Abstract
Although widespread declines in anuran populations have attracted considerable concern, the stochastic demographics of these animals make it difficult to detect consistent trends against a background of spatial and temporal variation. To identify long‐term trends, we need datasets gathered over long time periods, especially from tropical areas where anuran biodiversity is highest. We conducted road surveys of four anurans in the Australian wet–dry tropics on 4637 nights over a 16‐year period. Our surveys spanned the arrival of invasive cane toads (Rhinella marina), allowing us to assess the invader's impact on native anuran populations. Our counts demonstrate abrupt and asynchronous shifts in abundance and species composition from one year to the next, not clearly linked to rainfall patterns. Typically, periods of decline in numbers of a species were limited to 1–2 years and were followed by 1‐ to 2‐year periods of increase. No taxa showed consistent declines over time, although trajectories for some species showed significant perturbations coincident with the arrival of toads. None of the four focal frog species was less common at the end of the study than at the beginning, and three of the species reached peak abundances after toad arrival. Survey counts of cane toads increased rapidly during the initial stage of invasion but have subsequently declined and fluctuated. Distinguishing consistent declines versus stochastic fluctuations in anuran populations requires extensive time‐series analysis, coupled with an understanding of the shifts expected under local climatic conditions. This is especially pertinent when assessing impacts of specific perturbations such as invasive species.
Highlights
Dramatic declines of diverse taxa have led to concerns that we are in the midst of the Sixth Extinction Event (Eldredge 2001)
Many anuran species are declining rapidly due to habitat loss, climate change, pathogens, invasive species, among other threats and interactions (Kiesecker et al 2001; Pounds et al 2006)
Untangling causal influences of threatening processes on anuran abundance requires long-term datasets, gathered under standardized conditions (Dodd 2010); especially for the tropical habitats that contain a high proportion of anurans worldwide and are the sites of many catastrophic declines (McCallum 2007; Wake and Vredenburg 2008), few such datasets are available
Summary
Dramatic declines of diverse taxa have led to concerns that we are in the midst of the Sixth Extinction Event (Eldredge 2001). There are severe logistical impediments to documenting temporal changes in amphibian populations, because these animals often exhibit large spatial and temporal variation in abundance (Pechmann et al 1991; Wake and Vredenburg 2008). That stochasticity makes it difficult to differentiate unusual declines from “background noise”. Untangling causal influences of threatening processes on anuran abundance requires long-term datasets, gathered under standardized conditions (Dodd 2010); especially for the tropical habitats that contain a high proportion of anurans worldwide and are the sites of many catastrophic declines (McCallum 2007; Wake and Vredenburg 2008), few such datasets are available
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