together to evolve differences through time. All in all, MacArthur’s ideas became a route map for ecology.

The Homage came at the beginning of a golden age in ecology. In the 1960s and 1970s there was optimism that a unified theory of ecology and evolution—that would explain how natural selection acting through ecological laws produced the structure of nature—was close at hand. Hutchinson and MacArthur made competition between species the focus of this research program. The strength of competition between species, and the ways in which it could be avoided, came to be seen as the forces that controlled the number and type of species that could coexist and the evolutionary changes that would occur in similar species sharing a habitat. The hunt was on for a set of rules that governed the way animals and plants divided up resources and environmental conditions—niche space—between themselves.

Body size is crucial here. The extent of an animal’s niche, and so the number of species that can pack into any part of ecological space, depends on its body size. Besides trying to work out how an organism’s size affects its biology, scientists have spent decades looking at the question the other way around—trying to work out what determines body size. Whatever the determining factors are, they seem to favor smallness. There are mammals weighing everything from 2 grams to 100 million grams, but three-quarters of species weigh less than 1 kilogram. As for birds there are more species of warbler than eagle. There are more small tree species than large; the same is true for fish. This seems to be true however you slice up life: There are more species of insects than mammals, for example.

Hutchinson and MacArthur thought that niches had something to say about this too. To a browsing giraffe, a leaf is less than a snack. But the same leaf might feed one insect species nibbling at its edge and another that burrows in and eats it from within. Other insects could live on the tree’s bark, bore into its trunk or roots, or trick the plant into growing around them, creating a gall. Small animals use up less ecological space, and so more of them should pack into that space. The analogy that I used earlier, of packing balls into a crate to describe how trees fit into a forest, could apply here also, except that each ball represents a species, not an individual. More recent ecologists have analyzed

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