| Identifying the mechanisms by which similar species coexist is a central problem in ecology. The classical explanation for coexistence depends on resource partitioning driven by interspecific competition. Alternative processes that may also promote coexistence include predation, environmental heterogeneity and resource abundance.; The role of competition and other forces in permitting coexistence was studied among White-tailed Sea (Haliaeetus albicilla), Imperial (Aquila heliaca), Golden (A. chrysaetos), and Steppe Eagles (A. nipalensis) at the Naurzum Zapovednik (National Nature Reserve) in Kazakhstan. These species are so similar that many sources consider them ecological equivalents, and classical theory would predict that these species should compete intensely. To evaluate coexistence, spatially-referenced data on nests, reproduction, diet, and prey availability were collected and analyzed for each species. Intraspecific variation in these characteristics was also studied among Imperial Eagles.; Steppe Eagle diet and nesting behavior were distinct from tree-nesting species. All tree-nesting eagles bred in similar habitats, and although their nests were sometimes different in construction, location, and spacing, these differences were not interspecifically limiting and were not driven by competition. Eagle foraging responded to distributions of the most heavily used prey types, but not to distributions of other prey or other eagles. Thus, while eagle diet depended on nest location and eagle species, location was so important that neighboring heterospecifics often had diets more similar than did more distantly spaced conspecifics.; Intraspecific variation in nests and in diet of Imperial Eagles was as great or greater than interspecific differences. Diet selection of this species was independent of competition, tightly tied only to prey distributions, and positively associated with reproductive output.; Species-specific niche preferences and a diverse resource environment allowed coexistence of steppe- and forest-nesting eagles, and intraspecific demographic factors limited populations of forest-nesting species. Thus, although some resources were partitioned, alternative mechanisms allowed eagles to coexist without competing. Therefore, removal of any one or two species would likely have no demographic effects on remaining eagle populations. This work shows that in complex situations, multiple factors can work together to permit species coexistence without competition, even when classical theory would predict intense interspecific competition. |
