Australia's native frogs beat invasive toads

A green tree frog The frogs could be "Green Avengers" in the fight against toad invaders

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The tadpoles of Australia's native frogs can outcompete invasive toads, scientists say.

The toads are considered a threat to Australian wildlife, leading researchers to investigate methods to control their population.

A study into competition between wild amphibian young revealed that the presence of green tree frogs reduced cane toad survival.

Experts now suggest reintroducing the familiar frogs to suburban areas.

The results are published in the journal Austral Ecology.

Cane toads are native to South America but were introduced to Australia in 1935 to control sugar cane pests.

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They are now one of the International Union for the Conservation of Nature's (IUCN) top 100 invasive species and considered feral pests across north-eastern Australia.

Their large clutches of eggs, ability to migrate 40km per year and poisonous defence are outlined as some of the primary reasons for their population explosion.

However, research by Professor Richard Shine and colleagues from the University of Sydney has highlighted the weaknesses of the animals in early stages of development.

Amphibian 'wimps'

In 2011, the team reported that cane toad tadpoles cannibalise eggs to survive.

Continuing research into this area, Prof Shine highlighted that despite this behaviour, cane toad tadpoles are "wimps" when confronted with other species.

Previous laboratory studies have suggested that tadpoles of the green tree frog Litoria caerulea have a particularly strong impact on the survival rate of cane toads.

To fully understand the interactions between the two species in the field, Prof Shine and colleagues set up two temporary pools and studied how the frogs and toads developed.

Observations revealed that toad tadpoles grew more slowly and spent longer as larvae when they faced competition.

Start Quote

I think that it would be terrific to have giant green frogs sitting in people's mailboxes again!”

End Quote Professor Richard Shine University of Sydney, Australia

"If other larger tadpoles are present, the cane toad tadpoles don't get as much food - they can't compete successfully - and so they grow and develop slowly [and] often die as larvae," said Prof Shine.

"Even if they manage to metamorphose - turn into small toads - they tend to do so at a smaller size, and we know that little toads are very unlikely to survive."

Unlike many frog species, cane toads prefer to breed in disturbed waters but, according to Prof Shine, "there are several hardy city-dwelling native frog species that are happy to breed in the same kinds of places as do toads" including the green tree frog.

Averaging 10cm long and bright green in colour, L. caerulea is a well-recognised species in Australia where they are broadly distributed in western and northern areas down to coastal New South Wales.

But conservationists have reported that the frogs have suffered from pollution in urban areas and some habitat loss due to expanding human settlements.

In the study, the frogs did not prey on the tadpoles of cane toads and those that ate cane toad eggs were poisoned by the toxins they contain. However, the omnivorous frogs did compete for the same food sources as the toads.

Toad eggs (c) Michael Crossland Cane toad clutches contain thousands of eggs

"So, if we can encourage native frog populations back into suburban areas where they once occurred, they will be able to reduce survival and recruitment of cane toads from those same ponds," explained Prof Shine.

He told BBC Nature that he was "surprised by the consistency of the effects, and by how much the toads were troubled by the frog tadpoles". Overall, he was "delighted" with the findings.

"I think it's a great opportunity to involve school-kids [and] community groups in an action that will benefit local biodiversity - regardless of any effects on toads," he said.

"I think that it would be terrific to have giant green frogs sitting in people's mailboxes again!"

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