Fitter frogs from polluted ponds: The complex impacts of human‐altered environments

Abstract

Human‐modified habitats rarely yield outcomes that are aligned with conservation ideals. Landscapes that are subdivided by roads are no exception, precipitating negative impacts on populations due to fragmentation, pollution, and road kill. Although many populations in human‐modified habitats show evidence for local adaptation, rarely does environmental change yield outright benefits for populations of conservation interest. Contrary to expectations, we report surprising benefits experienced by amphibian populations breeding and dwelling in proximity to roads. We show that roadside populations of the wood frog, Rana sylvatica, exhibit better locomotor performance and higher measures of traits related to fitness compared with frogs from less disturbed environments located further away from roads. These results contrast previous evidence for maladaptation in roadside populations of wood frogs studied elsewhere. Our results indicate that altered habitats might not be unequivocally detrimental and at times might contribute to metapopulation success. While the frequency of such beneficial outcomes remains unknown, their occurrence underscores the complexity of inferring consequences of environmental change.

Evolutionary trade‐offs may interact with physiological constraints to maintain color variation

“…Interestingly, within individual differences in all of the color metrics between the first and second year occurred in the same direction as the differences in color metrics between males and females. Wood frog females generally take one year longer to reach sexual maturity than males (Berven 1982, Sagor et al 1998, and in a previous study, we found sex differences in age structure of reproductive individuals in our breeding ponds (Brady et al 2019). This suggests that the commonly reported sexual dichromatism in adult wood frogs could be explained, at least in parts, by demographic factors (i.e.…”

Replicated Landscape Genomics Identifies Evidence of Local Adaptation to Urbanization in Wood Frogs

“…Previous research on the effects of urbanization on wood frogs and other vernal pool amphibians provides some potential insights into the specific urban-related selection factors that may be causing the observed patterns. Roadside populations of wood frogs have been observed to have larger body sizes, higher fecundity, and better jumping performance than their nearby woodland counterparts (Brady et al In press). These indicators of greater fitness in roadside populations is juxtaposed by other observations of reduced growth rates for larval wood frogs from these environments (Hall et al 2017), suggesting road-associated effects may be variable across wood frog populations or life history stages.…”

“…Life history and behavioral characteristics such as short generation times (2–3 years; Berven 1990; Sagor et al 1998), small home ranges (Berven and Grudzien 1990; Groff et al 2017), and high rates of philopatry (Berven and Grudzien 1990) make wood frogs excellent candidate species for detecting evolutionary change over short time scales. Moreover, previous research suggests urban-associated landscape features such as roadways (Hall et al 2017; Brady et al In press) and open tree canopies (Skelly et al 2014) can influence wood frog fitness-related characteristics such as growth and fecundity rates, potentially as a result of, or a precursor to selection. In light of previous research on the evolutionary consequences of urbanization in other species, we tested 2 hypotheses: 1) genome-wide genetic diversity would be less in urban relative to rural populations and 2) signals of selection associated with urbanization would be detectable.…”

Salted roads lead to edema and reduced locomotor function in wood frogs

“…Individuals used in this analysis were collected during the inbound breeding migration (prior to arriving at ponds) using drift fences with pitfall traps. Edema was estimated from photos as described above and age was estimated using skeletochronology of toe clips (methods described in Brady et al 2019). We used a linear model with either prevalence or severity as the response variable, and the interaction between age and population type for fixed effects.…”

“…Frogs were placed on one end of a 0.91 X 5 m piece of brown paper and at least 5 consecutive jumps were recorded, or frogs were placed between two plastic walls (height: 34 cm; length: 3.4 m) separated by approximately 30 cm, such that frogs could only jump forward. Frogs were motivated to jump as needed by tapping their urostyle with a finger (Brady et al 2019). Marks were placed where frogs landed and the maximum distance between marks was recorded.…”