Quantitative evidence for the effects of multiple drivers on continental-scale amphibian declines

Abstract

Since amphibian declines were first proposed as a global phenomenon over a quarter century ago, the conservation community has made little progress in halting or reversing these trends. The early search for a “smoking gun” was replaced with the expectation that declines are caused by multiple drivers. While field observations and experiments have identified factors leading to increased local extinction risk, evidence for effects of these drivers is lacking at large spatial scales. Here, we use observations of 389 time-series of 83 species and complexes from 61 study areas across North America to test the effects of 4 of the major hypothesized drivers of declines. While we find that local amphibian populations are being lost from metapopulations at an average rate of 3.79% per year, these declines are not related to any particular threat at the continental scale; likewise the effect of each stressor is variable at regional scales. This result - that exposure to threats varies spatially, and populations vary in their response - provides little generality in the development of conservation strategies. Greater emphasis on local solutions to this globally shared phenomenon is needed.

A new parameterization for integrated population models to document amphibian reintroductions

“…Given the concomitant large-scale loss and degradation of wetland habitats (Davis and Froend 1999, Gibbs 2001, Zedler and Kercher 2005, it is not surprising that habitat modification is the most implicated source of amphibian declines (reviewed in Alford and Richards 1999). However, a suite of other interrelated factors have also been linked to amphibian declines and vary depending on context, species, and region (Grant et al 2016). …”

Overview of chytrid emergence and impacts on amphibians

“…They concluded that declines may be more widespread and severe here than previously recognized, but did not assign any specific cause to any of these declines. Grant et al [89] compared the spatial patterns and intensities of four threats-including chytridiomycosis-to declines in species occupancy for those US Geological Survey data and concluded that no single threat was consistent in explaining observed trends and that amphibian response to the threats varied spatially.…”

Predictors of breeding site occupancy by amphibians in montane landscapes

“…Examples of regional ecological variation are not uncommon for amphibians. Recently, it was reported that local amphibian population declines across North America were not attributable to any single continental or regional‐scale stressor and that the effect of each stressor varied across regions (Grant et al ). Moreover, numerous studies have identified direct and indirect negative effects of fish on wood frogs (Egan and Paton , Eaton et al , Herwig et al , Carlson and Langkilde ), whereas others have concluded that fish do not significantly affect wood frog population metrics (Schank et al , Davenport et al ).…”

“…Certainly numerous ecological successes have resulted from extrapolating inferences across regions and ecological gradients; however, region‐specific research may identify unique threats and ecological relationships important to the focal species’ persistence within the respective region (Grant et al ). Consequently, the efficacy of conservation and management plans, which will undoubtedly continue to be informed by out‐of‐region research, may hinge on their capacity to effectively integrate novel information.…”