Sunday, 28 August 2011

Road noise reduces avian richness and abundance in a Costa Rican park

Without roads, we would be unable to visit some of the world's natural marvels. We would be less able to support parks and reserves with funding from ecotourism, inspire hearts and minds with nature, or easily conduct scientific studies in far-flung locations. Unfortunately, without roads, we also wouldn't be adding chemical and noise pollution to habitats, fragmenting pristine areas of land, generating unpleasant and sometimes unhealthy vibrations through the ground, or causing collisions that injure or even kill animals.

Time and again, these and other negative impacts of roads have been documented by scientific research in a variety of habitats. One of the most recent of these studies--and one of the first to examine the impacts of roads in tropical habitats--quantified the impact of traffic noise on the richness and abundance of bird species in Costa Rica's Carara National Park, the western side of which is bordered by one of the biggest and busiest roads in Costa Rica. There, the average distance between the road and the forest edge is 5 m, causing noise levels in the nearby forest to approach a very unpleasant 100 dB.

(Map of the Carara National Forest in Costa Rica)

In order to quantify the impact of road noise on avifauna, scientists from The School for Field Studies (Alajuela, Costa Rica) conducted surveys at 16 permanent sample points--8 along trails near the road, and another 8 along trails farther away. During 10-minute sampling efforts conducted twice a day during both the wet and dry seasons, they counted all birds seen or heard within a 30-meter radius; they also conducted mist-netting at 6 nets within the "near" and "far" habitats. In addition to measuring ambient noise at each of the bird count locations, the researchers tallied passing vehicles at different times of day and classified the vehicles as either "heavy" (e.g., trucks) or "light" (e.g., cars and motorcycles).

(Some of the lush habitat in Carara National Forest, Costa Rica)

As anticipated, sampling points closer to the road had significantly higher noise levels than those farther away. Eight fewer bird species were detected in the louder areas than in the quieter ones (89 vs. 97, with a total of 129 unique species counted throughout the entire study). Although this difference in avian richness was not significant, there was a significant difference in relative bird abundance: an average of only 6 birds in the loud sites, compared to 8 birds in the quiet sites.

(Red-capped manakin, Pipra mentalis)
Noise from the road was found to attenuate more during the wet season than during the dry season, reflecting the sound-dampening effects of the leaves. However, traffic levels were also found to fluctuate with season: More vehicles (particularly those classified as "light") used the road during the dry season.

Because the road is an important conduit for traffic moving along the coast, it can't be shut for the sake of the birds. So what can be done to prevent the highway from making the neighboring habitat less suitable for bird use? The authors' first suggestion is to implement speed limits, which not only would diminish noise levels, but might also reduce the number of collisions between animals and passing cars. They also discuss the installation of barriers along the road--an expensive, large-scale project, but one that would have the added benefit of making trail use more pleasant for the 33,000 human visitors who visit the park annually and who, presumably, do not want to hear traffic noise any more than the birds do. Given the fact that "heavy" vehicles were consistently found to be louder than "light" vehicles, it might also be a good idea to limit truck access to the coastal highway--especially during the dry season when there are fewer leaves in the forest to buffer the birds from road noise.

The broader implications of the study are clear: In tropical environments as well as more temperate areas, noise from roads can have negative impacts on local bird communities. Where possible, managers should try to alter the roads and/or their traffic patterns in order to protect wildlife. In places where roads have not yet been built but have been suggested, hopefully planners can find a way to avoid sensitive habitats or include features that dampen sound transmission. In some places, voters may have the opportunity to prevent construction of proposed roadways, either by voting on initiatives themselves or by pressuring their lawmakers. Hopefully everyone will see the advantage of driving a few extra miles around important bird habitat, rather than taking a noisy shortcut that takes them too close to sensitive species.

Arévalo, J.E. and Newhard, K. 2011. Traffic noise affects forest bird species in a protected tropical forest. Rev. Biol. Trop. 59(2):969-980.

Thanks to the following websites for providing the images used in this post:


  1. This comment has been removed by the author.

  2. Another factor that may be reducing bird presence near the highway is higher predator's presence. The highways may facilitate access to predators.

    Interesting study!

    Speed limits should be enforced not only by signs, but also by placing structures that force vehicles to decrease their speed. Otherwise the speed may only change when there is a traffic officer present in the area, which is seldom possible.

    I agree that this enforcement in speed limit is justified not only by the findings of this study. First, for tourist safety, either when they are at the bridge to observe crocodiles, or when they are either at the bridge or along the road to observe flying macaws. Tourism is one of the main income sources in Costa Rica, therefore this enforcement is priority. Second, to allow vehicle entrance to the National Park, as currently it is quite difficult due to traffic speed and insufficient road signs. And third, to decrease wildlife mortality by collisions; even highly endangered species have been killed by traffic (including felines). Carara is a very important protected area as there are wildlife species present that are rare or endangered, not just in Costa Rica but in the world (manakins, spider monkeys, scarlet macaws, felines, etc.). In fact, there are tourists that come to the country only to visit this area in particular.

    Therefore, the enforcement of the speed limit is priority for economy reasons, conservation and people safety. It should be a priority for the Costa Rican Road Commission (MOPT: Ministerio de Obras Públicas y Transportes).

    Ma Isabel Di Mare, Ph.D. "Tati"