A Search for Stability Gradients in North American Breeding Bird Communities

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Abstract
To search for the existence of stability gradients in North American breeding land bird communities stability operationally defined as year-to-year persistence in species composition and distribution of species abundances was used. From the census data for 174 study plots, 9 indices were derived that estimate the annual variability of species composition, the species abundance distribution, diversity and breeding density. The resulting matrix of study plot by stability indices was used to estimate the correlation structure of the stability indices. The correlation matrix was, in turn, subjected to a principal components analysis to derive synthetic gradients of variation. Patterns of variation in these stability gradients associated with either geographic location or habitat type were determined. Three independent principal component axes reproduced most of the variation in the initial data and were interpreted as gradients of variation in species turnover, diversity and breeding abundance. The annual stability of community structure apparently responds independently to species and abundance variation. Despite the clarity of the derived gradients, few patterns emerged when the plots were ordinated by either habitat or geographic location. In general, grasslands showed greater annual variation in diversity than forested habitats, and, for some habitats, northern communities were less stable than more southern communities. Few of these patterns were very strong, and they must be interpeted cautiously.