Not all diversity patterns are rooted in space or time. Some are dimensionless. These show how diversity varies with other properties of organisms. Below, I give four examples. Marzluff and Dial (1991) study others (such as reproductive rate and longevity) connected with life history. You may think of more.

In including such patterns, I have stepped outside the bounds of this book's title. Yet, I believe that their discovery and explanation will occupy some of the most interesting pages of our journals during the next decade.

Body size

G. E. Hutchinson co-authored only one paper with his student R. H. MacArthur. It introduced the world to the body size pattern of species diversity (Hutchinson and MacArthur, 1959). Within a taxon, there are more intermediate-size species than either very large or very small ones. Figure 4.1 shows the pattern they found for dragonflies of the northeastern USA.

Hutchinson and MacArthur presented a theoretical explanation of the pattern. The explanation has not lasted because it depends on patch size being the same for all species regardless of body size. But the pattern itself remains fascinating.

May (1978) shows similar patterns among mammals, birds, beetles and butterflies. Also, he develops a reasonable, albeit speculative, overview of the distribution of body size among all species of animals. It, too, appears unimodal.