Shrub-Steppe Bird Assemblages Revisited: Implications for Community Theory

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Abstract
Wiens and Rotenberry''s shrub-steppe avifaunas, used to test predictions of competition theory, are characterized by low species number, variable total density of all species, and dominance by a few characteristic species. Variation in total species density is strongly affected by variation in densities of the dominant species, which can change dramatically from year to year. Analysis of 24 long-term censuses from a wide variety of North American habitats shows that these are not characteristics of breeding bird assemblages in general. A single study, that of a Connecticut [USA] swamp that flooded in two of the eight census years, conformed to the pattern set by the shrub-steppe avifauna. This study confirms that environmental perturbation can produce the variability found by Wiens and Rotenberry, but also illustrates the severity of perturbation necessary for the bottleneck model. Thus, the shrub-steppe avifauna may not be a good model for determining the role of competition in avian communities. Fitness-set analysis suggests that the dominant species in the shrub steppe may be limited by resources during the winter. Competition during the breeding season is better measured by factors reflecting reproductive success instead of winter survival. The importance of exploitation competition in structuring communities is a function of the frequency with which ecological bottlenecks occur; therefore a gradient of conditions can be described, ranging from severe annual resource limitation and intense competition to irregular catastrophes and little intervening competition. Knowledge of the natural history of the species of interest, especially the timing of resource limitation, is critical to designing proper tests of competition-based community theory.