do this; or rather, there are lots of them, but all have their caveats and exceptions, and all inspire argument, which at times has been bitter. None of the ecologists following in Hutchinson’s wake have quite been able to draw the teeth of his question. To quote one of them, we still do not know why there are 700 species of birds in North America and not seven.

Job Opportunities

In the Homage, Hutchinson suggested that one explanation for the number of different species lay in the range of possible biological professions and addresses. Belonging to a species is like having a job: It’s a specialized way of making a living, and an evolutionary choice that closes off other employment options. By becoming excellent at one way of life, through adaptation, animals and plants become inept in others. You would no more set a sheep to catch a rabbit than you would employ a plumber to cut your hair. Tropical orchids would struggle on the tundra. Cows are good at digesting grass, bad at ant eating; pangolins, vice versa. Species divide up resources, and each species can exploit some so well that it can monopolize them. But due to life’s ubiquitous trade-offs, the ability to hog some resources comes at the cost of being able to use all of them, leaving other jobs vacant. If all organisms use energy in the same way, maybe diversity reflects the number of different ways to get it.

The most fundamental difference in biological jobs is between those organisms that get energy from nonliving sources, called autotrophs, or self-feeders, and those that consume other organisms, called heterotrophs. Plants are the most obvious autotrophs—they use sunlight to build carbon dioxide into sugar—but not the only ones. Some microbes can fuel themselves from the chemical bonds in compounds such as ammonia and methane, using geothermal heat to break them apart. The first autotrophs, born in the ocean more than 3 billion years ago, probably did something similar.

Heterotrophs feed on sunlight by proxy, consuming the cells around them. This is how a food chain works, with autotrophs such as plants feeding herbivores and predators eating the herbivores and each



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