The Impact of Cane Toads on BiodiversityWhat impact do cane toads have on biodiversity in Australian ecosystems? Many people think it's a catastrophe when toads arrive - but what is the evidence?
We don’t know of any surveys to ask the Australian public WHY they hate cane toads so much – it would be an interesting exercise, and Rick has been looking for social-science collaborators who would want to ask exactly that question. But so far, no luck. So, all we have to go on is a general feeling from talking to lots of people about cane toads (taxi drivers, for example, almost always seem to have strong opinions about these animals!). And the main result to emerge is the YUK factor – people just don’t like cane toads, full stop. Some people worry about domestic pets (especially small dogs, like terriers) being poisoned by cane toads – and this is indeed a very real problem not just in Australia but in other parts of the cane toad’s introduced range (for example, in Florida). And we’ve heard all kinds of other stories too, like the worry about youngsters in remote communities licking cane toads as a form of drug abuse. So far as we know, these are just stories. If pushed to come up with a reason why cane toads are a problem in Oz (other than “THEY ARE SOOOO UGLY!”), most people talk about ecological impacts on the native fauna. That is, toads are widely believed to kill many native animals, and so cause a major ecological catastrophe in the areas they invade. Some of the community groups have suggested that toad invasion has incredibly serious effects on the native fauna. What’s the evidence for all this talk of ecological catastrophe and biodiversity impacts? The answer is: surprisingly little. Of course, that doesn’t mean that cane toads aren’t a major problem – but the evidence has been remarkably weak until some recent studies by TEAM BUFO and a few other groups. Part of the problem is that the kinds of native animals that are most likely to be affected by toads are big predators, like quolls (marsupial carnivores), large snakes, and goannas. They eat frogs, but can’t handle the poisons in cane toads, and so are killed when they try to eat the poisonous new froglike animal. And because big predators are fairly rare, and often secretive, we don’t know how many are out there in the bush BEFORE toads arrive – and so don’t know how many have been killed by toads. It’s crystal clear that many predators ARE killed by toads – we often find them lying dead with a toad in their mouth or stomach – but the question is, what proportion of the population are killed?
Goanna poisoned by a cane toad it tried to eatSometimes, it’s a lot! From work by Professor Thomas Madsen and Dr Beata Ujvari at Fogg Dam, as well as Dr Greg Brown’s regular counts, we know that most of the big floodplain goannas were killed by trying to eat toads, within a year or so of the toads first arriving. It’s likely that around 90% of the adult goannas may have died in this way – a huge impact. Surveys by Dr Sean Doody and his colleagues show the same situation on the Daly River. Dr Ben Phillips’ work on the Fogg Dam death adders indicates very high mortality also – perhaps 50% or so within a year of toad arrival. In her radio-tracking work, Dr Meri Oakwood has found similarly high mortality in quolls (carnivorous marsupials). And in our studies in upstream areas of the Daly and Victoria Rivers (but not in downstream areas like Fogg Dam), lots of freshwater crocodiles are killed when they try to eat toads.
Red-bellied Black Snake poisoned by a cane toadSo, we know that predators big enough to tackle a large adult toad are in big trouble when the toads arrive. Fortunately, things aren’t so awful for most of the smaller species. When the toads first arrived at Fogg Dam, we found quite a few dead predators, of a range of species. The most common victims were native tadpoles, who died when they tried to eat toad eggs. Even for the species most often killed, however, our surveys showed no real reduction in populations of those animals. Indeed, many of them increased in abundance, perhaps because of the lessened risk from big goannas, snakes and the like – the animals that had been killed by cane toad arrival.
Death Adder killed by a cane toadThe overall story then, is that when the toads arrive some species (especially big predators) are in real trouble. Others – especially the species that used to be prey for those big predators – probably benefit from the toads’ arrival. And lastly, there’s a lot of species for which toad invasion really doesn’t change much at all. For example, most Australian birds don’t seem to be affected by toads to any significant degree. There are scattered records of a few Australian birds dying after eating cane toads, and one study of toads eating baby birds (rainbow bee-eaters) inside nesting burrows. But the overwhelming majority of Aussie bird species don’t seem to be at any risk from cane toads. Indeed, Christa Beckmann’s work at Fogg Dam is showing that some of the local hawks benefit from toad invasion, because they can scavenge dead toads off the roads. The toads’ poison doesn’t seem to be a problem for most birds, perhaps because of their close evolutionary relationship to birds in Asia, where toads (not cane toads, but species with very similar poisons) are common.
Ibis are able to eat cane toads without a problemto to Home Page
|
