Frogs in the spotlight: a 16‐year survey of native frogs and invasive toads on a floodplain in tropical Australia

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.

Resource availability and sexual size dimorphism: differential effects of prey abundance on the growth rates of tropical snakes

“…Of these frogs, 38% were Litoria dahlii , 28% were Litoria bicolor , 23% Litoria nasuta and 5% were Litoria rothii . The remaining 6% consisted of variety of uncommon species (Brown & Shine ). During the same period, we captured 701 adult slatey‐grey snakes a total of 2026 times and 4602 adult keelbacks a total of 5372 times.…”

“…Temperatures at our study site are rarely low enough to limit snake activity, so temperature may not have constrained foraging or assimilation (Shine & Madsen ). Similarly, although rain is highly seasonal, the permanent body of water allows year‐round activity of both frogs and snakes (Brown & Shine ). The abundance of anurans has varied over the course of our study but bears no clear relationship with weather patterns (Brown & Shine ).…”

“…Similarly, although rain is highly seasonal, the permanent body of water allows year‐round activity of both frogs and snakes (Brown & Shine ). The abundance of anurans has varied over the course of our study but bears no clear relationship with weather patterns (Brown & Shine ). The net result of complex patterns in frog assemblages is a temporally varying prey resource for frog‐eating snakes, with species‐specific and sex‐specific impacts on snake growth.…”

The costs of parasite infection: Effects of removing lungworms on performance, growth and survival of free‐ranging cane toads

“…However, our results suggest that the parasites do have strong suppressive effects on their hosts, and hence, densities of toads would plausibly be higher in the absence of lungworms. Toad populations reach maximal levels within 3–4 years of initial establishment and then decline precipitously (Brown & Shine, ) as do many invader populations (Simberloff & Gibbons, ). Although Rhabdias has not been directly implicated in these declines, the timing does coincide with the arrival of the parasites (which lag behind the toad invasion by 1–3 years: Phillips et al., ) and thus they may contribute to the decreased toad abundance to some extent.…”

Impact of cane toads on a community of Australian native frogs, determined by 10 years of automated identification and logging of calling behaviour

“…There have been several other investigations of the impact of cane toads on various components of the northern Australian biota in general (reviewed in Shine 2010) and frogs in particular (Shine 2014;Brown & Shine 2016). Some have investigated the impact of toads on particular species in experimental arenas (Greenlees et al 2007) and others have investigated whether particular species prey on cane toads and are susceptible to their poison (Beckmann & Shine 2009).…”