Every biologist has his or her favorite branch of life—birds, bees, moths, water lilies, coral reef fish, or whatever. For Enquist it was trees. Ever since he began climbing them as a child, he had had a thing about them. They were impressive and important. If you stand in a forest, the world seems to be made of trees. He was also impatient, and botany is a good discipline for a naturalist in a hurry, because you don’t have to sit in a hide all day waiting for trees to come to you, and it doesn’t take a week to get your sample size into double figures. And for a young biologist beginning his career and looking for big questions to answer, botany was fertile ground. The main thrust of the science has been to classify and describe plants, and there is relatively little theory or mathematics in the discipline; new ideas in biology have tended to be developed and explored by researchers working on animals. Enquist decided to investigate scaling in forests, to see how the sizes of individual trees influenced the form of the whole forest.

For graduate school he went to work in Jim Brown’s lab at the University of New Mexico. More than a decade before Enquist joined his lab, Brown had decided that energy was the key to understanding biodiversity. He had begun his career in the 1960s as a physiological ecologist, studying the biology of energy in animals, looking at how the challenge of keeping warm affected their energy budgets and how it would affect their behavior and where they could live. One of his early studies was on the metabolic rate of weasels, and the cost of being long and thin, with a relatively large surface area—a weasel burns energy twice as quickly as a round animal of the same weight, he found. Lately, he had come to see energy as an organizing principle for the whole of nature. How organisms got energy and divided it between themselves could, he believed, explain biodiversity: Why different environments contain different numbers of species, why species live where they do, why certain species are found together or not, and why some are common and some are rare.

And the foundation of any investigation into energy and biodiversity, Brown believed, should be body size. Body size controls how much energy plants and animals need and so how much is left over for other individuals and species. Body size is also closely related to virtually everything else that ecologists are interested in, such as how much

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