According to A/Prof Mitchell, shorter wet seasons are likely to reduce hydroperiods (the number of days of standing water in swamps) leading to longer periods of aestivation (dry season hibernation) the tortoises would need to survive through. Image: Gerald KuchlingTHE world’s rarest turtle is under threat from climate change according to a study by UWA.
Thought to be extinct until its rediscovery in 1950, the Western Swamp Tortoise (WST), Pseudemydura umbrina, number only 70 individuals in the wild, located in four nature reserves in WA.
Declines in rainfall since the 1970s have shortened the periods of water available in swamps used by the WST, critically reducing its growing periods.
Lead author of the study, Assistant Professor Nicola Mitchell at UWA says, “The survival of the species is directly threatened by declining rainfall.”
According to A/Prof Mitchell, shorter wet seasons are likely to reduce hydroperiods (the number of days of standing water in swamps) leading to longer periods of aestivation (dry season hibernation) the tortoises would need to survive through.
Conversely there have been anecdotal reports that some reptiles can increase growth rates with higher temperatures and this may help it survive a longer period of hibernation.
The study tested how growth rates, food intake and foraging behaviour of hatchlings and juveniles varied with water temperature, to assess whether warmer water, expected with climate change, would compensate for the shorter growing season.
Hatchlings in heated ponds increased mass by an additional 78% and had a growth rate eight times that of juveniles. With unlimited food, hatchlings would reach critical mass necessary to survive first aestivation at least one month earlier under climates expected by 2050.
It may sound like good news for the tortoise but, says A/Prof Mitchell “it is hard to say whether there will be more food in a warmer world.
In a drier world, which we are expecting, there will definitely be less food, especially the high fat food sources, such as tadpoles, that help the WST put down fat stores to survive its aestivation.”
And those tortoises that have allocated all their energy to growth will be vulnerable to depletion of energy stores during it.
“The shorter hydroperiods are not great for the WST. The period when swamps are full is also critical for mating.”
The research is part of a larger study under the South-West Assisted Migration for endangered Populations Initiative (SWAMPI). It focuses on selecting the best release sites for captive reared tortoises.
According to A/Prof Mitchell, the current release sites are likely to become drier under current climate projections.
Long generation times, slow rates of reproduction and low genetic diversity also mean that the tortoise is unlikely to adapt rapidly to climate change.
“Human intervention is necessary to ensure its survival in the wild.”
