Geographic variation in incubation temperatures promoting viable offspring production in broadly co-distributed turtles

Organisms whose early life stages are environmentally sensitive produce offspring within a relatively narrow range of suitable abiotic conditions. In reptiles, development rate and survival are often maximized if incubation temperatures remain under 31°C, though this upper bound may vary within and among species. We addressed this expectation by comparing responses to egg incubation at 30°C versus 33°C in congeneric turtle species pairs with broad syntopic geographic distributions. In the two softshell turtles (Apalone spp.), the greatest changes in development rate and phenotypic variance were observed in the northernmost population, which had a low survival rate (40%) at 33°C. The presumably suboptimal temperature (33°C) for northern populations otherwise yielded 76%-93% survival rates and fast swimming speeds in more southern populations. Still, in one species, northern hatchlings incubated at 33°C matched the elevated speeds of their southern counterparts, revealing a countergradient response. In northern populations of the two map turtles (Graptemys spp.), survival was also reduced (28%-60%) at 33°C and the development rate (relative to 30°C) increased by up to 75%. Our experiments on divergent taxa with similar nesting ecologies substantiate that the optimal thermal range for offspring production is variable. These findings encourage further work on how population- and species-level differences relate to local adaptation in widely distributed oviparous species.

Impacts of invasive cane toads on an Endangered marsupial predator and its prey

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.