BioEd Online

Frogs in the Australian metropolis of Melbourne are having trouble getting together to mate, and the culprit is traffic noise, according to Kirsten Parris, an ecologist at the University of Melbourne. One species of frog is even changing the pitch of its love song to be heard above the roar of the road, she reported on 20 August at the International Congress of Ecology in Brisbane, Australia.

Parris visited many urban ponds and pools inhabited by frogs, measuring traffic noise, which is, unfortunately, at the same low frequencies as many frog mating calls. For a frog such as the onomatopoeic 'pobblebonk' (Limnodynastes dumerilii), she found that a call that could originally be heard by a female 800 metres away may only carry 98 metres above 60 decibels of traffic noise, an average value for Melbourne.

She has also discovered that the southern brown tree frog (Litoria ewingii) seems to be compensating for the traffic noise by increasing the pitch of its calls¹ (listen to before and after calls).

Parris suggests that installing noise barriers at strategic points around a road could help urban frogs to hear each other. Creating habitats where they thrive — such as ponds with sloping rather than steep sides — would also make sense, she adds. "Cities provide some of the last habitat for a range of frog species around the world. So if we only worry about conserving frogs and their habitats outside cities, some of these frogs may well go extinct."

"Some frog species," she says, "are very sensitive to environmental changes", but "others are quite adaptable and can persist in urban habitats if we gave them a bit of help".

However, Kris Kaiser, an ecology graduate student at the University of California, Los Angeles, who has also looked at the effects of anthropogenic noise on amphibians, sounds a note of caution on the subject of these amphibians' adaptability: "Frogs, unlike birds, are thought to have the frequency of their calls somewhat constrained by their anatomy."

"There is often a relationship between body size and frequency of call," she says, so the creatures' ability to compensate for traffic noise may be limited.

Glenn Cunnington, a biologist at Carleton University in Ottawa, Canada, says he's also heard frogs shifting the pitch of their calls in response to noise pollution. But, he says, "direct mortality and habitat loss likely represent greater threats to these species in urban environments".

"I don't think anyone is arguing that noise is the number-one cause of amphibian declines," says Kaiser, "but I think it has been underrepresented in the literature, and I think it is a threat that will continue to grow as humans continue to fragment the landscape".