Assisted Colonisation of the Western Swamp Tortoise

5 March 2021

Having learnt about the long-necked Oblong Turtle at our November meeting, we were privileged to hear from an expert on another local freshwater chelonian, the critically endangered short-necked Western Swamp Tortoise. That species is special to our Club: Since the 1850s it was thought to be extinct, but it was re-discovered in 1953 when a schoolboy brought it along to a Nats’ Club Wildlife Show in the Perth Town Hall. Our speaker, Bethany Nordstrom, a PhD student at UWA, spoke to us about the assisted colonization (AC) of this endangered reptile, in an attempt to ensure its survival.

Bethany first talked about previous AC projects around the world. AC is the intentional translocation of a species outside of its historic range to reduce the impacts of a threat, such as habitat loss, disease, exotic predators or climate change. It is recognized as a legitimate but inherently risky conservation strategy. A pioneering attempt in New Zealand in 1894 was an attempt to save the Kakapo and Kiwi from predation by the introduced stoat, by translocating them to Resolution Island. This failed because the stoats moved to the island themselves, but some lessons were learnt.

AC has been used over 140 times in Australia and New Zealand alone. The success rate has been one in four. Some Australian examples and the causes of their decline are the Tasmanian Devil (facial disease), the Koala (human impacts), the Northern Quoll (feral animals), and the Western Swamp Tortoise (climate change). Some marsupials, such as the Boodie, have been moved to offshore islands.

AC is inherently risky, and there has been much debate about its efficacy and safety. However, Bethany said that most of the literature is not based on empirical studies. Some say that it is a last resort for species that cannot adapt or migrate when their environment changes through climate change or habitat destruction. Others argue that there is too much uncertainty, that there is a long negative history of moving species, and that the translocated animals can become invasive. They argue that conservation measures such as captive breeding and habitat restoration are safer.

Even though there have been successful AC projects in Australia and New Zealand, there is ultimately no one strategy that will work for all species and ecosystems. It seems that AC works best with small populations that are unable to disperse on their own, in highly fragmented habitats. Any proposed AC scheme needs rigorous assessment beforehand and good monitoring afterwards.

The Western Swamp Tortoise Pseudemydura umbrina is small (15cm) and it is estimated that there are currently only 50 mature individuals in the wild. The female lays one clutch of 3 to 5 eggs a year. Their optimal thermal niche is 14-30°C, so they are only active in the cool, wet half of the year when they forage and mate. In summer, they aestivate in burrows or under leaf litter. Their habitat is seasonal clay wetlands on the Swan Coastal Plain, but much of this habitat has been cleared or drained. Predators (e.g., foxes) have a major impact too. So, their natural populations have been limited to two locations: Twin Swamps Nature Reserve (150 ha) and Ellen Brook Nature Reserve (80ha). Because of climate change, rainfall in the SW has declined by 20% since the 1970s, and this decline will continue. So, the water depth is reducing, and the wet (active) season is shortening.

In 1962, the population was estimated to be 200 in the wild, but it declined to only ~16 adults in 1982. In 1988 Dr Gerald Kuchling arrived from Austria to help with the captive breeding program. In 1991 Perth Zoo took over the breeding program, where they continue to have success. In an attempt to increase tortoise numbers in the wild, translocations to Mogumber and Moore River Nature Reserve were started in the 2000’s. Altogether, 800 tortoises have been released from the zoo at various sites.

However, Bethany says that the future is uncertain for the species, due to habitat fragmentation and drying climates. There is a focus on “climate proofing” in its remaining natural habitat, but the ability of the species to shift its distribution naturally has been lost. In 2016 there was an AC trial translocation to cooler environments in East Augusta and Meerup (near Northcliffe), where the microclimate will be suitable in a few decades. The Meerup group is not thriving however, probably because of low water temperature and timing of prey availability.

Bethany then talked about the future plans for the tortoise, including future AC trials to cooler southern sites. She discussed methods she will be using to detect the tortoises in translocation sites (They are difficult to find by looking.) and for ascertaining their prey availability. These can both be done using eDNA (environmental DNA) methods – that is, detecting traces of tortoise DNA in the water and traces of prey DNA in the tortoise faeces. Radio tracking is also done, with attached transmitters that record activity and temperature. She will also use predictive models (e.g., DEB: Dynamic Energy Budget) to explore the effect of body temperature and food availability on growth and reproduction, survival and longevity. This will be used together with climate predictions to model the future of these populations in AC sites. Her research will also provide a useful case study for other AC studies.

The Western Swamp Tortoise is one of the world’s most endangered chelonians and is the only member of its genus. As Bethany said, if they become extinct, we stand to lose a branch of the tree of life.

Mike Gregson

The recent devastating fire at the tortoise’s prime habitat at Ellen Brook Nature Reserve (EBNR), in which over
90 per cent of the reserve was burnt (during the February ‘Wooroloo fire’), highlights the urgency of finding new
solutions to the species’ vulnerability to going extinct in the wild.

We hope that the assisted colonization work being done now will ensure this unique and iconic little animal’s
future in the face of climate change. For more information on the remediation of EBNR, see the
Friends of the Western Swamp Tortoise’s Facebook page.

Tanya Marwood
(Vice-chair, Friends of the WST)