Environmental Influences on the Larval Density and Age-Class Distribution of Ascaphus truei near the Northern Extent of Its Range

Complex environmental factors influence the distribution of species within lotic systems. This is especially apparent for amphibian species that have a multi-year larval stage. Pre-metamorphic Coastal Tailed Frogs (Ascaphus truei) are found in fast-flowing headwater streams for up to four years. Although there is considerable opportunity for the spatial redistribution of larval frogs, there is relatively little known of the habitat ecology of the various growth stages. We compared the abundance of larvae near the northern extent of the species range to a collection of environmental factors hypothesized to influence population density. We also assessed spatial segregation at various developmental stages where relatively short summers and cool climates result in a long residence for larvae. The top ranked model for larvae abundance included covariates representing the wetted width and wetted depth of the stream. Capture rates decreased in a nonlinear fashion as wetted width increased, and rates increased in streams wider than 6 m. Capture rates nonlinearly increased when stream depth exceeded 20 cm. Older larvae were associated with greater slopes than younger larvae. Our results suggest little evidence for a relationship between elevation and cohort distribution. We recommend considering micro-scale influences on the presence and movement patterns of Coastal Tailed Frog larvae within streams.