Traffic noise killing off frogs

By Melissa Leong May 7, 2010
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The din from urban traffic is drowning out the mating calls of some Australian frogs.

SOME HIGHLY VOCAL species of frogs are sensitive to urban noise pollution. A new study shows it is affecting breeding cycles and that some frogs are increasing the pitch and frequency of their calls to be heard above the din.

“Being heard is important”, says ecologist Dr. Kirsten Parris at the University of Melbourne. “If the females can’t hear the male frogs then they have less chance of breeding successfully.”

As she will report at an upcoming conference, Kirsten has discovered that traffic noise in Melbourne is drowning out the mating calls of male pobblebonk frogs (Limnodynastes dumerilii). Historically, female pobblebonks can hear these calls from over 800 metres away, but their audible range has now been decreased to 14 metres, she says. Kirsten’s research was also detailed in the journal Ecology and Society last year.

Similarly, the southern brown tree frog (Litoria ewingii), also found in Melbourne, has to adapt to the noise. It ups its audible range from 19 metres to 24 metres in noisy urban environments by increasing the pitch of its call. Still, its usual range is 75 metres.

Adverse effects

Previous research has already shown the negative effects of urban noise pollution on wildlife. Exposure causes increased stress in mammals and alters the songs of birds and mating habits of some North American frogs.

A second study carried out by ecologists Dr. Conrad Hoskin and Dr. Miriam Goosem from James Cook University in Cairns, Queensland, discovered that smaller male common mist frogs (Litoria rheocola), seem to cope better with traffic noise pollution as compared to their larger counterparts because of their higher-pitched calls.

The study was conducted in and around a stream that runs alongside a highway in Kuranda, Queensland. The calls of 41 male common mist frogs were recorded. The research revealed that the size of males increases in populations living towards the forest’s interior, while the frequency and pitch of calls increases in populations that live nearer to the road.

Body size link

Miriam says that “body size influences call pitch with smaller frogs often calling at a higher pitch than larger individuals of the same species. It’s possible that larger males that can outcompete the smaller individuals choose stream habitat further from the road and the source of traffic noise.”

Besides noise, some species such as the endangered growling grass frog (Litoria raniformis) are threatened by habitat loss and drought, which hinder successful breeding. It’s plain that conservation policies have to be set up, and fast.  Leading the way are Kirsten and her team from the University of Melbourne, who are working towards developing a blueprint for the regional conservation of growling grass frogs by focussing on sound management policies to protect them in Melbourne’s growth areas.
“It’s often said that frogs are our canary in the coalmine, providing an early indication that all is not well in the environment”, Kirsten says. “Well, these canaries are falling off the perch, so maybe it’s time we started heeding the warning”.