We hear a lot about the many ways that we humans pollute our environment — ocean acidification, plastic trash accumulating in ocean gyres and on beaches, filthy air in cities and near factories, dirty water from industry and agriculture — the list of damage is enormous. All those (and more) affect the health, behavior, ecology, and survival, not only of humans, but also of all animals.
There’s another kind of pollution, however — one that we hear much less about — and that’s noise. We can sense something of the effects of noise, in a small way, just by going into a crowded restaurant where everyone is talking. That background makes it hard to hear distinctly, so everyone speaks more loudly, which makes it still harder to hear, and pretty soon everyone is shouting — everybody except the quiet couple in the corner who are trying to have an intimate conversation. I can’t even contemplate what it must be like to live near some noisy industrial complex or a busy highway or airport or a clattering railway, with nearly continuous noise bombarding the ears.
An increasing body of research is showing a variety of ways in which noise affects wildlife — their activity patterns, foraging success, reproductive patterns, and habitat use. Here are some examples.
Most bats use their acute hearing to locate and chase down their insect prey. Studies of two different species showed that when the bats were foraging in noisy conditions near a busy highway or an industrial compressor, their time spent foraging increased greatly, and their hunting success was halved.
The foraging of owls can also be significantly impaired near various sources of noise. Many species of owl forage principally at night, chiefly by using their extremely sensitive ears to detect and capture small mammals such as mice. In these owls, the right and left ears are shaped and located asymmetrically, giving them the ability to locate prey very precisely in both the horizontal and vertical planes. Experiments have demonstrated that noise interferes with the saw-whet owl’s ability to detect, attack, and capture mice: each decibel of increasing noise diminished all three phases of hunting by roughly 10 percent! Above the decibel level of a normal human conversation, the owls caught no prey at all. Long-eared owls and short-eared owls are similarly affected.
Fish sometimes also endure negative effects of noise. For example, three-spined sticklebacks were less successful in foraging when their environment was noisy. Loud noises are also reported to disrupt fish shoals, breaking up the protective aggregations and exposing them to predation.
The greater sage grouse lives in the sagebrush plains of Wyoming and nearby states; they are a native species of conservation concern. Males gather at display grounds called leks to show off and attract females, which visit the lek and “shop around” before choosing a male as a mate. Experiments using playbacks of road traffic or gas drilling noise reduced the number of males coming to leks by up to 75 percent, compared to quiet leks. Furthermore, the few males that remained had higher-than-normal levels of stress hormones. Stress is known to affect behavior in many ways and can impair reproductive success. The effect of these changes on female sage grouse has not yet been reported. However, other studies have shown that industrial noise reduced the ability of songbird males to attract females, so pairing success was reduced. Furthermore, in some cases, fewer eggs were laid and fewer fledglings were produced.
Several studies have shown that the abundance of songbirds is reduced near noisy industrial complexes, such as natural-gas extraction fields, as compared to quiet areas in nearby otherwise-similar habitat. Also, an experiment was set up in Idaho in a roadless area that had been used traditionally by migrating birds; a row of speakers created a “phantom road” that blasted out road-traffic noise during the fall migration season. After the phantom road was set up, a third of the migratory bird species just left the area entirely, and the remainder failed to gain the usual amount of fat needed for further migration.
In the American southwest, scrub jays are important consumers of pinyon pine nuts. The jays sometimes eat the nuts directly but they also cache them for later consumption. Not all the caches are retrieved, and some of the seeds germinate. Near an industrial site in New Mexico that made continuous noise, the scrub jays moved out; pine seed dispersal there was much reduced, so seedling recruitment to the pine population was diminished in that area, compared to a quiet area in similar habitat. In this case, the noise in the environment was having an impact not just on the jays but also a ripple effect through the community: changes in the density of pines would be associated with changes in the animals and plants that grow there.
Environmental noise can have a measurable effect on the vocalizations of wildlife. Several studies suggest that male birds of various species sing at higher pitches in noisy areas; however, one study indicates that females prefer males with lower voices. The mismatch of higher songs with the female preference for lower songs could clearly impair mating success and reproduction.
Harbor seal males defend underwater territories with roars, which may also attract females. In Glacier Bay, a recent study shows that the roars of harbor seal males are louder, shorter, and at higher pitches when there is cruise ship noise. This may impair the seals’ reproductive success.
The effects of military sonar on whales are now well documented. Sonar can generate extremely loud sounds, over two hundred decibels; these sounds can travel through water for hundreds of miles. Sometimes the whales are injured and bleed from the head. Many cases have been recorded of whales trying to avoid sonar noise; at the very least, their foraging has been interrupted. In some cases, dozens of whales have stampeded away from noise and stranded themselves on beaches, where they often die.
There is no doubt that noise can dramatically impact wildlife behavior and ecology in several ways. However, except in the cases of lethal effects, seldom have the full repercussions been documented. Nevertheless, noise is generally detrimental to wildlife well-being. When foraging is impaired, there are likely to be effects on health and survival. When mating is impaired, there are surely effects on population size. Furthermore, as human noise production continues to grow and spread, there are likely to be evolutionary changes in animal behavior, but we don’t know if the rate of adaptation can keep up with the rate of increase of noise.
It seems to me that humans are the noisiest creatures on the planet. We make more noise, more kinds of noise, in more places, more continuously throughout the day and the year, than any other species. The effects of all that racket on animal behavior and ecology are pervasive and widespread; the ultimate effects on survival and reproductive success of individuals and on populations need to be studied and understood.
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Friday. Her essays can be found online at onthetrailsjuneau.wordpress.com.