On the Causes of Tropical Species Diversity: Niche Overlap

It is suggested that the major factor causing the tropical increase in numbers of bird species is neither increased complexity of habitat nor, solely, increased specialization, but an increase in the similarity of coexisting species, reflected in a reduced "character displacement." This implies a reduction in the size of the exclusive portion of the species niche and is thus in accord with some of the more general predictions we have previously made on the relation between niche size and climatic stability.