Simulated climate change increases juvenile growth in a critically endangered tortoise

Abstract

Climate change can affect the availability of transient habitats upon which many species depend for growth and reproduction. In south-western Western Australia, declines in winter rainfall since the 1970s have shortened the hydroperiod of ephemeral swamps occupied by the Critically Endangered western swamp tortoise Pseudemydura umbrina reducing the length of the growing period of hatchlings and juveniles. Here, we tested whether the warmer water temperatures expected under global climate change could compensate for a shorter growing season. We increased pond temperatures of captive hatchlings and juveniles (1 and 2 yr old) by 1 to 2°C, and showed that growth rates and rates of food intake increased with temperature, with hatchlings in heated ponds increasing their mass by an additional 78% compared to hatchlings in unheated ponds. Hatchlings had a growth rate 8 times greater than that of juveniles. With an unlimited food supply, we predict that wild hatchlings will reach the critical mass (about 18 g) necessary to survive their first aestivation period at least 1 mo earlier under projected climate change by 2050. However, because shorter hydroperiods translate to longer periods for dry-season aestivation, small tortoises that have allocated all their energy to growth will be especially vulnerable to depletion of energy stores during aestivation.