Roads induce adapted populations of a salamander, but maladapted populations of a frog

Background/Question/Methods

The network of roads on the landscape is vast, and contributes a suite of negative ecological effects such as roadkill, habitat fragmentation, and pollution from runoff. For populations that occupy roaded landscapes, these effects also serve as potential agents of natural selection, and set the stage for contemporary evolutionary responses. Yet, while many studies have examined immediate consequences of roads, few have embraced the eco-evolutionary approach necessary to address long-term consequences that may be mediated by evolution. I investigated the influence of road adjacency and runoff on two species of amphibians: the spotted salamander and the wood frog. In particular, I asked whether road adjacency and runoff influence amphibian performance, and if so, whether this influence depends on population origin with respect to road adjacency. To answer these questions, I conducted field-based reciprocal transplant experiments coupled with roadsalt exposure experiments across ten populations—five roadside, five woodland—of the spotted salamander and wood frog. I measured a suite of performance traits including growth, development, and survival, throughout the aquatic life history stages of these two species.

Results/Conclusions

My data show that roads—even when situated in heavily forested landscapes—induce strong, negative effects on amphibians breeding in roadside wetlands. For both the spotted salamander and wood frog, rates of survival, growth, and development diminished substantially, both in roadside wetlands and in environmentally relevant concentrations of roadsalt. Yet the magnitude of these effects depended on the origin of the populations, while the direction of the effects varied with species. For the spotted salamander, roadside populations exhibited adaptive responses that could not be explained by egg size, suggesting a role for evolution. Yet for the wood frog, roadside populations fared poorly regardless of the environment in which they were reared. This devitalization suggests that for the wood frog, the negative influence of roads is inherited, possibly through maternal effects. Thus, for this species, the populations most susceptible to the negative influence of roads are those least capable of persisting there. When viewed together, these results highlight the complex nature of species’ responses to human modified habitats. That two species of amphibians breeding in identical habitats exhibit such divergent responses suggests that generalizations may not be possible. However, it is clear that eco-evolutionary approaches unveil otherwise cryptic patterns, and thereby provide critical insight toward understanding responses to environmental change.