How Much Nature Is Enough?

　　By Andrew C.Revkin

　　Even some ardent conservationists acknowledge that the diversity of life on Earth cannot be fully sustained as human populations expand�� use more resources�� nudge the climate and move weedlike pests and predators from place to place.

　　Given that some losses are inevitable�� the debate among many experts has shifted to an uncomfortable subject�� what level of loss is acceptable. The discussion is taking place at both the local and global levels�� How small can a fragment of an ecosystem be and still function in all its richness�� and thus be considered preserved﹖ And as global biodiversity diminishes�� is it a valid fallback strategy to bank organisms and genes in zoos�� DNA banks or the like�� or does this simply justify more habitat destruction﹖ Is nature on ice a sufficient substitute for the real thing﹖ Some conservation groups have strenuously avoided or even attacked such calculations and strategies. They say there is no safe diminution of habitat as long as human understanding of ecology is as sketchy as it is�� a fallback strategy is unthinkable. Furthermore�� banking nature in a deep freeze or database of gene sequences cannot capture context. For instance�� even if a vanished bird was someday reconstituted from its genes�� would it warble with the same fluency as its ancestors﹖ On the other side of the debate�� those considering what the smallest viable habitats are or how to expand archives as an insurance policy say that recent trends have proved that old conservation strategies are no longer sufficient. A few decades ago�� the issue seemed fairly uncomplicated�� identify biological “hot spots” or species of concern and establish as many reserves as possible. But the picture has grown murky.

　　Twenty�瞗our years ago�� Dr. Thomas E. Lovejoy and other biologists began a remarkable experiment on the fast�瞖roding fringe of rain forest near the Brazilian city of Manaus. They established 11 forest tracts�� ranging from 2.5 to 250 acres�� each surrounded by an isolating sea of pasture similar to what is advancing around most other tropical forests. Among the many findings�� an analysis published last week on birds in the lower layers of greenery found that it would take a fragment measuring at least 2��500 acres—10 times as large as the biggest one in the experiment—to prevent a decline of 50 percent in those bird varieties in just 15 years or so.

　　In the understated language of science�� the new study�� in The Proceedings of the National Academy of Sciences�� concludes�� “This is unfortunate when one considers that for some species�瞨ich areas of the planet�� a large proportion of remaining forest is in fragments smaller than 2��500 acres.”

　　In the face of this and other evidence�� a growing group of conservation biologists say�� try everything�� at the same time. “Clearly�� the most effective way to protect biodiversity is to protect natural areas�煛� said Dr. Peter H. Raven�� the director of the Missouri Botanical Garden�� “and to find those organisms most endangered in nature and somehow protect them in type�瞔ulture collections�� botanical gardens�� zoos�� seed banks or whatever.” But most important�� he said�� is to find ways to limit human pressures on the world’s last wild places by slowing population growth and using resources more efficiently. One pioneer of genetic deconstruction�� Dr. J. Craig Venter�� agrees with Dr. Raven. Dr. Venter has moved from sequencing the DNA of humans and other species to assaying genes in entire ecosystems�� most recently the waters of the �漇argasso Sea. In five 50�瞘allon samples gathered in February�� he said�� his team had found 1 million distinct genes�� quite a haul compared with the 26��000 or so of a human being. And that is the tiniest scratch in the surface�� he added. His is one effort among many. Britain has a Millennium Seed Bank�� a growing archive of all the country’s plants. The San Diego Zoo has its parallel Frozen Zoo�� an archive of thousands of DNA samples and cell lines from a host of species. Nonetheless�� given the overwhelming complexity of nature�� Dr. Venter added�� “we’re better off trying to preserve the diversity of what we have rather than trying to regenerate it in the future.”