The Extinction of Australian Freshwater Turtles?

Ricky Spencer

A juvenile Bellinger River Snapping Turtle (Myuchelys georgesi). Few remain after a disease brought the species to the brink of extinction in 2015

In a recent article in the international journal Conservation Biology, we demonstrate a theoretical basis for why headstarting programs are a necessary conservation strategy for stopping the declines of freshwater turtles, especially those primarily threatened by both adult mortality or removal (due to roads, predation, or harvest) and invasive predators affecting multiple life history stages. Once common widespread species are becoming locally extinct because the longevity of turtles has hidden the impact of these threats. Australia is now at the stage since post-European settlement where the effects of foxes and urban population sprawl (and associated infrastructure) are being observed through large declines and extinction events. Declines of up to 91% have been observed along the Murray River in south-eastern Australia. Wildlife diseases have become more prevalent over the last decade, which is a symptom of deteriorating water quality and climate change. The net effect of these threats are that freshwater turtle populations in Southern Australia are at high risks of extinction without active management.

EUROPEAN RED FOXES

The European red fox was introduced into many countries including Australia in 1845. Other successful releases followed in southern Australia in the 1870's and within 20 years, the red fox had achieved pest status. The expansion of the red fox population across mainland Australia followed the spread of rabbits and their distribution on mainland Australia has been limited by the northern tropics. Fox predation is having a serious impact on many native animals, and is a major contributor to extinction of some species. In the Murray River in Australia, mortality rates of eggs have increased to over 93%, which is likely to be replicated throughout the distribution range of foxes, because changes in nest predation rates are largely independent of fox density - meaning a single fox can have a similar impact on turtle nests as what high densities of foxes can. Long-term high levels of nest predation have resulted in extreme ageing populations and there are few techniques available for management to effectively eradicate foxes over a broad scale.

Citizen Science data from TurtleSAT showing locations of depredated nests (yellow) and dead turtles (green and blue) killed by foxes

Poison baiting is the only broad-scale management technique available in Australia, and our trials demonstrated that only intensive, large scale baiting can effectively reduce nest predation rates. Other common techniques, such as targeted shooting and fencing may also reduce impacts of foxes but are not cost-effective techniques for broad-scale management. Management of a population or a species under threat often focuses directly on reducing impacts on the life history stage(s) affected. In doing so, focus inevitably is directed to the threat, rather than on the impacts on the affected population. Plant biologists and conservationists have long criticized classical biocontrol for lacking quantitative assessments of effectiveness, especially post-release, yet invasive vertebrate pest management primarily focuses on reducing densities of invasive predators or herbivores. The core components of conservation policy to manage their impacts is to reduce predator numbers in an area using lethal methods. The actual efficacy (eg. reduced impact on target species or increases in biodiversity) of such programs are rarely assessed and success is determined by the number of carcasses, reduced activity of the target species or the number of baits taken. Efficacy of these programs is vital given the limited resources available for most conservation programs and the high costs associated with lethal control. AU$21.3m was spent on labor costs alone for red fox control in Australia in 1998–2003, but the benefits to native prey are largely not known.

INCREASING ADULT MORTALITY

Foxes also kill adult turtles they encounter on land. Australian turtles are resilient to high levels of nest predation for sustained periods and periodic levels of reduced nest predation and pulse recruitment can maintain population viability, but low levels of adult mortality can drive populations to extinction.

Besides foxes, mortality of Australian turtles has increased through disease. In February 2015, a mystery disease almost drove the Bellinger River Snapping Turtle (Myuchelys georgesi), in north-eastern New South Wales, Australia, to extinction in less than a month. The disease did not affect other turtle species, and the juvenile population of M. georgesi appears unaffected. The cause of the disease remains unknown, but has been suggested to be a novel virus and our analysis adds to the growing body of literature reporting that climate change is having a detrimental effect on organisms. The disease that has brought the species to the brink of extinction may be a spectacular climax to an already declining or stressed population. An unusual mortality event also occurred at the same time with the Johnstone River snapping turtles (Elseya irwini) in Far North Queensland, Australia. Similar to the Bellinger River Snapping Turtle, moribund animals were found lethargic with variable degrees of necrotising dermatitis and at the time of the turtle deaths, water levels were extremely low.

Water quality and drought are significant factor that has hastened population declines of turtles in South Australia. In early 2008, infestation of Murray River turtles, with the Australian tubeworm (Ficopomatus enigmaticus) was reported at the mouth of the Murray River in South Australia. This emergent condition in turtles is due to high water salinity in the region and the reported cases spread upstream until 2011. The worms form calcareous tubes on hard surfaces of turtles and potentially killed thousands of turtles, although the exact number is not known. At current levels of recruitment, it takes only 1% of the adult population (~2% of adult females) to be harvested from a population each year to increase the risk of extinction (over 200 years) to over 60%.

Eastern Long Neck Turtle encrusted by a marine tubeworm during the 2008-2011 drought in South Australia. Photo Credit: Deanne Smith (Alexandrina Wildlife)

Road mortality is another source of adult mortality that particularly targets nesting females as they emerge to nest. Our Citizen Science project, TurtleSAT, has shown extensive road mortality of Eastern Long-Neck Turtles in South Eastern Australia. Eastern Long-Necks are Australia’s most widely distributed turtles, yet their numbers have declined by 91% over the last 40 years in some areas.

Citizen Science data from TurtleSAT showing locations of roadkill Eastern Long Neck Turtles throughout south eastern Australia (since 2015)- From Spencer et al. 2017 Conserv. Biol.

PROGRESSIVE MANAGEMENT

With no recruitment in the region and limited dispersal opportunities due to the number of dams, the near-complete absence of turtles at many sites in South Australia is particularly disturbing because it was first predicted over 30 years ago, and low numbers have been subsequently reported. Several states have recently listed Murray River turtles as threatened or data deficient, but they are not listed at the federal level, thus few conservation initiatives are occurring. Species like Eastern Long-Neck Turtles (Chelodina longicollis) are not considered species of concern or trigger protocols associated with Environmental Impact Statements (EIS) for urban development. It takes community groups, such as Turtle Rescues NSW, to conduct last minute rescues as developers drain and fill in swamps and wetlands as urban development expands throughout the Sydney basin.

Loss of habitat from urban sprawl puts turtles at risk, but community groups, like Turtle Rescues NSW and Alexandrina Wildlife, relocate and rehabilitate turtles. Photo Credit: Top- NearMap. Bottom- Turtle Rescues NSW.

Restricted to only two wild populations, there are less than 200 endangered Western Swamp Tortoises remaining near Perth in Western Australia. Less than 50 individuals survived 30 years ago, but since 1988, a successful breeding program has allowed translocation of captive-bred juveniles to three sites. Similarly, the Bellinger River Snapping Turtles is now critically endangered under the Environment Protection and Biodiversity Conservation Act 1999. During the disease outbreak with the Bellinger River Snapping Turtle, I was part of a large rescue team that collected 16 healthy adult turtles before the disease reached the upper stretches of the River and these turtles are now part of a breeding program that will hopefully implement a successful headstarting program over the next few years. In general, headstarting is generally seen as a management tool of last resort when species crash and become critically endangered.

Captive breeding and headstarting has not been commonly used as a conservation strategy for freshwater turtles in Australia. High financial costs, as well as landscape level disconnectivity among populations, have probably restricted its use, and past population modelling suggests that conservation efforts are more effective when focused on reducing adult mortality. However, we clearly show that the criticism of headstarting as “halfway technology” is erroneous, especially in cases where external threats affect multiple life history stages of freshwater turtles. The “halfway technology” argument assumes that all perturbations or mitigating factors affecting turtle populations can be eradicated, but in Australia, factors that impact turtles in southern Australia are multi-factorial will never dissipate until populations are extinct, or technology to reduce threats from invasive predators becomes more effective. Headstarting should be the primary conservation tool for managing freshwater turtles in decline.

Headstarting programs with small captive populations of Galapagos tortoises and Burmese star tortoises have proven successful at restoring population numbers. But the value of headstarting as a management tool goes beyond critically endangered species. In cases where “common” turtles are declining, developing suitable harvest populations in situ is the key. Many common species of turtle occur in integrated wetlands and water treatment plants (e.g., constructed wetlands) throughout their range, and these facilities may provide a tool for low cost headstarting programs for widespread but declining populations. The reproductive potential of turtles in constructed wetlands represents a potential pre-existing resource for developing localized headstarting programs in situ. A simplistic model where relative densities of Eastern Long Neck Turtle are based on surface area of water demonstrates that all eggs/hatchlings collected from 1ha of water can service ~25ha of water in a region to maintain population growth at pre-European levels and completely eliminate the risk of population extinction. Our models also demonstrate that periodic increases in recruitment can sustain populations, potentially allowing populations in a region to be managed in a mosaic fashion. In other words, not all populations need to be actively managed each year. That is the key, their longevity ensures that they are resilient to annual variation in mortality, which provides management with the flexibility to manage populations over wide spatial and temporal scale.