SPATIAL SCALING OF SPECIES COMPOSITION - BODY MASSES OF NORTH-AMERICAN LAND MAMMALS

We describe the nonrandom assembly of the North American terrestrial mammalian fauna based on body size and spatial scale. The frequency distribution of body masses among species for the entire continental fauna was highly modal and right skewed, even on a logarithmic scale; the median size of the 465 species was approximately 45 g. In contrast, comparable frequency distributions for 24 small patches of relatively homogeneous habitat were essentially uniform, with approximately equal numbers of species in each logarithmic size class; the median sizes of the 19-37 species ranged from approximately 100 to 2,500 g. Frequency distributions for 21 biomes (large regions of relatively similar vegetation) were intermediate between the continental and local assemblages. This pattern of assembly indicates that species of modal size (20-250 g) tend not to coexist in local habitat patches and they replace each other more frequently from habitat to habitat across the landscape than species of relatively large or small size. We hypothesize that three mechanisms are necessary and possibly sufficient to produce this result: competitive exclusion of species of similar size within local habitats, differential extinction of species of large size with small geographic ranges, and greater specialization of modal-sized species owing to energetic and dietary constraints.