UWA PhD candidate Sophie Arnall came to speak about her research on assisted colonisation of the Western Swamp Tortoise, at the Darling Range Branch of the WA Naturalists' Club in April 2013. Below is a summary of her excellent and informative presentation. See 'Links' page for more information on her project.
Assisted colonisation of the western swamp tortoise
What should we do when a critically endangered animal faces the likelihood that its only habitat will soon be made untenable by climate change? Do we move it to a new location, where it hasn’t previously been found? Is such an action practical or ethical? It is to help answer these questions that UWA PhD candidate Sophie Arnall has been engaged on some groundbreaking research, with the Western Swamp Tortoise as her subject.
The most endangered reptile in Australia and one of the most endangered chelonians in the world, the Western Swamp Tortoise has been around since the Miocene, when most of Australia was under rainforest. This tiny freshwater tortoise (only 15 cm long) is sometimes called ‘the smiling tortoise’ because of its cheery upturned mouth. Discovered in 1839, it was only described in 1901 and was then presumed extinct until 1953. This all changed when keen schoolboy naturalist Robert Boyd presented two of them—which his cousin had found crossing a road in Bullsbrook—for inspection at the annual Wildlife Show held in Perth Town Hall. WA Naturalists’ Club (Darling Range Branch) member Eric McCrum remembers it well and tells us that Harry Butler was also present.
Since that fateful day, the tortoise has been studied by various scientists, including Gerald Kuchling and Andrew Burbidge. Andrew’s thesis established that the tortoise’s biorhythms are closely linked to our climate, with the wet winter-spring period spent feeding in shallow ephemeral swamps (the hydro-period) and the hot, dry summer-autumn in a semi-torpid state (aestivation) nestled in cracks in the dried wetland clay or under deep leaf litter. They await the rains before emerging; the eggs laid in late spring only hatch at this time, too. Then it’s time to feed up – the adults for breeding and the hatchlings to reach the crucial weight of 18g, below which they will not survive their first summer. Strangely, captive animals offered year-round water and food will not breed well, so these tortoises really do depend on the right wet-dry cycle to survive and thrive!
Now that the changes in the climate of south-western Australia threaten the life-cycle of this small yet charismatic animal, more innovative research and ideas are needed to prevent it from slipping into extinction. There are only 150-200 animals in the wild in a few locations near Perth and less than 50 of those are breeding adults. Perth Zoo holds a colony and has bred more than 700; juveniles are released to monitored reserves most years. However in years like 2010, released animals had to be returned to the zoo because climatic conditions were so poor.
What makes this animal a good candidate for assisted colonisation (the human movement of at-risk species into areas that are predicted to be climatically suitable long term)? The answer lies in its restricted range, special habitat requirements, inability to migrate unassisted, low fecundity and low current population, as well as a source of captive animals. We also have 30 years of data on the animal in the wild and in captivity—including data that shows it survives well at new sites.
How do we select sites for colonisation? With such a restricted range, one can’t use empirical modelling to infer its requirements. Mechanistic models appear to be the only way to proceed, since they can predict a species’ response to an environment, via its physiology and behaviour. Sophie’s research has therefore focussed on the tortoise’s gut, thermal and metabolic physiology and has led to a ‘thermal response curve’ for this animal. The data was then applied to the question: where could we put the tortoise?
Now other researchers came into the picture and ‘Project SWAMPI’ —a collaboration between universities, zoos and government departments around the country and internationally—really geared up. A preliminary hydrology model was developed that made predictions which fell close to actual data. A model called NicheMapper™ was used to run a behavioural routine that Sophie developed based on her physiological data and this predicted in general terms where the tortoise might live in the south-west. An approach called Dynamic Energy Budgeting (DEB) was used to predict the tortoise’s survival and reproduction, with a 96% fit to actual data. Much useful information has been gained from the model. For instance it predicts a tortoise at normal weight could fast in aestivation for 588 days and that the average lifespan of a Western Swamp Tortoise is 90 years. The latter was obviously not known because no one has been studying them for that long! Although the models are only in a preliminary stage, another, more frightening, prediction is that if the hydro-period in their habitat is shortened by any more than one month, small hatchling tortoises will not survive and that particular population may eventually die out.
This is the first time that anyone has explicitly modelled for assisted colonisation—an idea which was considered purely theoretical as recently as the late 80s. It is hoped that this work will not only help save the Western Swamp Tortoise but be a template for other species, particularly those in wetlands, such as the Sunset Frog (Spicospina flammocaerulea). Meantime the project is about 6 months from pinpointing possible colonisation sites, though areas around Donnybrook, Busselton and Augusta are being considered.