The effect of willow proximity on species richness and abundance of birds nesting in aspen <i>Populus tremuloides</i> woodlands.

Abstract

Riparian zones are widely recognized as important sources of high species diversity. High levels of food resources and habitat complexity in riparian zones attract large numbers of avian species, particularly in arid regions. However, less is known about the importance of riparian proximity in wet montane regions. The landscape near the Rocky Mountain Biological Laboratory (RMBL) in Gunnison Co., Colorado is a wet, montane area with valleys that encompass large riparian regions surrounded by willow shrubs (Salix subcoerulea, S. pseudocordata, S. brachycarpa.) and aspen (Populus tremuloides) groves. These willows provide food and nesting resources to a variety of bird species that nest in aspen woodlands, including the Red-Naped Sapsucker (Sphyrapicus nuchalis), a keystone species. As one travels further from riparian areas, elevation quickly increases, leading to variation in microhabitat. The impacts of elevation and willow proximity distance on bird diversity may be hard to disentangle. We investigated the effects of riparian proximity on the species richness and abundance of breeding birds in aspen groves. To control for elevation, we conducted bird censuses at from three different elevations (high, medium and low, 20 point counts each). Willow patches were categorized as “minor” (< 6500 m2) and “major” (≥ 6500 m2) patches. Distance from major willow patches had a large effect on avian species richness at low elevations (P = 0.017), a marginally significant effect at medium elevations (P = 0.08) and no effect on Glass 2 species richness at high elevations (P = 0.95). Our results stress the importance of preserving willows around riparian regions, particularly at low elevations, where habitat destruction caused by development and grazing is most prominent.