Abstract
Recent research has revealed that impacts of some invasive species are chronic. Invasive cane toads Rhinella marina have apparently caused rapid and severe population-level declines of the Endangered northern quoll Dasyurus hallucatus across tropical Australia; however, more targeted, quantitative impact data are needed to disentangle this from other threats such as fire regimes, disease, feral cats and dingos. Moreover, repeatable counts before, during, after and long after toad invasion are needed in order to determine if short-term impacts are chronic vs. transitory. We used game cameras to monitor 2 quoll populations and their prey over a 5 yr period spanning the invasion of the toxic cane toads in 2 gorges in northwestern Australia. We predicted severe declines in quolls with the toad invasion, and predatory release of 2 prey species of quolls, a rodent and a smaller marsupial. Quolls declined quickly upon arrival of toads, becoming undetectable in one gorge and barely detectable in the other. Identification of individuals via unique spot patterns confirmed that the declines in detection rates were due to changes in relative abundance rather than decreases in activity. Despite quoll declines we found no evidence of mesopredator release; small mammals generally declined as toads arrived. Our research confirmed rapid population-level declines of quolls, and possibly smaller mammals, associated with arrival of invasive cane toads. Importantly, our surveys provide a baseline for future surveys to determine whether these short-term impacts are chronic or transitory, and whether recovery requires assistance from managers.
Highlights
Animal diversity is globally imperilled due to the 5 main anthropogenic threats of habitat loss, introduction of invasive species, overharvest, pollution and climate change
The study of invasive species and their impacts has exploded in recent decades, we are in urgent need of understanding longer-term effects of species invasions on biological communities
Assuming that declines occurred in quoll populations, our study provides a baseline for monitoring any recovery of quolls in the future, and for determining whether toadinduced declines are chronic or transitory
Summary
Animal diversity is globally imperilled due to the 5 main anthropogenic threats of habitat loss, introduction of invasive species, overharvest, pollution and climate change. The ideal design for determining the ultimate extent of the impacts of invasive species on biological communities requires monitoring the absolute or relative abundance of impacted populations before, during, after and long after the invasion This is not often possible due to the typical lack of pre-invasion data and because monitoring longer-term impacts requires sustained funding and effort. Since their introduction into the northeast of the country in 1935, toads have been steadily spreading westward, a journey that will end within the ~5−10 yr when they will reach the west coast (Doody et al 2018), providing nearly 100 yr of opportunity for scientists to measure their impacts (Shine 2010). Assuming that declines occurred in quoll populations, our study provides a baseline for monitoring any recovery of quolls in the future, and for determining whether toadinduced declines are chronic or transitory
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