Geographic range structure in North American landbrids: variation with migratory strategy, trophic level, and breeding habitat

We investigated the relationship between abundance and geographic range str ucture of 258 North American landbirds. For this purpose we used six measur es of range structure based upon fractal geometry and geostatistics, and th ree ecological characteristics that can influence avian distribution. Perma nent residents (PRs) that were abundant showed little fragmentation of thei r abundance surface at the periphery of their breeding range. Conversely, c ommon Neotropical migrants (NTMs) exhibited low fragmentation of their cent ral populations. The abundance surface was smoother For PRs than NTMs or sh ort-distance migrants (SDMa), indicating that changes in abundance occurred more gradually across space for this group. The areas of high abundance fo r grassland species had little demographic fragmentation, but other populat ions showed little spatial autocorrelation in abundance. Species that bred in late-successional forests were relatively rare compared to species breed ing in other habitat types. Among carnivores, PRs had a higher average abun dance than either NTMs or SDMs. Although carnivores had more distributional gaps within their ranges than other trophic groups, the number of gaps did not differ between rare and abundant species, indicating that increased ab undance did nor change their presence/absence distribution maps. Knowledge of patterns and variations of geographic range structure among species may provide insights into processes that shape and maintain the biodiversity of a continent.