Interspecific Aggression and the Limiting Similarity of Close Competitors: The Problem of Size Gaps in Some Community Arrays

Starting from MacArthur's hypothesis that interspecific aggression will occur whenever one of the species can expect to gain from this behavior, a model is built explicating the circumstances under which the limiting size similarity cannot be expected to be solely caused by resource competition. We develop the model from considerations of shorebird and waterfowl communities which inhabit open defensible terrain and in which gaps occur in the sequence of size similarities. In shorebird communities there exists a gap of two or three species between the cranes and the sandpipers; in waterfowl communities the gap is three species. In both communities natural history data suggest that interspecific aggression by the largest member of the sequence has eliminated intermediate sized species. Similar gaps do not occur in the rail-heron community, which inhabits an indefensible habitat.