Environmental correlates of avian diversity in lowland Panama rain forests

Abstract

Aim The composition of communities is known to be influenced by biogeographical history, but also by local environmental conditions. Yet few studies have evaluated the relative importance of the direct and indirect effects of multiple factors on species diversity in rich Neotropical forests. Our study aims to assess drivers of change in local bird species richness in lowland tropical rain forests.Location Thirty-two physiographic subregions along the corridor of the Panama Canal, Panama.Methods We mapped the distributions of all forest-dwelling bird species and quantified the environmental characteristics of all subregions, including mean annual rainfall, topographic complexity, elevational variability, forest age and forest area. Plant species richness, believed to be correlated with structural complexity, was estimated by interpolation through kriging for subregions where data were unavailable.Results The study region has a strong rainfall gradient across a short distance (65 km), which is also accompanied by steep gradients in plant and bird species diversity. Path analysis showed that precipitation strongly affected plant species diversity, which in turn affected avian diversity. Forest age and topography affected bird diversity independently of plant diversity. Forest area and its proportion occurring in the largest two fragments of each subregion (habitat configuration) were also positive correlates of bird species richness.Main conclusions Our results suggest that plant species richness, known to be influenced in part by biogeographical history and geology, also affects bird species assemblages locally. We provide support for the hypothesis that bird species richness increases with structural complexity of the habitat. Our analysis of the distributions of the region's most disturbance-sensitive bird species showed that subregions with more rainfall, more complex topography and older forests harboured not only richer communities but also more sensitive species; while subregions with the opposite characteristics usually lacked large fractions of the regional forest bird community and hosted only common, widely distributed species. Results also emphasize the importance of preserving forest diversity from habitat loss and fragmentation, and confirm that larger, continuous forest tracts are necessary to maintain the rich avian diversity in the region.

Causes of habitat loss in a Neotropical landscape: The Panama Canal corridor

“…In our study area, low mean annual precipitation means drier seasonal forests compared to thicker "jungles" in more humid subregions (Condit, 1998;Rompre et al, 2007). Likewise, in Amazonia, deforestation is greater in seasonal areas and regions with low precipitation (Fernside and Ferraz, 1995;Steininger et al, 2001;Laurance et al, 2002a).…”

“…Our results, excluding Colon, which is the only large city on the Caribbean coast of the entire country (Suarez, 1981;MIVI, 2005), indicate urbanization clearly occurs in the drier subregions of the study area. Complex topography is often associated with more mature-intact forests than areas with flatter terrain, because ease of access for harvesting and development is limited (Kerr and Packer, 1997;Steininger etal., 2001;Veldkamp and Lambin, 2001;Laurance et al, 2002a;Rompre et al, 2007). In this study, areas with flatter terrain tend to occur on the drier slope where forests are usually younger and more disturbed by human activities (Condit, 1998;Pyke et al, 2001).…”

“…We calculated mean annual rainfall for each subregion from the ACP data (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003). Rain gauges from the ACP were not available for all subregions, therefore we interpolated precipitation data from isohyets available from atlases; these data were concordant with other studies in the same area (Condit, 1998;Pyke et al, 2001;Santiago and Mulkey, 2005;Rompre et al, 2007). Variability in elevation was determined by calculating maximum altitude above sea level (ASL) in each subregion minus minimum altitude (ASL), which was often the altitude of the canal waters.…”

“…Biodiversity increases from the Pacific to the Caribbean along a steep rainfall gradient (Condit, 1998;Condit et al, 2001;Pyke et al, 2001). For example, forest bird species richness increases from 4 dry-forest specialists to more than 170 wet-forest specialists along this gradient (Ridgely and Gwynne, 1989;Angehr, 2006;Robinson et al, 2004;Rompre et al, 2007). The same pattern occurs in tree species richness (Condit, 1998;Pyke et al, 2001).…”

“…Studies of birds, for example, show that both fragmentation and habitat loss reduce diversity (Robinson et al, 2000(Robinson et al, , 2004Laurance et al, 2002b;Gaston et al, 2003;Waltert et al, 2005;Rompre et al, 2007). Major factors associated with fragmentation and habitat loss, and therefore loss of biodiversity, are high levels of human population density (Cincotta et al, 2000;McKee et al, 2003;Scharlemann et al, 2004), density of roads (Maki et al, 2001;Laurance et al, 2002aLaurance et al, , 2006Gutzwiller and Barrow, 2003;Glennon and Porter, 2005), changes in agricultural practices (Meyer and Turner, 1992;Huston, 1993), logging (Kohleretal., 2002;Rudel, 2005), and hunting (Brashares et al, 2001;Laurance et al, 2006).…”