Polliwog Wars

Wriggling, squiggling tadpoles may seem unlikely warriors in the fight against invasive species. In Australia, however, the limbless larva could help drive down populations of a poisonous exotic toad that is killing native animals.

In 1935, Australian sugar cane growers imported the cane toad (Bufo marinas) from Central and South America to hunt down insect pests. Since then, the toads have spread across more than a million square kilometers, biologists Elisa Cabrera-Guzmán, Michael Crossland and Richard Shine of the University of Sydney report in the Journal of Applied Ecology. Huge numbers of predators—from snakes to foxes—have died after eating the toads, which have glands that produce a toxic brew.

Cane toads lay at least 30,000 eggs per clutch in shallow pools. Studies have suggested, however, that toad tadpoles often don’t do well in pools that also have native frog tadpoles, which tend to hatch earlier and grow larger. To see just how well the native frogs might help contain the toads, the Sydney researchers created dozens of tadpole competition arenas from plastic bins filled with water. Then, they matched toad tadpoles against tadpoles from eight kinds of native frogs, and waited up to 85 days to see what happened.

“The presence of native tadpoles strongly affected cane toad tadpoles in ways likely to reduce organismal fitness,” the researchers report. In bins containing six of the native species, for instance, toad tadpoles were shorter, lighter, had higher death rates and spent longer times as larva. That’s probably because the natives outcompeted the toads for food and other resources. The results “are broadly encouraging for a potential role of native frogs in reducing toad densities,” they conclude, although results in the wild could differ.

Now, the challenge is how to get more frog tadpoles into pools already conquered by the toads. One strategy is to strengthen existing efforts to get Australians to build pools to attract native frogs, boosting populations. Another is to transplant native tadpoles into pools used by toads. Such approaches, when joined with two other “biocontrol” strategies – encouraging hungry ants to prey on young toads, and planting native vegetation that the toads don’t like – could create an “integrated approach to toad control,” the researchers note. “All three of these approaches have a common theme,” they add. “That we can manipulate native species to intensify their negative impacts on the invader.” David Malakoff | January 4, 2011

Source: Cabrera-Guzmán, E., Crossland, M., & Shine, R. (2010). Can we use the tadpoles of Australian frogs to reduce recruitment of invasive cane toads? Journal of Applied Ecology DOI: 10.1111/j.1365-2664.2010.01933.x

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