Niche compression: Two tests of an hypothesis using narrowly sympatric predator species

The compression hypothesis predicts that when two species occur together in narrow sympatry, individuals in the overlap zone will use a smaller range of habitats and a larger or unchanged range of prey than individuals in allopatry. This report attempts to validate the assumptions underlying this hypothesis, and describes two tests of the predicted resource shifts using narrowly sympatric species of predatory marsupials (Antechinus spp.). In the first test, using A. stuartii and A. flavipes , there was little correspondence between observed and predicted changes in diet and habitat use (as measured by indices of niche breadth) for populations of either species. One index of niche breadth (Levins' B) revealed no trends in resource use, but a proportional similarity index relating resource use to resource abundance consistently indicated that the niche breadths of each species were greater in allopatry than in narrow sympatry. In the second test, using A. stuartii where it occurred alone and in narrow sympatry with A. swainsonii , there was again little correspondence between observed and predicted changes in resource use for seasonal or overall data. Differences in magnitudes of niche breadths between allopatric and narrowly sympatric A. stuartii were smaller in this test than in the first. While these findings provided no support for the compression hypothesis, they suggested three situations in which the predictions of the hypothesis may generally not apply. These are:(i)  when the predators are active, dietary generalists (diet cannot expand in sympatry); (ii)  when one species is a markedly superior competitor to the other (the dominant species suffers little or no niche compression in sympatry); and (iii)  when the species compete by interference rather than exploitation (one species is evicted from sympatry or both show compression in habitat use and diet).