Climatic Instability and the "Ecological Saturation" of Bird Communities in North American Grasslands

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Abstract
The prospect that grassland breeding bird communities may be ecologically saturated was examined using data from 33 plots at 13 sites distributed through 5 types of temperate North American grasslands. Tallgrass, mixed-grass, shortgrass and Palouse prairies and northern shrubsteppe were all characterized by a similarly low number of breeding species, low species diversities, relatively uniform total breeding population densities and a similar degree of spatial packing of species. Standing crop biomass, however, was highest in the more productive tallgrass sites, intermediate in shortgrass and Palouse areas, and low in shrubsteppe. Small species predominated in shrubsteppe; medium-sized species, in shortgrass prairies; and the avifauna of tallgrass sites was equally divided between small, medium and large species. The number of breeding bird species which can successfully and persistently exploit grassland habitats is limited by recurrent but unpredictable, large magnitude variations in climate and thus production, coupled with the absence of suitable refugia which could harbor suboptimally adapted populations during such periods of resource stress.