ON THE RELATIONSHIP BETWEEN RANGE SIZE AND LOCAL ABUNDANCE - BACK TO BASICS

In many taxa, the sizes of species' geographic ranges and their averag e local abundances at sites where they occur are positively correlated . We develop a simple population model that predicts this relationship which does not rely upon spatial dynamics. The simple, but fundamenta l, fact which should link distribution and abundance is that the distr ibution of species i is given by the number of sites at which its intr insic rate of increase, r(i) > 0, whereas its equilibrium local abunda nce should often vary directly with r(i). Any factor which tends to in crease r(i) across all sites will simultaneously enlarge the number of sites potentially occupied, and increase abundance at each occupied s ite. If species differ principally in their response to density-indepe ndent factors influencing birth and death rates, but are similar in de nsity-dependence, then in a broad range of circumstances one should ob serve a range-abundance correlation. The model also predicts particula r circumstances in which there is no, or even a negative, correlation between range and abundance. The general occurrence of this correlatio n in a broad range of taxa may reflect the operation of a number of di stinct mechanisms.