By Matt Walker
Editor, Earth News
Owls must hear to hunt
Noise pollution is becoming a major threat to the welfare of wildlife, according to a scientific review.
Sounds produced by vehicles, oil and gas fields and urban sprawl interfere with the way animals communicate, mate and prey on one another.
The sounds are becoming so ubiquitous that they may threaten biodiversity, say the review's authors.
Even the animals living in protected National Parks in the US are being exposed to chronic levels of noise.
Writing in the journal Trends in Ecology and Evolution, three scientists based in Fort Collins, Colorado, US detail the extent to which noise pollution is now harming wild animals.
In Canada, the number of frog species falls as traffic density increases
In Africa, the species richness of primates and carnivores falls within 30m of roads
In The Netherlands, 60% of woodland bird species avoid roads
Dr Jesse Barber and Dr Kevin Crooks of Colorado State University and Dr Kurt Fristrup of the US National Park Service reviewed all recent scientific studies examining the issue.
They found that man-made noise is already causing a catalogue of problems.
"Many animal species evolved hearing sensitive enough to take advantage of the quietest conditions; their hearing is increasingly compromised by noise," Dr Barber told the BBC.
That intrusion can have a significant impact on the way wild animals communicate.
Great tits (Parsus major) sing at higher frequencies in response to urban noise, so they are better able to hear each other.
But not all animals are able to adapt in this way.
Urban great tits have to sing at a higher pitch
Female grey tree frogs (Hyla chrysoscelis) exposed to the sounds of passing traffic take longer to locate and find calling males, while European tree frogs (Hyla arborea) call less overall.
Crucially, both species appear unable to change their calling habitats to overcome the din from the roads, potentially compromising their ability to reproduce.
Noise pollution can also effect the ability of many animals such as owls and bats to find and hunt their prey.
Laboratory studies have shown that gleaning bats, which locate prey by the sounds they make, avoid hunting in noisy areas.
That can place gleaning bats at a higher risk of extinction, as noise pollution increasingly corrupts once habitable areas.
83% of land on the continental US is within just over 1km of a road
At this distance, the sound of an average car is 20dB
At the same distance, average trucks and motorcycles project 40dB of noise
For example, one gleaning bat species, the Bechstein's bat (Myotis bechsteinii), is less likely to cross roads than other bat species that forage in open areas, suggesting the noise of the traffic could fragment their hunting grounds. The bat occurs across Europe including in the south of the UK.
In the Amazon, terrestrial insectivores, which also hunt using sound, especially avoid areas where roads are being constructed.
"Noise pollution is so ubiquitous that it may be a factor in some large-scale declines in biodiversity," says Dr Barber.
The problem appears to be getting worse.
In the US alone, between 1970 and 2007, the US population increased by approximately one-third.
Traffic on US roads tripled, to almost 5 trillion vehicle kilometres per year, while air traffic also more than tripled between 1981 and 2007, say the reviewers.
Shipping noise has similarly increased, according to recent reviews of the effects that artificial noise has on marine mammals such as whales.
Gleaning bats rely on picking up the telltales sounds made by prey
Even National Parks are becoming increasingly affected.
Despite being protected against the sprawl of towns and cities and other forms of development, noise carries into the parks from surrounding roads and planes flying overhead.
"Quiet places are especially vulnerable to noise intrusions, because even distant sources can have an impact," says Dr Barber.
Systematic monitoring by the Natural Sounds Program, a research exercise carried out by the US National Park Service, confirms the extent of the noise intrusion.
Noise is audible during more than one quarter of daylight hours at more than half of 55 sites in 14 National Parks studied to date.
At 12 sites, anthropogenic noise can be heard more than half the time.
SUFFER THE ANIMALS
Birds with lower frequency calls are more likely to avoid traffic noise
The din from overhead aircraft disturbs the behaviour of harlequin ducks and goats
Greater sage grouse and mule deer steer clear of noise-producing oil and gas developments
Much more needs to be done to mitigate the problem, says Dr Barber.
"Noise mitigation techniques include quieter road surfaces, noise barriers, appropriate signage in protected areas and most importantly, restriction of motorised travel in protected natural areas," he says.
Otherwise, there may be few quiet places left.
"Naturally quiet backgrounds are an imperilled acoustical resource in many parts of the world," Dr Barber says.