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Headstarting strategy saved an endangered wallaby population

A team of conservation scientists led by UNSW Sydney has managed to bring a group of wallabies back from the brink of extinction using a strategy known as “headstarting.”

The wallabies were placed within a protected area to grow into adulthood without the threat of their main predators,  feral cats, and then released back into the wild.

Study lead author Alexandra Ross explained that juvenile wallabies under three kilograms, which is smaller than a rugby football, are easy prey for feral cats.

“Previous studies have shown that more than half of these young bridled nailtail wallabies were killed by feral cats before they could reach adulthood,” said Ross.

“But when you look at the numbers of adults, the survival rate goes up to 80 percent – which shows that size is a good predictor of survival.”

“So we figured if we can just get them through that tough period – when they’re still little and an easy size for a cat to prey on – by putting them in feral-free protected areas, then we could make a positive difference to the population numbers.”

Out of 56 wallabies that were raised within the protected area between 2015 and 2018, 89 percent survived long enough to be released back into the wild. 

Study co-author Professor Mike Letnic said that headstarting is a cost-effective intervention when compared to other, more complex strategies that involve the creation of large nature reserves, like the one created in Sturt National Park in 2019.

“Aly’s headstarting project involved fencing off an area about 10 hectares which was big enough to hold about 30 or 40 wallabies at a time,” said Professor Letnic.

“We’re basically growing them from football size to medicine ball size before releasing them back into the wild, which can take anywhere from a few months to a year.”

“For the most part they’re fending for themselves in the headstart exclosure just like they do in the wild, except without the threat of feral animals. But they’re not completely protected – they can still get eaten by eagles which means there is still some predator recognition.”

According to Ross, the population of bridled nailtail wallabies more than doubled following the three years of headstarting in Avocet Nature Refuge, which is the largest increase that had been observed in this particular population since monitoring began in 2011.

“Before we started the headstarting strategy, we estimated the core Avocet population at 16 individuals,” said Ross. “When we did a recount in 2018 after three years of gradually releasing headstarted wallabies that had reached the right size, the estimate of the total population of bridled nailtail wallabies – both inside and outside the headstarting exclosure – was 47.”

“This clearly demonstrates the effectiveness of the headstart exclosure as a conservation strategy.”

The researchers calculated how the wallaby population would fare with or without varying lengths of headstarting scenarios. The projections showed that, if headstarting was ceased, the population would become extinct within two to 52 years.

“What this tells us is that until we find a way to eliminate feral cats in the wild, headstarting may be the only way to keep this population at a sustainable level.”

The wallaby headstarting project was the very first of its kind for a land-based mammal, and raises new hope for other potential endangered species among which the size of juveniles may be a factor in population survival.

“One of the great things about headstarting is it’s relatively cheap, doesn’t interfere too much with animals’ awareness of predators, and can get good results in a short time,” said Ross.

“And there are plenty of other mammal species around the world that could benefit. Any species that is particularly vulnerable in the early life stage could potentially thrive under a headstarting strategy.”

Up until now, headstarting has been used with some success with birds, fish, reptiles, and seals, and there is no reason why it shouldn’t also be implemented for terrestrial mammals, concluded the researchers.

The study is published in the journal Current Biology.

By Chrissy Sexton, Staff Writer

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