Salted roads lead to oedema and reduced locomotor function in amphibian populations

Human activities have caused massive losses of natural populations across the globe. Like many groups, amphibians have experienced substantial declines worldwide, driven by environmental changes such as habitat conversion, pollution, and disease emergence. Each of these drivers is often found in close association with the presence of roads. Here we report a novel consequence of roads affecting an amphibian native to much of North America, the wood frog (Rana sylvatica). Across 38 populations distributed from southern to central New England, we found that adult wood frogs living adjacent to roads had higher incidence and severity of oedema (indicated by obvious bloating caused by subcutaneous fluid accumulation) during the breeding season than frogs living away from the influence of roads. This effect was best explained by increased conductivity of breeding ponds, probably caused by runoff pollution from road salt used for de-icing. Oedema severity was negatively correlated with locomotor performance in more northerly populations. Interestingly, northern populations experience more intense winters, which tends to result in more de-icing salt runoff and increased energetic demands associated with overwintering cryoprotection needs. Thus, this emerging consequence of roads appears to impose potential fitness costs associated with locomotion, and these effects might be most impactful on populations living in regions where de-icing is most intense. Together, our findings reveal a novel set of impacts of roads and runoff pollution on wood frog physiology and performance, which seem likely to contribute to population decline. Given the global prevalence of roads and increasing salinisation of freshwater habitats, oedema and related impacts could be widespread consequences faced by amphibian populations across much of the planet's temperate zones.

Automated summary

Human activities have caused massive losses of natural populations across the globe, and amphibians are among the groups that have experienced substantial declines. Roads, which are often associated with habitat conversion, pollution, and disease emergence, have been found to have a novel consequence on wood frogs in North America. Wood frogs living near roads had higher incidence and severity of oedema during the breeding season compared to those living away from roads. This effect was explained by increased conductivity of breeding ponds, likely caused by runoff pollution from road salt used for de-icing. Oedema severity was negatively correlated with locomotor performance in northern populations, which experience more intense winters.