New Island Home for Rare Frogs

In a bid to keep alive an ancient species of frogs, forty New Zealand frogs now have a new island home.

The frogs on the move are Hamilton’s frogs, which are now only found on one island off the coast of the South Island of New Zealand, Stephens Island. The new colony has been set up on a predator-free island in the same area.

“Basically it’s all about not having your eggs in one basket,” says Roy Grose, a manager at New Zealand’s conservation agency, the Department of Conservation.

Preparation for the move of the frogs to the new island of Nukuwaiata has been extensive.

The site for the “frog bank” (as the area where the frogs live is called) has been selected for its mix of forest above and scree below. “We found a rock tumble and this one’s got forest over the top so we are presuming it’s probably ideal,” says Mike Aviss, who has led this project for DOC. (University of Otago scientists also assisted with the frog transfer).

The temperature and humidity of the frogs’ new home was monitored for two years before the frogs were moved. “We found out that the Nukuwaiata site is fractionally moister and fractionally warmer, so we figured that this will do us,” says Aviss.

The DOC team also installed a boardwalk over the frog bank in order to the frogs safe from human visitors.

Rigorous attention to detail with the tiny frogs is needed because there are only about 350 frogs of the Hamilton’s frogs left in the world. The Hamilton’s frogs are protected by a fence from their only natural predator on Stephens Island, the tuatara, but when there was only a single location for the frogs, there was always a risk that the whole species could have been wiped out.

Already three species of New Zealand frogs have become extinct, with only the Hamilton’s frog, the neigbouring Maud Island frog, and the North Island’s Archey’s and Hochstetter’s frogs remaining.

The New Zealand frogs are relics of the country’s Gondwanaland past: they lack external ears, don’t croak, are nocturnal and hatch directly into froglets without going through a tadpole stage. The Hamilton’s frogs also have their own unique “fingerprint” — markings above their lip.

If the transfer to Nukuwaiata is a success then DOC plans to move another 40 or so Hamilton’s frogs. “What we are hoping will come out of this transfer is that we’ve got a good population of 80 frogs,” says Aviss. “At the same time we are taking 80 out of the Stephen’s Island frogs and therefore giving them more space to improve their breeding as well.”

The Hamilton’s frogs project follows the successful transfer of a group of Maud Island frogs, another species vulnerable because of their single location, to a new home. DOC moved 300 frogs from the 20,000 population on Maud Island to Motuara Island. The move, says Mike Aviss, appears to have worked. “They are probably increasing going by the places we find them like in the grass, on the edges of the bush and that sort of stuff.”

That the Maud Island frogs are reproducing on their new home is a relief for the conservationists: figuring out the sex of these small frogs is tricky and is a matter of sifting the smaller ones from the bigger ones (females are slightly bigger than males.)

The mix at the new home for the Hamilton’s frogs should be about right, says Aviss. “Probably 70 pct of those frogs we are confident about saying that’s a male or female.”

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