THE RELATIONSHIP BETWEEN DISTRIBUTION AND ABUNDANCE IN A PATCHY ENVIRONMENT

Average density per patch occupied often increases with number of patches occupied. Explanations for this pattern include the idea that extinction probability varies among species as a function of the number of sites they occupy and the hypothesis that widely distributed species can use more kinds of resources than rare species. These ideas can be compared by developing a statistical model of the consequences of patch selection by individual organisms. The model predicts a nonlinear increase in the average number of individuals per patch as the proportion of patches occupied increases. According to the model, satellite species have very narrow niches, but core species need not have wide niches to obtain access to many patches. In addition, satellite species should have higher variability in their densities and have higher probabilities of going extinct in the set of patches occupied. A numerical simulation that incorporates the use of resources into the model predicts that patchy environments of high productivity with many kinds of resources available should have higher slopes than environments of low productivity. This prediction was substantiated by comparing the distribution-abundance relationships of bird communities in boreal forests at 2 latitudes. -Author