Abstract
The construction of roads near protected forest areas alters ecosystem function by creating habitat fragmentation and through several direct and indirect negative effects such as increased pollution, animal mortality through collisions, disturbance caused by excessive noise and wind turbulence. Noise in particular may have strong negative effects on animal groups such as frogs and birds, that rely on sound for communication as it can negatively interfere with vocalizations used for territorial defense or courtship. Thus, birds are expected to be less abundant close to the road where noise levels are high. In this study, we examined the effects of road traffic noise levels on forest bird species in a protected tropical forest in Costa Rica. Data collection was conducted in a forest segment of the Carara National Park adjacent to the Coastal Highway. We carried out 120 ten minute bird surveys and measured road noise levels 192 times from the 19th to the 23rd of April and from the 21st to the 28th of November, 2008. To maximize bird detection for the species richness estimates we operated six 12 m standard mist nets simultaneously with the surveys. The overall mist-netting effort was 240 net/h. In addition, we estimated traffic volumes by tallying the number of vehicles passing by the edge of the park using 24 one hour counts throughout the study. We found that the relative abundance of birds and bird species richness decreased significantly with the increasing traffic noise in the dry and wet season. Noise decreased significantly and in a logarithmic way with distance from the road in both seasons. However, noise levels at any given distance were significantly higher in the dry compared to the wet season. Our results suggest that noise might be an important factor influencing road bird avoidance as measured by species richness and relative abundance. Since the protected forest in question is located in a national park subjected to tourist visitation, these results have conservation as well as management implications. A decrease in bird species richness and bird abundance due to intrusive road noise could negatively affect the use of trails by visitors. Alternatives for noise attenuation in the affected forest area include the enforcement of speed limits and the planting of live barriers.
Highlights
Clevenger et al 2003, Forman et al 2003)
Bird abundance and road noise: Overall, we recorded a total of 129 species of birds during the entire study (Appendix 1 for the species list), with 89 species recorded in the “close” trail compared to 97 in the “far” trail site
Species richness significantly decreased with road noise both in the dry and the wet seasons (ANCOVA, F1,30=9.84, p=0.0041, Fig. 2)
Summary
Clevenger et al 2003, Forman et al 2003). As a result, animal populations can be isolated due to either the impediment to their movements (e.g. mortality) or their avoidance behavior towards roads (Jaeger et al 2005). Road-derived noise often masks the vocalizations produced by some species of birds whose call frequency overlaps with that of the traffic (Parris & Schneider 2009). There has been a recent notable increase in the number of articles that address the effects of trafficked roads on wildlife, the vast majority of those have been conducted in temperate zones (Parris & Schneider 2009, Fahrig & Rytwinski 2009, Baskaran & Boominathan, 2010). We hypothesize that bird species in a tropical forest would be affected by traffic noise as discussed above. If this were the case, we predict that bird abundance and species richness will be inversely related to road noise. Since the results presented here have conservation as well as management implications, we discuss alternatives for noise attenuation in the affected zone
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