What are the biggest threats faced by wild animals living in human habitats? Ask a random sampling of people, and you’ll be sure to hear a few culprits mentioned repeatedly-habitat destruction, pesticides, hungry domestic pets, poachers.
All of these are correct, and increasing awareness of these issues has led to the development of many successful laws and mitigation measures aimed at protecting a variety of species, great and small. However, despite the fact that it is pervasive, there is another important threat that most people are unaware of; not only can it impact animals throughout their lives, but it can also have equally negative effects on humans. This threat is noise – specifically, anthropogenic noise, or noise caused by humans.
Loud noise damages hearing abilities. Probably the most obvious impacts of noise are hearing impairment and deafness-as anyone who has stood too near the speakers at a loud concert will tell you. These maladies result when sound waves cause physical damage to one or more parts of the inner ear. Injuries may stem from single, extreme acoustic traumas (which are not often a problem for wildlife), or from chronic exposure to moderate to high intensity noise. Over time, this can lead to perforated eardrums and the irreplaceable loss of hair cells, which are required to detect the presence of sound waves. In nature, even short-term hearing deficiencies-such as those caused when a passing vehicle “masks” other sounds-can be problematic; animals that can’t detect predators will be easy prey, while those that fail to hear a potential mate might miss out on a breeding opportunity.
Loud noise influences behaviour. As demonstrated by the pivotal great tit study, animals in noisy environments may act differently than those in quieter areas, and this may not always be to their advantage. Among acoustically communicating animals, individuals may attempt to counter the effects of noise by vocalising at a different pitch, at a higher volume, over a different length of time, or even, as seen in European robins, at a different, quieter time of day. This sounds like a clever solution to the noise problem until you consider the fact that the males of many of these species use acoustic signals to attract mates; variations in signal properties might make them seem less attractive to females, and therefore might reduce their breeding success. This possibility is receiving increasing support from both laboratory and field studies on a variety of species.
Because animals often rely on acoustic cues to alert them to the presence of a predator, many species seem more nervous in noisier environments. Both rats and chaffinches, for instance, spend less time eating and more time scanning for potential danger when exposed to ambient noise. Over extended periods of time, this could lead to nutritional deficits. Predators may also be adversely impacted by noise; studies on bats have shown that highway traffic noise reduces their foraging efficiency. Recent work among avian communities breeding near natural gas extraction facilities found that nest predation rates were lower in noisier sites due to the absence of predatory western scrub jays. Thus, the presence of sound pollution seems to cause some species to completely vacate affected habitats-to the benefit of those who stay behind.
Noise can alter community structure. The story of the western scrub jays has one additional complexity: These birds are a key disperser of pinyon pine seeds, and therefore play an important role in shaping the habitat. Areas abandoned by these birds will have fewer new pine saplings, likely allowing other species to take root instead. This could ultimately lead to a complete restructuring of the community.
Despite the length of time over which noise research has been conducted, this threat is not understood nearly as well as, for instance, habitat loss or poaching. That’s because much of the work has been done by scientists working in isolation, and on a diverse array of focal organisms. Further, many studies have been conducted under laboratory conditions, and investigate the effects of noises unlike those that would actually be experienced in anthropogenic environments. As a result, while our current knowledge offers tantalising glimpses of the effects that noise may have, more work is needed to connect the patterns reported in scientific literature to what is actually happening in the wild, and to which species.
From a conservation perspective, the most important question is whether noise pollution reduces the health and reproductive success of wildlife-the two factors responsible for maintaining the success and stability of populations over the long-term. Studies on great tits and eastern bluebirds suggest that, for these species at least, the answer is “yes.” This indicates that managers and conservationists need to take noise into consideration when approving land use schemes and developing mitigation plans. For instance, they might veto installation of a noisy road near or through a national park, or at the very least require that it be flanked by sound-reducing walls. Lawmakers might consider imposing volume restrictions-a tactic that has been employed in industrial settings in order to improve employee welfare. Engineers could possibly redesign machinery so that it meets desired noise standards. In wilderness areas, humans and their noisy activities might even be banned during the breeding season, when animals are likely to be most sensitive to acoustic disruption.
There are a variety of potential solutions to the noise problem, and given human sensitivities to sound pollution, many will likely benefit us as well as wildlife. In order to develop good plans for minimising noise, it will be vital for researchers to collect more data on noise-exposed animals living in the wild. Perhaps even more important, though, is making people aware of the harmful effects of their acoustic activities. Rachel Carson’s 1962 book Silent Spring helped protect wildlife by alerting the public to the harmful effects of pesticide use; a campaign to highlight the negative impacts of sound pollution might be similarly effective in avoiding an equally dangerous noisy spring.
Photographs: Michael Meinhardt, Gamma infinity