My biogeographic studies focus on the widely held belief that species are most abundant in the center of their range and decline toward the edges. I call this the "abundant center" hypotheis. Although this assumption is printed in most biogeography textbooks and forms the basis of dozens of hypotheses about species' responses to climate change, gene flow, optimal habitat preservation and population dynamics, I don't believe it holds true in natural systems. This belief stems from my research in three areas:

1. Theoretically, it is very difficult to summarize in 1 dimension (i.e., abundance vs. distance from range center) a 2-dimensional pattern (i.e., abundance over species' range area). The exception is for species that essentially have 1-dimensional ranges, such as intertidal plants and animals.

2. My review of direct studies of population distributions showed that even with very generous assumptions, few studies conclude that abundant center distributions occur. Moreover, fewer studies sampled over enough of the species' range to truly account for species' overall abundance distributions. See Sagarin & Gaines. 2002. The "abundant center" distribution: to what extent is it a biogeographic rule? Ecology Letters. January 2002.

3. My own empirical research of distributions of abundance of 12 intertidal invertebrate species over most or all of their range (see map at left) showed that only 2 had abundant center patterns, while most showed high, or even highest, abundance right at the range edges. The figure below shows abundances vs. position in the range for all species combined (top panel) and for each species separately (lower panels). The insets show the shape of the abundance distribution as determined by a shape-fitting statistical procedure.