The plants that saved the numbat
Toxic shrubs have unwittingly aided the numbat in WA, creating sanctuaries for the endangered animal.
Tim Low lives in a state of perpetual surprise at everything wild and alive. His response is to write searching books, Australian Geographic articles and this blog. His book Where Song Began (Penguin, 2014) recently became the first nature book ever to win the Australian Book Industry Award for best general non-fiction. Tim’s newest book is called The New Nature.
IT LOOKED NERVOUS, as well it might. Numbats were nearly wiped out by foxes and cats, and the one I was watching in Western Australia’s Dryandra Woodland needed to be alert to survive. After scratching at one patch of ground for termites to eat, the numbat would listen carefully before scampering to another, often passing shrubs with yellow and red pea flowers. Odd as it may seem, these plants, known as sandplain poison (Gastrolobium microcarpum), have been central to the numbat’s survival.
Many pea bushes in the genus Gastrolobium are highly toxic, containing the active ingredient in dingo, fox and rabbit bait, namely sodium monofluoroacetate, known as 1080. These plants are concentrated in south-western Australia, where with long exposure to the plants, many animals have evolved high 1080 tolerance.
Brushtail possums in this region can survive 150 times as much 1080 as brushtails in the east. Predators such as goannas have also evolved resistance, presumably by feeding on insects and other prey that browse these plants. Ingested 1080 may take a day or two to be eliminated, which means that a fox or cat dining on possum in south-western Australia runs a risk of being poisoned.
Female numbat. (Photo credit: Gnangarra/commons.wikimedia.org)
This toxin has apparently limited foxes in the south-west, explaining why numbats, woylies, western quolls and red-tailed phascogales survived there and nowhere else. These mammals once lived as far east as inland New South Wales, but hung on only in the corner of Australia where 1080 plants abound. At their low point, numbats survived only at Dryandra and Western Australia’s Perup Nature Reserve. Sandplain poison is Dryandra’s main shrub.
Today, under the Western Shield program, national parks and other south-west reserves are baited with 1080, upping the aid conferred by the plants. While elsewhere in Australia the baits are dangerous to fauna, 1080 baits are safe for feral animal control in this region because native animals are so tolerant – quolls and goannas won’t die if they gulp down a bait.
The plants have helped wildlife in another vital way. As they’re lethal to cattle and sheep, places with high densities were quickly found to be useless for grazing. Many of these high-density areas became reserves – which are often rich in rare wildflowers – as a result.
Unfortunately, the benefit these plants bestow is weakening. When I re-visited Dryandra last year, I was told that numbat and woylie numbers have plummeted due to cats. Less inclined than foxes to take baits, feral cats have taken over as the main numbat enemy. What Australia desperately needs is an effective way to curb cats. 1080 plants provided numbats with a lifeline, but much more is needed if they are to have a secure future.