Fox predation of turtle nests

by Stuart Dawson.  Turtles are good examples of r-strategists.  They produce many young that experience high mortality (compared with K strategists, such as humans, which invest heavily in each individual offspring).  Most people would know that many turtles are killed as hatchlings, but did you realise that they are often predated even before they even hatch?

Since the red fox has been introduced and become established in Australia, they have been recorded predating nests of both freshwater and marine turtles.  Thompson (1983) reported that 96% of turtle nests in the Murray River were raided; 93% of which was by foxes.  At the 2014 Australasian Wildlife Management Society meeting in Brisbane, where he won the awarded the 2014 Practitioners Award, Mike Butcher presented data revealing that 80-100% of loggerhead turtle nests monitored at Gnaraloo Station in WA were predated by foxes in the absence of fox control.  Understanding the behaviour of these predators may help to control their impacts.

A recent study investigated factors that influence the fate of turtle nests.  Stuart Dawson compared the fate of four artificial ‘nest’ treatments at a site where foxes regularly dig up freshwater turtle nests: buried chicken eggs that were sprayed with water from a turtle pond, just the chicken eggs, just the pond water, or patches of soil that were simply dug up and then re-filled.

Nearly half (46%) of the 580 artificial ‘nests’ were excavated by predators within 2 months.  ‘Nests’ with eggs present or those sprayed with the turtle pond water were more likely to be dug up, but even the soil that had just been turned over was ‘investigated’  by foxes (38% of 145 ‘nests’ monitored).  Stuart also found effects of distance from the water’s edge and vegetation density.  Artificial nests close to the water and in the open were more likely  to be dug up by foxes.

This study highlights how foxes use visual and olfactory cues to find buried eggs.  Is this likely to reveal the cues that they are using to find turtle nests too?  Hatching turtles also communicate with each other to coordinate their hatching, and this ‘chirping’ may also provide predators with auditory cues to locate turtle nests.

The freshwater oblong turtle Chelodina colliei.
The freshwater oblong turtle Chelodina colliei.

Studies of turtle populations under heavy fox nest raiding pressure have shown a lack of juvenile recruitment into the population.  The long life expectancy of turtles means that this lack of recruitment may be masked by relatively stable adult numbers.  This may be a classic case of extinction debt, since with little recruitment,  populations may eventually collapse.  Careful management may therefore be required to reduce nest raiding by foxes.

A worrying sight - adult female turtles are also vulnerable to predation when they come onto land to lay their eggs. Carapaces of 25 freshwater turtles found outside a fox den.
A worrying sight – adult female turtles are also vulnerable to predation when they come onto land to lay their eggs. Carapaces of 25 freshwater turtles found outside a fox den.

Can we use this information to help increase turtle conservation?  Using artificial ‘nests’ obviously does not replicate all the conditions of real turtle nests, but it does indicate environmental factors that make it easier for foxes to locate nests.  In the absence of adequate fox control measures, this information may help with developing methods to ‘disguise’ nests.

Stuart Dawson is currently studying his PhD at Murdoch University, where he is investigating how habitat disturbance could influence the presence of the greater bilby.

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