Wildlife struggle in an increasingly noisy world

We live on an ever more-populous planet, pulsating with human-generated noises of every description. The most ubiquitous noise-making structures we produce are traffic-laden roads (Fig. 1), which already criss-cross much of the Earth and are projected to increase in length by some 25 million km by midcentury: enough to encircle the planet more than 600 times (1). For wildlife, the challenges of living in a world increasingly swamped by such infrastructures are only going to worsen.

Fig. 1.

Logging truck in Borneo. Image courtesy of Rhett Butler (Mongabay.com). (Inset) Two-minute sonogram illustrating the frequencies (vertical axis) and durations (horizontal axis) of sound from 32 passing vehicles. Image courtesy of LA Listens, originally published on CoLab Radio.

Road ecology as a discipline (1–4) has been galvanized by the realization that we are presently experiencing the most explosive era of infrastructure expansion in human history (1). A growing body of research is revealing the myriad ways that roads can affect wildlife and ecosystems, sometimes opening a Pandora’s box of environmental problems, such as illegal hunting, encroachment, wildfires, and land speculation (5–7). Even where such activities are controlled, roads can still cause marked local changes in the abundance and behavior of wildlife, via edge effects, road kill, and vehicle noise and pollution (2–4).

One of the challenges facing road ecologists is to understand the specific causes and mechanisms underlying the highly varied responses of different wildlife species to roads (8). Are certain species shunning roads and their vicinity because of a desire to avoid road clearings or edge effects, or do they fear the sound or sight of fast-moving vehicles? And, are species that persist near roads relatively healthy and unaffected, or do they suffer from stresses provoked by recurring road-related disturbances?

In PNAS, Ware et al. (9) use a “phantom road” approach to disentangle the effects of road noise from other consequences of road creation. The authors established a ∼0.5-km-long array of loudspeakers along a ridgeline in a conifer forest-shrubland mosaic in southwestern Idaho, and then played road noises at a volume comparable to a suburban street (increasing ambient noise from an average of 37 dB to a louder 48 dB). Along their phantom road, they captured migrating songbirds in the autumn, using mist-nets, as the birds were moving southward toward their wintering grounds.

Ware et al.’s (9) study elegantly incorporated two types of experimental control. The first was a matched ridgeline in the same area that lacked road noise. For the second control, they turned the speakers on their phantom road on and off at 4-d intervals, so that the road effectively appeared and disappeared acoustically. This process allowed them to infer the effects of road noise on the abundance and behavior of birds with an unusually high degree of confidence.

Another key element of the Ware et al. (9) study is that they focused on migrating birds. Migration is an energetically stressful time, when individuals need to maximize their foraging and resting times during brief feeding stopovers while avoiding predators. On their fixed breeding territories, some bird species can partially overcome road noises by shifting the frequencies of their calls (10); some breeding frog species do likewise (11). But for actively migrating birds, the options are limited: aural cues, such as vehicle sounds, cannot be ignored because they could mask the sound of an approaching predator. Hence, continuous or unpredictable road noises might force stressed-out migrating birds to spend more time being vigilant and less time foraging, thereby increasing their risk of starvation.

The study by Ware et al. (9) produced several notable findings. First, based on mist-net captures, overall bird abundance declined by 31% on average near the phantom road, with 5 of 21 captured species exhibiting significant declines in abundance. Second, even among individuals that chose to remain near the pseudo-road, body condition declined substantially: by one full SD, on average, compared with unperturbed birds.

The notion that road noises can disrupt the foraging behavior of birds was bolstered by a laboratory experiment on white-crowned sparrows, one of the more common migrators that suffered lower body condition near the phantom road. In the laboratory, birds exposed to increasingly intense road noises (55 and 61 dB) spent progressively more time being vigilant (raising their heads and looking around) and less time feeding than did birds without road noise (32 dB). Moreover, the authors (9) found no evidence that the sparrows habituated to simulated road noises, suggesting that learning to ignore certain noises is too risky a strategy when one is migrating through a potentially predator-rich environment. Other conceivable explanations for the authors’ findings (such as a possible reduction in insect prey in noisy places) seem less compelling, given that they found that both fruit- and insect-eating birds were negatively affected by road noise.

The phantom-road study by Ware et al. (9) suggests that the rapidly expanding footprint of roads and other infrastructure across the planet might be invisibly degrading habitat quality for noise-sensitive species. Notably, there is no reason to presume that these findings would be confined only to conventional road systems. For example, might sensitive marine species, such as echolocating cetaceans and migratory fish, avoid noisy regions, such as high-volume shipping lanes or areas where naval vessels regularly pierce the oceans with high-intensity sonar? Could echolocating bats be distressed by roaring airplanes or even by the steady whine of wind farms or other infrastructure (12)? For that matter, might even hiking trails frequented by quiet ecotourists or researchers reduce local wildlife activity, as has been observed in protected areas in California (13) and Sumatra, Indonesia (14)?

Another intriguing possibility is that species that use low-frequency infrasound for long-distance communication—such as elephants and cassowaries (15)—might be especially vulnerable to road noises. Low-frequency sounds travel further than do those at higher frequencies and are less likely to be blocked by vegetation and other obstacles. For species that use infrasound, the halo effects around roads could potentially be enormous in extent. This seems broadly consistent with the observation that bird species that produce low-frequency calls show stronger avoidance of roads than do those that call at higher frequencies (16).

Despite the intriguing implications of the Ware et al. (9) study, it is apparent that vehicle noise is not the only thing that can induce wildlife to avoid roads. In the Amazon rainforest, for example, even narrow dirt roads with very little traffic (<five vehicle passes per day) can markedly reduce the local abundance (17) and road-crossing movements (18) of some understory bird species. Even more dramatically, wider clearings (∼200 m), such as those typical of major highways, can completely halt the movements of strongly forest-dependent species, preventing experimentally translocated birds from returning to their territories and lifelong mates (19).

And of course, when new roads lead to an influx of poachers and encroachers, as happens far too often in remote frontiers and many developing nations, all bets for wildlife are off. In the Brazilian Amazon, 95% of all forest destruction occurs within 5 km of a road (20), whereas in the Congo Basin, signs of hunters increase—and forest elephants decline—up to 40 km away from major roads (6). These are among the worst evils in the Pandora’s box of road impacts.

However, the findings of Ware et al. (9) suggest that, even where the effects of roads are far more controlled, their impacts on wildlife can be significant. This is an alarming prospect in a world ever more beset by human-induced noises. For example, nowhere in Costa Rica’s iconic La Selva Biological Reserve can one avoid hearing the incessant thrum of a nearby highway. Many other nature reserves are suffering a similar fate, underscoring the urgency of limiting new roads in protected areas and devising strategies to limit noise disturbances where roads already exist. For wildlife and for humans too, quiet places on the Earth are becoming increasingly rare and precious.