This Week's Citation Classic

The general goal of science is to find general laws that explain patterns observed in nature. An excellent example of this is found in this week's citation classic: James H. Brown. 1984. On the Relationship between Abundance and Distribution of Species. The American Naturalist, Vol. 124, No. 2. (Aug., 1984), pp. 255-279.

We are all aware that some species are common in some areas, but absent in others. Zebras are not naturally found in upstate New York, nor are polar bears found in Africa. But where a species is found, how is it distributed across is range? Are they distributed evenly? Patchily? More in the center than at the edges?

Until Brown's paper, few scientists systematically studied the relationship between an organism's abundance across its range. Brown synthesized distribution and abundance data from taxa as diverse as vascular plants, intertidal invertebrates, terrestrial arthropods, planktonic crustaceans, and terrestrial vertebrates to provide a general theory to explain species' biogeography.

Using this data, Brown observes that populations of most species show the highest density at the center of their range with density decreasing as you approach the perimeter.

"I now propose a single general explanation for both of these patterns: the relatively symmetrical, monotonic decrease in abundance from the center of the distribution toward all boundaries, and the positive correlation between local population density and extent of spatial distribution among similar species."

This general theory stems from three assumptions:

First, the abundance and distribution of each species are limited by the combination of physical and biotic environmental variables that determines the multidimensional niche. Second, spatial variation in these environmental variables is somewhat stochastic but autocorrelated, so that nearby sites tend to have more similar environmental conditions than more distant ones. Third, closely related, ecologically similar species differ in no more than a very few niche dimensions.

Thus Brown was able to look at the distributions and densities of just a few organisms and predict the distribution/density relationship for all of them. Naturally there are some exceptions (Brown predicts them too), but, generally speaking, Brown's theory has been confirmed over the intervening two decades.

James H. Brown is one of the founders of the emerging field of Macroecology, the study of the ecological phenomena at large spatial scales, and the editor of the excellent text: Scaling in Biology. He was elected to the National Academy of Sciences in 2005.