By VERLYN KLINKENBORG
As I walked back the other night from dinner at a lodge near the Van Diemen Gulf on the north coast of Australia, I accidentally stepped on a toad in the dark. When I looked down, I realized there were toads all around me and that they were cane toads — Bufo marinus — natives of Central and South America. Tens of thousands were released in Australia in the 1930s to control a beetle that preyed on sugar cane, another introduced species.
The toads have marched slowly ever since from the Queensland cane fields into New South Wales and the Northern Territory, reaching the country around Darwin, on the north central tip of the continent, only a couple of wet seasons ago. Cane toads are poisonous, from tadpole to adult. They kill whatever eats them, including birds, reptiles and carnivorous mammals.
Cane toads are only one of the pressures on Australia’s small and increasingly endangered species — others include large grazing (non-native) herbivores, ferocious late-season wildfires and feral cats.
It’s estimated that there are between 4 million and 12 million feral cats in Australia, the progeny of former house cats. Just in the Kimberley — a region of northwestern Australia that is about the size of California — feral cats are eating as many as 300 million small mammals, especially small nocturnal marsupials, a year.
What is happening is a population crash. Scientists surveying native mammals in northern Australia, widely regarded as an oasis of biodiversity, report that they are finding it almost impossible to catch native mammals. During a recent study, it took an average of 1,000 trap-nights to trap 3 mammals.
The scale of this crisis is partly the result of Australia’s unusual and particularly vulnerable ecology. It has always been a predator-poor country — no bobcats, no weasels — so the effect of feral cats has been especially devastating. And though there are many poisonous reptiles in Australia, the advent of a new poisonous amphibian — one so apparently edible as a cane toad — has completely upset nature’s balance. Some birds, crows especially, have already learned how to flip cane toads over and eat their stomachs, avoiding the poisonous glands near the head. But nothing is really stopping the cane toads.
And in most places, nothing is stopping the cats. There is an exception: Australia’s native dogs, the dingoes. They, too, are under attack. Since dingoes sometimes kill sheep, the owners of pastoral stations have tried to exterminate them by using poison bait — a practice once called “dog stiffening.”
The Australian Wildlife Conservancy has purchased several environmentally significant properties across the continent, and on several of them they have stopped killing dingoes. Studies by the conservancy have shown, preliminarily, that when dingoes are present, cat numbers drop sharply. It’s not clear whether the dingoes are killing the cats or driving them out of the neighborhood. But the effect is a rebound in the numbers of other small mammals and reptiles.
The result resembles what scientists discovered in Yellowstone when gray wolves were reintroduced in the mid-1990s. The wolves killed intermediate predators, coyotes mostly, which in turn caused a rebound in the number of small mammals, including voles, gophers and ground squirrels.
What makes Australia’s population crash especially problematic is that most of the species that are disappearing — the marsupials eaten by cats — are seldom visible even when they’re abundant. Their absence goes unnoticed, so it’s hard to rally public sentiment. And thanks to the cane toad, even some larger species are dwindling. Not long ago, on a drive along the Arnhem Highway, heading east out of Darwin, you would have passed a number of pythons warming themselves on the asphalt. Now, they’re a very rare sight.
In a profound sense, the landscape of northern Australia is rapidly losing its biological resilience. It can’t begin to be restored until these non-native creatures are eliminated, no matter how uncaring it may sound to mount a campaign to kill feral cats. What Australia should not do is introduce another non-native predator and create other unintended and disastrous consequences.
Until feral cats and cane toads have vanished, there can be no hope of real recovery. The only good news is the familiar good news: Nature rebounds quickly whenever it gets a chance. But there’s no rebounding once entire species of marsupials and reptiles and birds have vanished for good.
No comments:
Post a Comment