land animals need, how quickly their populations grow, and how many young they produce.

The ecological importance of body size makes it of practical as well as academic interest. To conserve a species, we need to understand it. As the biologist William Calder wrote: “A conservation biologist trying to prepare a protection plan without data on the species’ biology is like an insurance underwriter issuing a policy in ignorance of the applicant’s age, family status and medical history.” Yet conservationists often work in almost perfect ignorance of what they are trying to save. It is likely that at least 90 percent of species, and possibly a much higher proportion, have not yet been discovered, described, and named. And our knowledge of most of those that have stops with their existence—we have only a pressed leaf or a bug pinned out in a draw. No one knows how widely they are spread, how great their numbers are, what they eat, what parasites assail them, how many young they produce and how often, what foods or habitats they prefer, or how they behave. Of the species we do know something about, the majority are birds, mammals, and flowering plants. Of the most diverse groups, such as insects, fungi, and bacteria, we know practically nothing. Most species live in tropical forests, through which it is difficult to travel and in which it is difficult to spot things. The number of biologists and the resources they have to fund their discovery and description of species are both limited, particularly in tropical countries, which tend to be among the poorer nations. Our destruction of biodiversity outpaces our knowledge of it.

We need rules of thumb that, in the absence of detailed knowledge, can help us predict which species are most vulnerable to threats such as habitat destruction or hunting and so which are most in need of conservation. Rules based on body size have two great advantages. Size is informative, and it is easy to measure. Even if we know nothing else about a species, we almost always know how big it is. It’s been said that ecologists report body size in the same way that journalists report their subjects’ ages—practically as a reflex. It takes only a moment to weigh an animal; taking such a measurement requires little equipment or expertise, its accuracy can usually be trusted, the animal doesn’t have to be alive to be weighed, and if it is, you don’t need to interfere

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