A number of definitions of metacommunities exist in the literature. Built upon theoretical ideas, none of these definitions provides a rigorous empirical foundation for evaluation of the size, extent, or composition of the metacommunity. Lacking such a definition, estimates of important properties of metacommunities needed to test different hypotheses for local community dynamics are not available. Several empirically based rules for constructing metacommunities were evaluated using large databases on distributions of species across geographic regions. Using each local community as a focal point, distances in geographic space, species composition space, and environmental space were calculated and used to develop metacommunity descriptions. For North American birds, geographic distance was a poor criterion to use to construct metacommunities because communities close to one another in geographic space often occupied different parts of environmental space (e.g., a desert site close to a mountainous area).
Results/Conclusions
Similarity in species composition among sites decreased with in increasing geographic distance, while distances among sites in environmental space increased with increasing geographic distance. Both curves, however, leveled off at a distance of around 2000 km. Using this as a distance for the maximum diameter (or major axis) of a metacommunity, and applying a cut-off distance in environmental space, metacommunities for each local community were constructed and examined for geographical and internal consistency. With the resulting sets local communities, several measures of divergence among local communities and migration among them were used to evaluate assumptions of metacommunity models of community assembly. Evidence was found for dispersal limitation that was related to environmental differences among sites. Although complications with measuring geographic distances existed for a data set on
