs. For example, Daphnia adaptively change their resource allocation between somatic growth and reproduction in response to cues from invertebrate predators, which consume the smaller size classes, and fish, which consume the larger size classes (Figure 9). Forming the wrong defense would increase vulnerability. Different types of defense differ in their costs and their efficiency. In zooplankton, for example, vertical migration is a predator-avoidance mechanism that is linked to relatively high costs. Morphological defenses can have lower costs but also lower efficiencies. Both traits may evolve (or in the case of inducible defenses: may be expressed) in response to environments that differ in predation risk.
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