An ongoing research project from the National Park Service Natural Sounds Program is about to publish a groundbreaking paper that outlines the many ways that even moderate increases in human background noise can create major impacts on animals.  The study proposes a new metric for use in bioacoustics research, the “effective listening area.”  This is the area over which animals can communicate with each other, or hear other animals’ calls or movements; as might be expected, animals focus especially on listening for sounds at the very edges of audibility, so that even a small increase in background noise (from a road, wind farm, or regular passing of airplanes) can drown out sounds that need to be heard.  The authors note analyses of transportation noise impacts often assert that a 3dB increase in noise – a barely perceptual change – has “negligible” effects, whereas in fact this increased noise reduces the listening area of animals by 30%. A 10dB increase in background noise (likely within a few hundred meters of a road or wind farm, or as a private plane passes nearby) reduces listening area by 90%.

In addition to introducing this important new metric, the paper provides a good overview of previous research that has addressed the impacts of moderate noise on various animals, including bats, antelope, squirrels, and birds.  The paper will be published next year, though an “in press” version is available for download.  A recent BBC article also covered this important new work.  A full detailed lay summary of this paper, as previously published on AEI’s science research page, appears below the fold:

Animal listening area and alerting distance reduced substantially by moderate human noise

Barber, Crooks, Fristrup. The costs of chronic noise exposure for terrestrial organisms. Trends in Ecology and Evolution, 2010. [ARTICLE IN PRESS AVAILABLE HERE]

This is one of the most important and interesting papers published this year. It emerges from the partnership between the National Park Service’s Natural Sounds Program and researchers at Colorado State University, who have been working together for several years to create metrics that are more useful for assessing the biological impacts of moderate human noise on wildlife. The assessments presented here, along with the paper’s review of recent studies documenting changes in foraging, anti-predator behavior, reproductive success, density and community structure in response to noise, form the basis for a much more cogent assessment of noise impacts on wildlife.

The focus of this research is the ways that moderate increases in background noise can affect animals. The authors note that both air and road traffic in the US has tripled in recent decades; even in National Parks, noise is audible 25% of the time in more than half of sites assessed. Over 80% of US land area is close enough to roads for some low-frequency masking to occur, with passing trucks and motorcycles increasing the sound levels at 1km to 40dB(A).

One of the new proposed metrics is the effective “listening area.” Changes in listening area has impacts involving both communication and what the authors term “adventitious” sounds (sound made largely by movement, which are important in predator/prey awareness). For predators, the listening area is the area over which they might hear the rustling of their prey, and for prey, it is the area in which they are alerted to approaching predators, whether wingbeats or footsteps; in each case, the most important sounds are those right at the edge of audibility. Social groups rely on hearing each other’s alarm calls or songs inviting mating; reduced listening areas for these calls result in effectively reducing the number of individuals in any given communication network. Each animal has its own auditory acuity, so can hear at different distances; but for all animals, increasing background noise will reduce their natural listening area in predictable ways. The authors note analyses of transportation noise impacts often assert that a 3dB increase in noise – a barely perceptual change – has “negligible” effects, whereas in fact this increased noise reduces the listening area of animals by 30%. A 10dB increase in background noise (Ed. note: likely within a few hundred meters of a road or as a private plane passes nearby) reduces listening area by 90%.

Throughout, the authors stress the difficulties in separating noise-related effects of traffic from other related forces, such as habitat fragmentation or threat responses; there is also a fascinating short section citing studies showing behavioral changes in response to quiet recreation, sometimes even more dramatic than those caused by vehicles. Though only a few studies have directly addressed this ambiguity, many studies cited here provide strong indications that acoustic masking is a significant factor in behavioral changes near roads. Among the findings that caught my eye/ear:

• Masking affects not only audibility, but understanding: “thresholds for discrimination between calls of the same species were consistently higher than were detection thresholds for the same calls.” Not surprising, but easy to forget: background sound often obscures the words being said, though we can still hear the voice.
• Bats that listen for ground movements of their prey hunt more in quiet areas than noisy ones; similarly insect-eating birds are more likely to avoid noisy areas than other birds.
• Masking can also make it more difficult for animals to tell what direction a call (such as a mating call) is coming from
• Pronghorn antelope showed a marked shift in proportion of time spent foraging and in vigilance (looking around) when closer to roads: foraging dropped from 45% of the time to 35%, while vigilance increased from 40% to over 50%.
• Two key studies of increased vigilance in clearly noise-triggered contexts: Ground squirrels showed a marked increase in vigilance behavior when hearing squirrel alarm calls at a site in a wind farm than in a quiet site (including a slightly less “relaxed” non-vigilance baseline state), and a lab study with chaffinches found that the mean time spent pecking (eating) between times scanning the area decreased when noise was introduced.

The authors conclude by stressing: “Chronic noise exposure is widespread. Taken individually, many of the papers cited here offer suggestive but inconclusive evidence that masking is substantially alter- ing many ecosystems. Taken collectively, the preponderance of evidence argues for immediate action to manage noise in protected natural areas….The costs of noise must be understood in relation to other anthropogenic forces, to ensure effective mitigation and efficient realization of environmental goals. Noise pollution exacerbates the problems posed by habitat fragmentation and wildlife responses to human presence; therefore, highly fragmented or heavily visited locations are priority candidates for noise management. Noise management might also offer a relatively rapid tool to improve the resilience of protected lands to some of the stresses imposed by climate change.”