Turtles are great evolutionary survivors. With their iconic shells and ponderously slow pace of life, they have plodded through 220 million years of natural selective pressures. In the face of forces that have ended many living lines – including dinosaurs – the overall turtle solution of being gently inoffensive yet well protected has held strong.
But now, a combination of human-induced changes has created a downward spiral so powerful that – without strategic intervention – much of the great turtle lineage will have disappeared by the close of the 21st century. Nearly half of all turtle fauna are threatened or extinct in the wild.
Australia is not immune from these global trends: six of our freshwater turtle species are listed as nationally threatened under the Environment Protection and Biodiversity Conservation Act. Recent research has also highlighted that other turtle species may soon find themselves added to the list. Around the Murray River region for example, the Eastern Long-necked Turtle has declined in abundance by 90% since the mid-1970s, and no juvenile recruitment has occurred for over a decade.
Threats to Australian freshwater turtles are numerous. But collectively their potency lies in the ability to permeate every aspect of the turtle’s life history, from egg to adult. The natural history of turtles involves high but fluctuating rates of egg and juvenile mortality balanced by repeated reproductive episodes over a very long lifetime, in which threats to adult survival are low. That is, young turtles die easily, but many more are born, and once they reach adulthood they have a good chance of living a long life. Unfortunately, we have sent this selective regime awry in many places. Eggs and young are being depleted, but adult mortality is also increasing.
The wondrous shell that has been so successful at holding back nature’s vicissitudes is no match for motor vehicles, and adult turtles frequently become road kill victims as they disperse throughout riverine and wetland habitats. They are also struck by boats, drowned in fishing nets and die in falls from weirs.
During the recent “millennium drought”, there were reports of mass turtle mortality as historically permanent wetlands dried up: a harbinger of what may become commonplace under climate change predictions. And in the lower lakes of South Australia – where salinity rose during the drought – many turtles perished after becoming entrapped by massive growths of estuarine tubeworms on their shells.
Mortality rates in the egg stage appear to be even bleaker: over 90% of them are dug up and eaten by European foxes around the Murray River. Feral pigs fulfil a similarly devastating predatory role in more northern regions.
The unique life history traits of turtles also serve to mask the extent of threat to populations. Their longevity means that adults can persist in regions at relatively high levels for decades. But these apparently healthy populations may in fact be imperilled by chronic reproductive failure.
“Living-dead” populations composed entirely of old turtles with limited reproductive capabilities create a dangerously false illusion of prosperity. This problem is compounded by the cryptic nature of juvenile freshwater turtles – young turtles hide too well to be counted, and often scientists only know how many adults there are in a population. However, if managers wait until numbers of adult turtles are greatly reduced before initiating conservation measures, recovery will be extremely difficult or impossible.
Extended inter-generation intervals also need to be taken into consideration when judging the success of conservation actions. A turtle nesting this year may have hatched some half a century ago, when conditions affecting survival were vastly different from today. And where decades of failed recruitment have occurred, even immediate and aggressive intervention will not compensate for the inevitable (though hopefully temporary) future drop in adult population numbers.
Globally, the turtle conservation crisis is probably more widespread than the well-publicised amphibian decline phenomenon. However, perhaps partly due to the long generation time of turtles, it has not yet engendered the same level of concern. Amphibians are often described as unique indicator organisms because they are so sensitive to environmental perturbations. But turtles, with their long life spans and historical resilience to dramatic environmental changes, can also tell us a lot about the health of our ecosystems.
The story of the slow but steady turtle winning the race against a fleet hare is familiar to us all. But a contemporary – sadly real – version of this fable would see the turtle in a race against a tide of anthropogenic change. In this setting, their slow pace presents a major hindrance to winning the race.
If we’re to turn the tide back in favour of the turtle, we have to recognise that managing a vertebrate with a hundred-year lifespan presents some unique challenges. Conservation actions need to be proactively directed at every age class and across multiple habitats. They need to be augmented by careful long-term monitoring and research that provides a deeper understanding of turtle biology and the threats they face.
These advances can only be achieved through dedicated scientists who are able to devote much of their professional careers to ensuring the survival of one of Earth’s most ancient citizens.