Demography and Population Dynamics of Amphibians in Desert Mountain Canyons

Abstract

Changes in the distribution and abundance of species reflect variation in underlying demographic rates, including survival, reproduction, growth, and dispersal. Understanding how natural and anthropogenic processes affect demography and dynamics of species with patchy distributions can be challenging, but it is critical for developing reliable conservation strategies for landscapes that are changing at unprecedented rates. We studied amphibians that inhabit isolated, intermittent streams in mountain canyons of the desert southwest, a region where drought conditions have prevailed over the last 30 years and where the availability of surface water is likely to decrease further in response to changes in climate. Specifically, we used detection-nondetection data from 22 years of biannual visual encounter surveys with recent capture-recapture data to better understand survival, growth, and metapopulation dynamics of lowland leopard frogs (Lithobates yavapaiensis) and canyon treefrogs (Hyla arenicolor). We used a spatially-explicit metapopulation model to describe the distributional dynamics of leopard frogs in eight canyons in southern Arizona, and found that local frog populations were more likely to go extinct during periods of drought, particularly at sites that lacked deep or spring-fed pools that held water reliably. Leopard frogs were also less likely to colonize unoccupied sites when larval or dispersal periods were drier than normal or when sediment levels in pools increased following high-elevation wildfires. In one of the two watersheds we surveyed, occupancy by leopard frogs declined markedly in the latter half of the study, and we observed no frogs in this watershed after 2015. We used frequent capture-recapture surveys (≥1 survey every two weeks) based on in-situ photographs to demonstrate that the apparent extirpation was due, at least in part, to drought-mediated decreases in post-metamorphic survival. Seasonal and spatial variation in survival were governed largely by surface-water availability, and survival was particularly low at sites where water levels were lowest. Finally, seasonal variation in demographic rates was not limited to survival, as we found strong evidence of temporal variation in somatic growth of post-metamorphic canyon treefrogs. Growth rates were highest during the summer monsoon season, when both ambient temperatures and water levels in pools were typically high. Given the effects of climate and surface-water availability on demography of leopard frogs and treefrogs, distribution and abundance of these species may decline if future changes in climate reduce the quantity or quality of aquatic resources available in mountain canyons. Moreover, because leopard frog populations tend to be small and geographically isolated, they are unlikely to persist at a regional scale if increased drought frequency or severity limits the ability of individuals to disperse among populations through an increasingly arid landscape.