Examining the Individual- and Community-Level Effects of Natural and Anthropogenic Stressors on Larval Amphibians
Both natural and anthropogenic stressors are encountered regularly in nature. Predators and pathogens are fundamental components of ecological communities, while chemical contamination commonly enters aquatic environments via drift or runoff. The presence of each of these stressors may have individual- or community-level impacts. Individuals may be forced to make physiological tradeoffs upon exposure to stress that can be addressed through metabolic rate. Examining metabolic rate provides insight into energy allocation, a process that may be altered by the addition of a stressor. I examined how two stressors, the pathogen ranavirus and the pesticide carbaryl, affected the metabolic rates of larval amphibians. Moreover, I used both acute and chronic carbaryl exposures to explore temporal dynamics. I found that an acute exposure to carbaryl reduced routine metabolic rate by up to 36% relative to controls, but I did not observe any metabolic effects of chronic exposure. In the ranavirus experiments, infection reduced wood frog routine metabolic rates by 31%. Lower routine metabolic rates suggest there are energetic tradeoffs associated with exposure to a stressor, as individuals may alter behavior to compensate for the energy lost to detoxification or tissue repair. At the community level, predators and pathogens have the potential to influence each other via their interactions with victims and initiate density- and trait-mediated effects. I used ranavirus and predatory dragonflies to examine these effects using a semi-natural mesocosm experiment. I found that lethal (free-ranging) predators reduced ranavirus infection prevalence by 57 to 83% compared to treatments with no predators or caged predators. These results suggest that dragonflies could play an important role in reducing disease risk within amphibian communities (i.e. healthy herd hypothesis). The reduction in infection prevalence appeared to be driven by tadpole density rather than tadpole behavior. Survival and infection prevalence were relatively high in both the caged- and no-predator treatments, but low in the lethal-predator treatment. Tadpole activity was lower in both caged- and lethal- predator treatments relative to the no-predator treatment, indicating that behavior alone did not have a strong influence on transmission. Predators also initiated a trophic cascade as a result of reduced tadpole density, though ranavirus did not. Periphyton biomass was 117 and 68% higher in the lethal-predator treatment than in the no-predator and caged-predator treatments, respectively. The results of the mesocosm experiment provide support for the healthy herd hypothesis in amphibian communities and underscore the importance of examining the interactions between stressors. Collectively, these experiments demonstrate that stressors have wide-ranging consequences for individuals and communities, and are capable of influencing one another. Given that larval amphibians are regularly exposed to combinations of stressors, and that amphibians are in decline worldwide, it will be critical to examine their impact moving forward.
Hoverman, Purdue University.
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