Frog sex in the city

Tree frogs defy the trend of urban decline

Central Melbourne used to be a Mecca for frogs, but now there is only one species left.

Southern brown tree frogs can still be heard calling to attract females for mating in parks throughout inner Melbourne, including the Royal Botanic Gardens and Fitzroy Gardens.

A survey conducted at 104 ponds across Melbourne found a total of nine frog species, but revealed the southern brown tree frog to be the sole inner-city survivor.

An important factor in the loss of other frog species from central Melbourne is the steep walls of bluestone or concrete surrounding many ponds, according to Dr Kirsten Parris of Deakin University and the Royal Botanic Gardens Melbourne.

“Most frogs around Melbourne can’t climb vertical surfaces, so young frogs would become trapped in these ponds and drown,” Kirsten said.  “But the southern brown tree frog can climb, using its large sticky toes – that’s what makes this frog special.”

A second reason for the decline of frogs in the city is that urban ponds are isolated from each other by roads, houses and factories.  If a population dies out, other frogs cannot arrive safely to start a new population.

“Frogs cannot cross busy roads without being squashed.  I found that the number of frog species at a pond drops as the number of roads around the pond increases” Dr Parris said.

“There are two simple things we can do to bring more frogs back to central Melbourne – replace steep pond walls with gradual slopes, and use a carefully-designed program to reintroduce the tadpoles of some species that used to live there.

“This way, late-night revellers in the city will be able to hear a varied chorus of frisky male frogs calling to woo their women and perpetuate the species in an unlikely urban habitat.”

Kirsten is one of 15 early-career scientists presenting their work to the public and media as part of Fresh Science 2004. The scientist who best meets the requirements of the program will win a study tour to the UK courtesy of British Council Australia.

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