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This article could leave the impression that the evolutionary significant unit (ESU) is the de facto concept employed for all listing decisions under the Endangered Species Act. In fact, the ESU has not been used in the vast majority of recent listing decisions under the act. Nor should it be. The act allows the National Marine Fisheries Service and the U.S. Fish and Wildlife Service to list distinct vertebrate population segments. What qualifies as “distinct” is the million-dollar question. I would argue that the ESU concept falls short of the goals of the act by ignoring populations that are distinct because of their ecological or cultural significance. Evolutionary significance is critical for preservation of biodiversity, but it isn’t the only factor that should be considered.

J. Alan Clark
Seattle, Wash.

It seems symptomatic of something (I won’t speculate what) that conservation biologists are using the term “evolutionary significant unit,” which implies a value judgement. Scientists in other fields would have been careful to use a value-neutral term, such as “evolutionary distinct unit.” The definitions proposed quite properly seek to identify population groups that are distinct from each other. That, clearly, is a hard enough nut to crack for now.

Charles Masi
Golden Valley, Ariz.

In our modern, finite world, where human population grows at a rate of 1 billion people a decade, we are constantly applying Band-Aids to all the consequent tribulations. What else could an “evolutionary significant unit” or a “management unit” be but a new form of Band-Aid applied to ever-decreasing slivers of once pristine habitat? A tiger did not evolve to be in a zoo. Once a habitat is altered by the direct or indirect pressures of human-population growth, the species therein must either evolve or perish. Tigers are functionally extinct, as is almost any species larger than a bread box. Will we ever awake to the nightmare of our own overpopulation?

Russ Agreen
Denton, Md.