The amphibian fungal pathogen Batrachochytrium dendrobatidis (Bd) has received considerable attention due to its role in amphibian population declines worldwide. Although many amphibian species appear to be affected by Bd, there is little information on species-specific differences in susceptibility to this pathogen. We used a comparative experimental approach to examine Bd susceptibility in 6 amphibian species from the United States. We exposed postmetamorphic animals to Bd for 30 days and monitored mortality, feeding rates, and infection levels. In all species tested, Bd-exposed animals had higher rates of mortality than unexposed (control) animals. However, we found differences in mortality rates among species even though the amount of Bd detected on the different species' bodies did not differ. Of the species tested, southern toads (Anaxyrus terrestris) and wood frogs (Lithobates sylvaticus) had the highest rates of Bd-related mortality. Within species, we detected lower levels of Bd on individuals that survived longer and found that the relationship between body size and infection levels differed among species. Our results indicate that, even under identical conditions, amphibian species differ in susceptibility to Bd. This study represents a step toward identifying and understanding species variation in disease susceptibility, which can be used to optimize conservation strategies.

The role of plasticity in shaping adaptations is important to understanding the expression of traits within individuals and the evolution of populations. With increasing human impacts on the environment, one challenge is to consider how plasticity shapes responses to anthropogenic stressors such as contaminants. To our knowledge, only one study (using mosquitoes) has considered the possibility of induced insecticide tolerance. Using populations of wood frogs (Lithobates sylvaticus) located close to and far from agricultural fields, we discovered that exposing some populations of embryos and hatchlings to sublethal concentrations of the insecticide carbaryl induced higher tolerance to a subsequent lethal concentration later in life. Interestingly, the inducible populations were located >800 m from agricultural areas and were the most susceptible to the insecticide. In contrast, the noninducible populations were located close to agricultural areas and were the least susceptible. We also found that sublethal concentrations of carbaryl induced higher tadpole AChE concentrations in several cases. This is the first study to demonstrate inducible tolerance in a vertebrate species and the pattern of inducible and constitutive tolerance among populations suggests the process of genetic assimilation.

Understanding population responses to rapid environmental changes caused by anthropogenic activities, such as pesticides, is a research frontier. Genetic assimilation (GA), a process initiated by phenotypic plasticity, is one mechanism potentially influencing evolutionary responses to novel environments. While theoretical and laboratory research suggests that GA has the potential to influence evolutionary trajectories, few studies have assessed its role in the evolution of wild populations experiencing novel environments. Using the insecticide, carbaryl, and 15 wood frog populations distributed across an agricultural gradient, we tested whether GA contributed to the evolution of pesticide tolerance. First, we investigated the evidence for evolved tolerance to carbaryl and discovered that population-level patterns of tolerance were consistent with evolutionary responses to pesticides; wood frog populations living closer to agriculture were more tolerant than populations living far from agriculture. Next, we tested the potential role of GA in the evolution of pesticide tolerance by assessing whether patterns of tolerance were consistent with theoretical predictions. We found that populations close to agriculture displayed constitutive tolerance to carbaryl whereas populations far from agriculture had low naïve tolerance but high magnitudes of induced tolerance. These results suggest GA could play a role in evolutionary responses to novel environments in nature.

Coinfections are increasingly recognized as important drivers of disease dynamics. Consequently, greater emphasis has been placed on integrating principles from community ecology with disease ecology to understand within-host interactions among parasites. Using larval amphibians and two amphibian parasites (ranaviruses and the trematode Echinoparyphium sp.), we examined the influence of coinfection on disease outcomes. Our first objective was to examine how priority effects (the timing and sequence of parasite exposure) influence infection and disease outcomes in the laboratory. We found that interactions between the parasites were asymmetric; prior infection with Echinoparyphium reduced ranaviral loads by 9% but there was no reciprocal effect of prior ranavirus infection on Echinoparyphium load. Additionally, survival rates of hosts (larval gray treefrogs; Hyla versicolor) infected with Echinoparyphium 10 days prior to virus exposure were 25% greater compared to hosts only exposed to virus. Our second objective was to determine whether these patterns were generalizable to multiple amphibian species under more natural conditions. We conducted a semi-natural mesocosm experiment consisting of four larval amphibian hosts [gray treefrogs, American toads (Anaxyrus americanus), leopard frogs (Lithobates pipiens) and spring peepers (Pseudacris crucifer)] to examine how prior Echinoparyphium infection influenced ranavirus transmission within the community, using ranavirus-infected larval wood frogs (Lithobates sylvaticus) as source of ranavirus. Consistent with the laboratory experiment, we found that prior Echinoparyphium infection reduced ranaviral loads by 19 to 28% in three of the four species. Collectively, these results suggest that macroparasite infection can reduce microparasite replication rates across multiple amphibian species, possibly through cross-reactive immunity. Although the immunological mechanisms driving this outcome are in need of further study, trematode infections appear to benefit hosts that are exposed to ranaviruses. Additionally, these results suggest that consideration of priority effects and timing of exposure are vital for understanding parasite interactions within hosts and disease outcomes.

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