Since then, many efforts have been made to refine Erwin’s estimates. Most have tended to revise the numbers downward. (Among other things, Erwin probably overstated the proportion of insects dependent on a single host plant.) Still, by all accounts, the figure remains shockingly high: recent estimates suggest there are at least two million tropical insect species and perhaps as many as seven million. By comparison, there are only about ten thousand species of birds in the entire world and only fifty-five hundred species of mammals. Thus for every species with hair and mammary glands, there
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