Stressors

Rapid warming in polar regions is causing large changes to ecosystems, including altering environmentally available mercury (Hg). Though subarctic freshwater systems have simple vertebrate communities, Hg in amphibians remains unexplored. We measured total Hg (THg) in wetland sediments and methylmercury (MeHg) in multiple life stages (eggs to adults) of Wood Frogs (Rana sylvatica) and larval Boreal Chorus Frogs (Pseudacris maculata) from up to 25 wetlands near Churchill, Manitoba (Canada), during summers 2018?2019. We used egg mass counts for Wood Frogs from 24 wetlands (2015–2019) and per-ovum MeHg concentrations to estimate site-level MeHg flux by metamorphs from wetlands to the terrestrial environment. Total Hg in wetland sediment was unrelated to MeHg concentrations of amphibian larvae, but sediment THg increased with from coastal tundra vegetation to inland boreal forests. Methylmercury concentrations of Wood Frog eggs (geometric mean = 35.9; range: 6.7–77.9 ng/g dry weight [dw]) exceeded previous reports for amphibians, including from sites contaminated by industrial sources of Hg. Methylmercury concentrations of adult Wood Frogs (298.9 ng/g dw) was also higher than that for frogs included in a recent assessment of MeHg in amphibians across the contiguous United States. Within wetlands, MeHg concentrations of Wood Frog larvae were strongly correlated with MeHg concentrations in eggs earlier in the summer and concentrations increased with each life stage. We estimate there would have been 1971.8?3286.4 ng MeHg exported from wetlands by Wood Frog metamorphs, which is 3.4?5.6 times more MeHg than inputted by eggs. Collectively, these data provide an initial assessment of Hg concentrations, body burdens, and dynamics in subarctic food webs that are expected to experience large changes from climate warming.

Climate change is a primary threat to biodiversity, but for many species, we still lack information required to assess their relative vulnerability to changes. Climate change vulnerability assessment (CCVA) is a widely used technique to rank relative vulnerability to climate change based on species characteristics, such as their distributions, habitat associations, environmental tolerances, and life-history traits. However, for species that we expect are vulnerable to climate change yet are understudied, like many amphibians, we often lack information required to construct CCVAs using existing methods. We used the CCVA framework to construct trait-based models based on life history theory, using empirical evidence of traits and distributions that reflected sensitivity of amphibians to environmental perturbation. We performed CCVAs for amphibians in 7 states in the north-central USA, focusing on 31 aquatic-breeding species listed as species of greatest conservation need by at last 1 state. Because detailed information on habitat requirements is unavailable for most amphibian species, we used species distributions and information on traits expected to influence vulnerability to a drying climate (e.g., clutch size and habitat breadth). We scored species vulnerability based on changes projected for mid-century (2040?2069) from 2 climate models representing “least-dry” and “most-dry” scenarios for the region. Species characteristics useful for discriminating vulnerability in our models included small range size, small clutch size, inflexible diel activity patterns, and smaller habitat breadth. When projected climate scenarios included a mix of drier and wetter conditions in the future, the exposure of a species to drying conditions was most important to relative rankings. When the scenario was universally drier, species characteristics were more important to relative rankings. Using information typically available even for understudied species and a range of climate projections, our results highlight the potential of using life history traits as indicators of relative climate vulnerability. The commonalities we identified provide a framework that can be used to assess other understudied species threatened by climate change.

Wildfire regimes are changing rapidly with widespread increase in the intensity, frequency, and duration of fire activity, especially in the western United States. Limited studies explore the impacts of wildfires on aquatic taxa and few focus on lentic habitats that are essential for amphibians, many of which are of conservation concern. We capitalized on existing pre-fire surveys for anuran species and resurveyed a random subset of wetlands across a gradient of soil burn severity to investigate the short-term effects of wildfire on a relict population of wood frogs in the southern Rocky Mountains. We also investigated whether maps created to support rapid post-fire emergency response activities (i.e., USFS BAER program) accurately characterize soil burn severity around small habitat features (i.e., ponds) that serve as important amphibian breeding and rearing habitat. We found that wood frog breeding persistence following fires was strongly influenced by the percentage of their terrestrial habitat (100m buffer surrounding breeding ponds) that was burned. Wood frog colonization probability of previously unoccupied ponds was low (~ 0.10) and unaffected by soil burn severity. Importantly, we found that remotely sensed data typically produced to predict flooding and erosion at broad (catchment) scales is a poor representation of the amount and variation in soil burn severity surrounding small habitat features (e.g., ponds), suggesting that additional field sampling is necessary to understand wildfire responses for species that rely on small habitat features. Understanding short-term geographic- and species-specific variation in response to wildfires provides the basis to explore time to recovery (e.g., when wood frogs return to burned breeding sites) or to determine if declines in breeding distributions intensify over time.