Healthy herds and peaked packs: How infection alters inducible defenses against predators

Background/Question/Methods

Recent analytic models and empirical tests have shown that predators can decrease infection levels in prey populations by selectively killing infected and susceptible individuals; however, other studies have not found this effect. While such variation has been attributed to predator preference and density, specialist vs. generalist strategies in the predator and parasite, virulence of the pathogen and immunity against it, and host demographics, there has been less focus on how infections change prey traits that alter predation risk. Alterations of inducible defenses that are caused by infections could change vulnerability to predation, potentially leading to healthy herds if these changes make infected and susceptible individuals more vulnerable, peaked packs it these changes make infected individuals less vulnerable, or some intermediate. To understand how trait changes in the prey population might affect patterns of predation, we exposed wood frog (Rana sylvatica) tadpoles to predator and no-predator environments crossed with the presence or absence of a pathogenic fungus (Batrachochytrium dendrobatidis [B. d.]).

Results/Conclusions

Exposure to B. d. altered morphological and behavioral inducible defenses of tadpoles to predators. B. d. induced a tadpole morphology that was similar to that of predator-induced tadpoles, suggesting that infected animals would be better able to escape predators. Changes to behavioral defenses as a result of B. d. were time-specific. Early in the experiment, B. d. exposed animals were more active than unexposed animals. Moreover, tadpoles that contracted an infection were more active, while resistant animals were less active than controls. These trends were not apparent later in the experiment. Because higher activity typically increases the risk of predation in these systems, infected individuals would be most vulnerable to predation and resistant individuals would be least vulnerable. Collectively these data suggest that inducible defenses against predators can be altered by infection in ways that could either increase or decrease their risk of predation, and, because these traits change over the course of an infection, the effect of predation on infection in the prey may be time-dependent as well. In this particular case, an understanding of the relative magnitude and effect of trait changes on vulnerability to predation would be necessary to predict how B. d. exposure influences predation on tadpoles.