The Combined Effects of Road Salt and Biotic Stressors on Amphibian Sex Ratios

The effects of road salt (NaCl), predation, and competition on the growth and community interactions of spotted salamanders (Ambystoma maculatum) and wood frogs (Lithobates sylvaticus)

Road deicing salts are frequently used in northern regions of the world during the winter and early spring months. As a result, a significant portion of road runoff into surrounding aquatic habitats contains road deicing salts. Previous studies found road salt contaminations in vernal pools that pond-breeding amphibians commonly use, including spotted salamanders (Ambystoma maculatum) and wood frogs (Lithobates sylvaticus). Studies have examined the impact of road salt on both amphibian species, but to our knowledge no previous studies have examined how road salt impacts the interspecific competition between both amphibians. We hypothesized that road salt would negatively impact growth and survivorship of both amphibian species. During the spring and summer of 2017, we conducted an outdoor mesocosm experiment in which we created eight experimental conditions with three main factors: presence/absence of NaCl (1000 mg/L Cl^-), presence/absence of interspecific competition between the two amphibian species (A. maculatum and L. sylvaticus), and presence/absence of predatory dragonfly larvae (Family Libellulidae). Our experiment revealed that salt delayed hatching and increased deformity in spotted salamander hatchlings. Additionally, salt reduced salamander survivorship by 62% and frog survivorship by 30%. Wood frog tadpoles and road salt interacted to diminish salamander survivorship a further 80% beyond salt alone, likely through an increase in interspecific competition. Road salt increased the larval period of salamanders and decreased the proportion metamorphosed by the end of the experiment. Dragonfly larvae reduced salamander survivorship by 35%, whereas they increased wood frog tadpole development rates. Dragonfly larvae and salt interacted to alter tadpole denticle size, with salt negating the impact of dragonfly larvae. Thus, we found that salt interfered with aquatic predatory chemical cues. Overall, the results of this study suggest that management strategies should be implemented in order to reduce the impact of road salts on freshwater aquatic ecosystems.

Distance to coastline modulates morphology and population structure in a coastal amphibian

Salinization due to sea-level rise and marine submersions is expected to strongly impact coastal ecosystems. Exposure to salinity can negatively impact biodiversity especially in coastal wetlands. To understand comprehensively the consequences of environmental salinization on coastal biodiversity, it is essential to document how coastal species currently respond to exposure to salinity. In this study, we investigated how variations of environmental salinity relative to the distance to the ocean influence population structure (age ratio and sex ratio), and individual hydro-mineral balance (osmolality) and morphology (size, mass, condition) in the western spadefoot toad (Pelobates cultripes) in two populations from the French Atlantic coast. We show that distance to coastline strongly influences exposure to salt on a small spatial scale. Some variables (e.g., abundances, osmolality, morphology) responded similarly in both sites and may be related to salt deposition due to landward sea-spray. Interestingly, other variables (sex ratio and age ratio) displayed site-specific responses and appeared to be linked to the salinity of breeding sites. Distance to the shoreline appears to be a critical population structuring factor in this coastal salt-tolerant species. Future studies should investigate how distance to shoreline—and thus environmental salinity—can ultimately affect individual performances and fitness.