Ecology in Action

Exploitative interactions can be understood in terms of their lethality and intimacy. Predators and parasitoids cause highest lethality, parasites and parasitoids have highest intimacy with their hosts, while grazers are low on both scales. Exploiters can regulate the populations of their hosts directly by killing or injuring them, or through nonconsumptive processes such as increasing their prey’s stress level and thereby reducing reproductive rates, as has been implicated for the snowshoe hare. Exploiters can also regulate community processes indirectly; for example bats and birds eat arthropods in the forest, which reduces leaf damage by herbivorous arthropods. Prey and hosts use constitutive defenses, such as thorns in plants, and large body size in Serengeti grazers, against exploiters. Some species have evolved induced defenses; for example some plants release toxic chemicals following herbivore attack. The outcomes of exploitative interactions can be predicted by the Lotka–Volterra predation model, which, in its most basic form, predicts that the relative abundance of predators and prey will cycle. A simple model of disease transmission can explain how disease spreads in host populations based on the ease of transmission, the amount of time the host is infectious, and the population size of the host. Both models make numerous simplifying assumptions. Ecologists can incorporate biological complexity into these models, which makes them more realistic, but also more difficult to understand and apply.