A Shrinking Gene Pool

   The number of plant and animal species on earth—variously estimated at 5 million to 30 million— is fast dwindling, in a mass extinction of proportions not seen since the dinosaurs died off 65 million years ago. Among contemporary birds and mammals alone, which make up only some 13,000 species in all, the last century has seen the disappearance of about one species a year.
   Many biologists believe that this rate of extinction is at least a hundred times faster than it would be were human activity removed from the equation. The effects of civilization have been particularly harsh on islands, lakes, and other environments that are isolated or closely circumscribed. In Polynesia, for example, hunting and logging have eliminated about half of all the bird species.
   Even as some species of plants and animals vanish, however, others are observed for the first time. To date, only about one and a half million are actually known to science—that is, they have been named and described. unknown species crop up constantly. A study of nineteen trees in a Panamanian forest, for example, uncovered 950 species of beetles; more than three-quarters of them were previously uncatalogued.
   Given this apparent biological plenty, the loss of species might seem a small problem, especially as so many seem to be minor vari­ations on a theme, such as the Panamanian beetles. But many scientists assert the need for maxi­mum diversity within the world's total genetic possibilities—the gene pool. These researchers fear that the accelerating rate of spe­cies extinction could reverse nature's climb toward ever greater multiplicity and diminish the overall viability of life on the planet.