Spiders keep cane toad numbers in check
FROM CROCODILES TO SNAKES and quolls, many Australian species have died trying to eat the indestructible cane toad. But it seems that some native spiders have found a way to consume large numbers of the large toxic pest.
Spider expert Dr Robert Raven, from the Queensland museum, says three families of spider – Australian tarantulas, wolf spiders and racing stripe spiders – may be helping to keep northern Queensland cane toad numbers in check.
“That spiders eat cane toads is not uncommon or unknown,” says Robert. “The interesting thing is size of the toads they are able to take. In one instance, the toad was bigger than the opening of the spider’s burrow.”
Natives fight back: spiders vs. cane toads
Although the Australian tarantula (Theraphosidae sp.) is able to kill and eat larger toads, says Robert, the more common wolf spider (Lycosidae sp.) may be a better candidate for reducing the number of cane toads.
“Tarantulas and wolf spiders both kill dogs and cats quickly, in around thirty minutes to an hour with one bite,” says Robert. “However, tarantulas are quite low in number, and they’re becoming rarer because of humans.”
Introduced into northern Queensland in 1935, the cane toad releases a fatal toxin to vertebrates, causing rapid heartbeat, excessive salivation, convulsions and paralysis.
Robert believes that although the dangers to vertebrates are known, research needs to be done on the effects of the toxin on invertebrates, which at this time are unknown.
“With cane toads, the toxins have to be ingested for them to be effective,” says Robert. “However, the spider, in order to eat, vomits a proteolytic enzyme into the animal, which may cause the toad venom to [become inactive].”
Native animals picky cane toad eaters
Although cane toads are devastating for a small number of large predator species – including goannas, blue tongue skinks and quolls – their impact doesn’t appear to be permanent, says toad expert Professor Rick Shine, from the University of Sydney.
“Even these species eventually seem to recover a few decades after the toad’s first arrival, so the long-term impact of cane toads probably isn’t as great as that of animals like rabbits or cats,” says Rick.
Other native Australian invertebrates are able to eat cane toads, including meat ants and water beetles, which consume millions of toads every year, according to Rick.
Other natives that prey on cane toads include the Australian crow (Corvus orru), which has adapted by eating them from the underbelly to avoid the venom, and the saw-shelled turtle (Wollumbinia latisternum), adds Robert.
Ants keep toad tadpoles in check
Studies both in the lab and around breeding ponds in the field have revealed that ants, in particular, can significantly reduce the number of young cane toads.
“What we don’t yet know is how a reduction in the numbers of small toads ends up influencing the number of adult toads in an area,” says Rick. “But, we are hoping to conduct research on that question also.”
When it comes to conservation, Rick believes that increases in predator numbers may help explain why toad numbers drop after they have been in one area for a long time. “Basically, the native predators recognise that there is a new free food source and start to exploit it.”
However, despite being hopeful that a combination of methods will see a drastic reduction in cane toad numbers, Rick warns that “eradicating toads from Australia simply isn’t possible.”