Nonlethal effects of parasites can alter the behavior of wildlife hosts. For instance, several studies have shown that infection with parasites such as acanthocephalans or trematodes reduces how much their wild hosts eat. This can have important consequences for the ecosystem as a whole because infected hosts often play an important role in regulating the populations of other organisms.

For example, studies of sticklebacks infected with the tapeworm Schistocephalus solidus have shown that infected fish eat less and grow more slowly than uninfected fish. This can have a significant impact on the stickleback population because infected fish are less likely to survive to adulthood and reproduce. In addition, infected fish are more vulnerable to predation, which can further reduce the stickleback population.

The reduction in food consumption by infected hosts can also have a negative impact on the other organisms that rely on them for food. For example, if infected sticklebacks are an important food source for birds, then the decline in the stickleback population could have a negative impact on the bird population as well.

The nonlethal effects of parasites can therefore have a significant impact on the ecosystem as a whole. By reducing the amount that their hosts eat, parasites can alter the balance between different species and change the flow of energy through the ecosystem.