Curious case of the coastal tailed frog (ascaphus truei) near the northern extent of its range

Ascaphus truei, an ancient species of frog, migrated into British Columbia, Canada, following the last ice age. A. truei is of conservation interest because forestry practices, and the associated infrastructure, may reduce habitat quality. There is also concern that a warmer and drier climate will change the distribution and abundance of this species. I used two genetic methods, microsatellites and next-generation sequencing, to compare the genetic diversity of A. truei from the northern extent of its range in British Columbia, CA to southern Washington, USA. Both methods suggested a dramatic reduction in diversity across the northern portion of the species’ range. The phylogeography suggests a northern range expansion from a single refugium. I used DNA metabarcoding to compare the gut contents of larvae across three stream reaches and two development classes near the northern extent of A. truei's range. Gut contents differed between stream reach but did not differ among development class. Wetted width and ultimately stream volume may influence differences in gut content among stream reaches. I also quantified the relationship between an index of larvae density and environmental factors hypothesized to influence population density near the northern extent of the range. I assessed the segregation of larvae at various developmental stages. Wetted width and wetted depth correlated with differences in the abundance of larvae. Older developmental stages were captured in stream reaches with greater slopes than younger stages. Management should minimize modifications of stream structure, such as in-stream siltation due to road building. We should maintain habitat connectivity and gene flow to ensure the continued migration and adaptive capacity of A. truei.

Environmental Drivers of Variation in Labial Teeth Number in Ascaphus Tadpoles

ABSTRACTAnuran tadpoles exhibit extraordinary diversity in mouthpart morphology, reflecting phylogenetic relationships, life history specializations, and ecological adaptations. In this study, we investigate patterns and potential environmental drivers of variation in labial teeth counts across and within the coastal tailed frog (Ascaphus truei) and the Rocky Mountain tailed frog (A. montanus), which together constitute the basal frog family Ascaphidae. Tailed frogs occupy cold, fast-flowing, high gradient streams in mesic forests of western North America, with differences in environmental conditions that reflect their coastal and continental distributions. Despite a very strong relationship between the number of labial teeth and developmental (Gosner) stage, we found significant relationships between tooth counts and site-specific environmental predictors after controlling for stage. The most important environmental predictor varied between the species, with percent canopy cover for A. truei and average benthic substrate size for A. montanus. There were also differences in the overall tooth counts between species, with A. truei having more teeth in all rows compared to A. montanus. These findings demonstrate that differences in oral morphology in response to local environmental conditions can override the homogenizing effects of gene flow in both Ascaphus species, likely through a combination of local adaptation and phenotypic plasticity. Follow-up investigations should include the assessment of community level variability across sites, and experimental tests to confirm functional relationships between environmental variation and oral morphology, in addition to partitioning genetic from plastic mechanisms.

Do Dams Also Stop Frogs? Assessing Population Connectivity of Coastal Tailed Frogs (Ascaphus truei) in the North Cascades National Park Service Complex

We investigated the effects of three hydroelectric dams and their associated lakes on the population structure and connectivity of the coastal tailed frog, Ascaphus truei, in the North Cascades National Park Service Complex. Three dams were erected on the Skagit River in northern-central Washington state between 1924 and 1953 and subsequently changed the natural shape and movement of the Skagit River and its tributaries. We collected 183 individuals from 13 tributaries and generated a dataset of >2,500 loci (unlinked SNPs) using double digestion restriction site-associated DNA sequencing (ddRADseq). An analysis of molecular variance (AMOVA) identified ~99% of the genetic variation within groups, and the remaining variation among groups separated by dams, or the Skagit River. All populations exhibited low FST values with a maximum of 0.03474. A ‘de novo’ discriminant analysis of principal components revealed two populations with no geographic cohesiveness. However, testing groups that were partitioned a priori by the dams revealed distinctiveness of populations down-river of the lowest dam. Coalescent-based analyses of recent migration suggest that up to 17.3% of each population is composed of migrants from other populations, and an estimation of effective migration rates revealed high levels of migration heterogeneity and population connectivity in this area. Our results suggest that although the populations down-river from the lowest dam are distinguishable, a high level of A. truei population connectivity exists throughout the North Cascades National Park Service Complex.