Seasonal drought and its effects on frog population dynamics and amphibian disease in intermittent streams

Abstract

Chytridiomycosis, caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd), has contributed to amphibian declines globally, but drivers of outbreaks vary locally. Here, we explore the role of drought in population and host-disease dynamics of the endangered stream-breeding foothill yellow-legged frog (Rana boylii). In central California (USA) where severity of seasonal drought is increasing, we observed the non-native, Bd-tolerant and lentic-adapted North American bullfrog (Lithobates catesbeianus) extend into streams when flood disturbance was minimal. Analysis of skin swabs revealed that prevalence and load of Bd infection among bullfrogs was low.Yet, among the native frogs, prevalence and load intensified as the seasonal drought progressed and surface flow became intermittent. When temperatures decreased in autumn and frogs concentrated at a reduced number of water points, we found dozens of dead foothill yellow-legged frogs (2018)(2019). Necropsies suggested chytridiomycosis as the likely cause of death. Despite recent lethal outbreaks, foothill yellow-legged frog population abundance appeared resilient based on comparison to prior decades when no die-offs were observed. Wet-dry mapping of the stream channel and retrospective analysis of hydrologic records revealed that the native frogs spawn away from perennial pools, a behaviour that may allow them to avoid bullfrogs and predatory fish. In an ecological trade-off, tadpoles face the risk of the stream drying before metamorphosis. Fluctuations in population size thus corresponded to extremes of inter-annual variation in streamflow that limit recruitment rather than disease outbreaks. We conclude that hydrologic constraints, which climate change may exacerbate, appear to override the stressors of non-indigenous species and chytridiomycosis.

Ecological consequences of shifting habitat mosaics within and across years in an intermittent stream

“…While bullfrogs are known to occur in the Coyote Creek catchment, they were not detected in the study area in from 2014 to 2016. However, we detected a small number of bullfrogs in 2017 that have since become more widespread, probably due to a particular sequence of wet and dry years (Kupferberg et al, 2021). Initial bullfrog presence followed the wet winter of 2017, where higher flows prevented tadpole desiccation in the summer and autumn (Kupferberg et al, 2021), while subsequent lower precipitation years prevented winter scouring and facilitated range expansion of this more lentic species (Kupferberg, 1996).…”

“…While bullfrogs are known to occur in the Coyote Creek catchment, they were not detected in the study area in from 2014 to 2016. However, we detected a small number of bullfrogs in 2017 that have since become more widespread, probably due to a particular sequence of wet and dry years (Kupferberg et al, 2021).…”

To treat or not to treat? Experimental pathogen exposure, treatment, and release of a threatened amphibian

“…The initial arrival of Bd to R. draytonii's range may have initiated a selective sweep against alleles that did not confer immune defenses against the pathogen, leaving extant populations in an enzootic state. This transition from epizootic to enzootic conditions has been described for other amphibian species, with population-level outcomes ranging from slow, continued declines to recruitment-mediated stability (Briggs et al, 2010;Kupferberg et al, 2021;Retallick et al, 2004;Scheele et al, 2015). Multiple southern California R. draytonii populations exhibit signals of recent genetic bottlenecks, possibly the result of a chytridiomycosis-induced selective sweep (Richmond et al, 2011).…”

Ecohydrological interactions during drought

“…The papers span a range of water‐dependent environments: aquatic (Kupferberg et al, 2022; Sabatino et al, 2022; Saltarelli et al, 2022; White et al, 2022), floodplain (Nguyen et al, 2022), terrestrial (Belmonte et al, 2022; Castellanos et al, 2022; Gao, Wang, Li, et al, 2022; Gao, Wang, Wang, et al, 2022; He et al, 2022; Lindh et al, 2022; Parolari & Paschalis, 2022; Xu et al, 2022), wetland and pool interface (Cartwright et al, 2022; Mattei et al, 2022), and regulated, perennial, and intermittent (Di Sabatino et al, 2022; Kupferberg et al, 2022; Saltarelli et al, 2022; Sarremejane et al, 2022; Wegscheider et al, 2022) systems across both modelled‐, field‐, and lab‐based studies to address process research, cascading effects, and novel methodological approaches in ecohydrological research.…”

“…In the first research article of this special issue, Kupferberg et al (2022) examined the relationship between drought and host‐disease and host‐disease and population dynamics for the endangered stream‐breeding foothill yellow‐legged frog ( Rana boylii ) in California, USA. Foothill yellow‐legged frogs were observed spawning away from perennial pools in an ecological trade‐off between potential drying of spawning habitat and avoiding predators.…”