Climate change-induced shifts in survival and size of the worlds' northernmost oviparous snake: A 68-year study.

Because of their dependence on ambient temperature ectothermic animals can serve as sentinels of conservation problems related to global warming. Reptiles in temperate areas are especially well suited to study such effects, as their annual and daily activity patterns directly depend on ambient temperature. This study is based on annual data spanning 68 years from a fringe population of Grass Snakes (Natrix natrix), which is the world's northernmost oviparous (egg-laying) reptile, and known to be constrained by temperature for reproduction, morphology, and behavior. Mark-recapture analyses showed that survival probability was generally higher in males than in females, and that it increased with body length. Body condition (scaled mass index) and body length increased over time, indicative of a longer annual activity period. Monthly survival was generally higher during winter (i.e., hibernation) than over the summer season. Summer survival increased over time, whilst winter survival decreased, especially during recent decades. Winter survival was lower when annual maximum snow depth was less than 15 cm, implying a negative effect of milder winters with less insulating snow cover. Our study demonstrates long-term shifts in body length, body condition and seasonal survival associated with a warming climate. Although the seasonal changes in survival ran in opposite directions and though changes were small in absolute terms, the trends did not cancel out, but total annual survival decreased. We conclude that effects of a warming climate can be diverse and pose a threat for thermophilic species in temperate regions, and that future studies should consider survival change by season, preferably in a long-term approach.

Small-scale population divergence is driven by local larval environment in a temperate amphibian.

Genomic variation within and among populations is shaped by the interplay between natural selection and the effects of genetic drift and gene flow. Adaptive divergence can be found in small-scale natural systems even when population sizes are small, and the potential for gene flow is high, suggesting that local environments exert selection pressures strong enough to counteract the opposing effects of drift and gene flow. Here, we investigated genomic differentiation in nine moor frog (Rana arvalis) populations in a small-scale network of local wetlands using 16,707 ddRAD-seq SNPs, relating levels of differentiation with local environments, as well as with properties of the surrounding landscape. We characterized population structure and differentiation, and partitioned the effects of geographic distance, local larval environment, and landscape features on total genomic variation. We also conducted gene-environment association studies using univariate and multivariate approaches. We found small-scale population structure corresponding to 6-8 clusters. Local larval environment was the most influential component explaining 2.3% of the total genetic variation followed by landscape features (1.8%) and geographic distance (0.8%), indicative of isolation-by-environment, -by-landscape, and -by-distance, respectively. We identified 1000 potential candidate SNPs putatively under divergent selection mediated by the local larval environment. The candidate SNPs were involved in, among other biological functions, immune system function and development. Our results suggest that small-scale environmental differences can exert selection pressures strong enough to counteract homogenizing effects of gene flow and drift in this small-scale system, leading to observable population differentiation.

Occurrence of Batrachochytrium dendrobatidis in Sweden: higher infection prevalence in southern species.

The chytrid fungus Batrachochytrium dendrobatidis (Bd) has caused worldwide declines in amphibian populations. While Bd is widespread in southern and central Europe, its occurrence and distribution in northernmost Europe is mostly unknown. We surveyed for Bd in breeding anurans in Sweden by sampling 1917 amphibians from 101 localities and 3 regions in Sweden (southern, northern and central). We found that Bd was widespread in southern and central Sweden, occurring in all 9 investigated species and in 45.5% of the 101 localities with an overall prevalence of 13.8%. No infected individuals were found in the 4 northern sites sampled. The records from central Sweden represent the northernmost records of Bd in Europe. While the proportion of sites positive for Bd was similar between the southern and central regions, prevalence was much higher in the southern region. This was because southern species with a distribution mainly restricted to southernmost Sweden had a higher prevalence than widespread generalist species. The nationally red-listed green toad Bufotes variabilis and the fire-bellied toad Bombina bombina had the highest prevalence (61.4 and 48.9%, respectively). Across species, Bd prevalence was strongly positively, correlated with water temperature at the start of egg laying. However, no individuals showing visual signs of chytridiomycosis were found in the field. These results indicate that Bd is widespread and common in southern and central Sweden with southern species, breeding in higher temperatures and with longer breeding periods, having higher prevalence. However, the impact of Bd on amphibian populations in northernmost Europe remains unknown.

Urban environment and reservoir host species are associated with Batrachochytrium dendrobatidis infection prevalence in the common toad.

Human-induced changes of the environment, including landscape alteration and habitat loss, may affect wildlife disease dynamics and have important ramifications for wildlife conservation. Amphibians are among the vertebrate taxa most threatened by anthropogenic habitat change. The emerging fungal pathogen Batrachochytrium dendrobatidis (Bd) has caused extinctions and population declines in hundreds of anuran species globally. We studied how the urban landscape is associated with the prevalence of Bd infections by sampling 655 anurans of 3 species (mainly the common toad Bufo bufo) in 42 ponds surrounded by different amounts of urban habitat (defined as towns, cities or villages). We also examined the association between Bd infections and a potential reservoir host species (the moor frog Rana arvalis). We found that 38% of the sites were positive for Bd with an infection prevalence of 4.4%. The extent of urban landscape was negatively correlated with Bd infection prevalence. However, the positive association of Bd with the presence of the possible reservoir species was substantially stronger than the urban effects. The body condition index of B. bufo was negatively associated with Bd infection. This Bd effect was stronger than the negative effect of urban landscape on body condition. Our results suggest that urban environments in Sweden have a negative impact on Bd infections, while the presence of the reservoir species has a positive impact on Bd prevalence. Our study also highlights the potential importance of Bd infection on host fitness, especially in rural landscapes.

Effects of host species and environmental factors on the prevalence of Batrachochytrium dendrobatidis in northern Europe.

The fungal pathogen Batrachochytrium dendrobatidis (Bd) poses a major threat to amphibian populations. To assist efforts to address such threats, we examined differences in Bd host infection prevalence among amphibian species and its relations to both local environmental factors in breeding habitats and landscape variables measured at three scales (500, 2000 and 5000 m radii) around breeding sites in southernmost Sweden. We sampled 947 anurans of six species in 31 ponds and assessed their infection status. We then examined correlations of infection prevalence with canopy cover, pond perimeter and pH (treated as local-scale pond characteristics), and the number of ponds, area of arable land, area of mature forest, number of resident people and presence of sea within the three radii (treated as landscape variables). The Bd infection prevalence was very low, 0.5-1.0%, in two of the six anuran species (Bufo bufo and Rana temporaria), and substantially higher (13-64%) in the other four (Bombina bombina, Bufotes variabilis, Epidalea calamita, Rana arvalis). In the latter four species Bd infection prevalence was positively associated with ponds' pH (site range: 5.3-8.1), and negatively associated with areas of mature forest and/or wetlands in the surroundings. Our results show that the infection dynamics of Bd are complex and associated with host species, local pond characteristics and several landscape variables at larger spatial scales. Knowledge of environmental factors associated with Bd infections and differences in species' susceptibility may help to counter further spread of the disease and guide conservation action plans, especially for the most threatened species.