Microhabitat Pattern and the Structure of Forest Bird Communities

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Abstract
A forest-stimulation model was used to generate an extensive, yet detailed, data set comprising tree size classes measured on a 9-ha area of contiguous forest over a period of 750 years. Principal-components analysis served to develop a statistical characterization of the forest stand as a two-level construct in which the stand encompasses a dynamic pattern of microhabitat variety and abundance. This statistical framework was used to examine the role of microhabitat pattern in structuring random assemblages of bird species defined by niche ellipses in principal-component space. An increasing variation in microhabitat structure through successional time accounted for realistic trends in species richness, dominance, equitability, and turnover in forest bird communities. The statistical distribution of microhabitat availability in mature forest provided for an inverse relationship between niche position (species preferring unusual microhabitats have high niche position) and abundance; this same distribution within potentially usable niche space predicated the occurrence of more rare species than common species. The spatial distribution of microhabitat variety in mature forests generated an increase in species richness with increasing sample area. This study illustrates a two-level approach to studies of forest bird communities. At the stand level, among-quadrat variation in microhabitats accounts for the variety and abundance of bird species supported by the stand. Within landscapes, among-stand variation in microhabitat availability represents another source of variation contributing to the distribution of bird species. Our results help to explain why habitat area is such a useful predictor of species abundance and suggest the conditions under which estimates of the stand-level distribution of microhabitats might be useful predictors of species-abundance patterns at the landscape scale.

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