But in a sweeping analysis of the ecosystems of the United States and Canada, a team of scientists suggests that 13 of what they call the continent's 116 large "ecoregions," hold as much biodiversity as the Everglades and are even more imperiled. Moreover, the report suggests, North America in general harbors a far more critical share of the world's biological diversity than has been generally recognized, with fully 32 of its ecoregions harboring "biodiversity that elevates them to a globally outstanding ranking."
The study appears to be the most extensive ever conducted for the region. It examined all of the continent north of Mexico (where a related study is expected to be completed in coming months) in an effort to catalogue biological richness, and threats to it, in a variety of ways. Previous studies have tried to locate "hot spots" where clusters of endangered species survive, or to identify numbers and types of species in relation to protected parks and refuges.
The new analysis, called "A Conservation Assessment of the Terrestrial Ecoregions of North America," combines several of these factors. The report catalogues rare and endemic (highly localized) species, and sheer richness of species habitat by habitat. But it also looks beyond individual species for what the authors called "globally outstanding ecological phenomena," like the huge migration of caribou on Canada's Arctic tundra, or the dazzling array of species among Hawaii's honeycreepers, birds that evolved from a single common ancestor to fill available food and other habitat niches.
The report also seeks to highlight ecosystems that are rare on earth, especially those under severe threat. One such system is the southeastern pine forest, centered in north Florida, where expansive open, sunlit groves of old growth longleaf pine once supported one of the richest assemblages of forest-floor plants on earth, along with such now-endangered species as the red cockaded woodpecker and gopher tortoise. These forests have been reduced to about 2 percent of their former range.
Others include the "central tall grasslands" region in and around the present-day corn basket of Iowa, where only a few remnants of the largest tallgrass prairie on earth survive; and four ecoregions along and near the highly developed Southern California coast that together make up one of only five habitat groups defined by "Mediterranean" climates on earth. Although they occupy only small fragments of the earth's surface, the Mediterranean habitats -- hot and dry in summer, cool and wet in winter -- hold fully 20 percent of the globe's plant species.
The study was prepared by the United States and Canadian branches of the World Wildlife Fund, a conservation organization that has conducted similar studies in Russia and South America, and has others in progress for Mexico and Asia. Dr. Eric Dinerstein, chief scientist at the World Wildlife Fund, U.S., who has led or worked on this and previous studies, said the picture that emerged of "how rich many of these North American regions are when you stack them up against ecoregions in other countries," came as a surprise even to him.
"We always tend to equate biodiversity only with places like Brazil, or Indonesia," Dinerstein said. "But as you look more deeply, you begin to realize that it's as if North Americans have won the biological lottery, but forgot to look at the ticket."
In a bleak counterpoint, however, the report's authors charged that across broad areas of the East and California, "the United States is doing a worse job of protecting its biological resources than many poorer countries with fewer resources for biodiversity conservation."
As one example, Dinerstein said, "There are several countries in Asia protecting much larger areas for tigers than we've set aside for our large carnivores."
Dinerstein acknowledged that tropical rain forests in particular had garnered attention for their brimming diversity. "Perhaps 50 percent of all living things live in tropical rain forests," he said. "But if an overriding goal of conservation is to preserve representations of all living things, we also have to ask, 'what about the other 50 percent?' "
The new analysis, he said, showed that some of the richest ecosystem-types outside the tropics occur in North America. "If you ask where in the world will you find the richest warm deserts, the answer is the Namib-Karoo in Africa and the Chihauhuan in North America. If you ask where are the richest temperate forests in the world in terms of plant species, or snails, or salamanders, the answer is the Hunan-Setzuan in China, and the Appalachian-Blue Ridge forests of North America."
Unlike some previous attempts, this study analyzed the biodiversity status of the continent not by political boundaries -- states, provinces, or nations -- but by ecoregions. Many of these regions are larger than entire states. They were broadly defined as "a relatively large area that contains a geographically distinct assemblage of natural communities."
The definition, like any delineation of an "ecosystem," is less than precise, particularly in terms of where boundaries lie. But the units were based on strong similarities in climate, geology and plant species, and appear to correlate with widely held notions of where boundaries of major systems tend to lie: the Everglades themselves are one ecoregion, the Sonoran Desert another, the rain-soaked and soaring coastal temperate rain forests of the Pacific Northwest and British Columbia another. A few are geographically disconnected pieces, like the lush heights of forested "sky island" mountains that rise, here and there, out of the dry Arizona and New Mexico deserts.
Dr. David Wilcove, senior ecologist at the Environmental Defense Fund, praised the new study as "the first really comprehensive overview of the status of ecosystems in the U.S. and Canada." But he said he also had some scientific reservations about parts of it, particularly the more subjective inclusion of "globally outstanding" phenomena, like the "intact predator-prey relationships" between predators like wolves and grizzly bears and their hoofed prey in the Canadian Arctic.
"I'm just not sure how you can objectively define some of these things," he said. "For instance we know that a few grizzly bears and wolves have moved into the northern Cascade Mountains. Does that mean there are now intact predator-prey relationships there? If not, when do they become intact? And if caribou migrations are globally outstanding, how do we distinguish them from, say, the fabulous bird and butterfly migration through a place like Cape May, N.J.?"
Wilcove, who was an author of a related, more limited study of endangered-species "hot spots" in the United States that was published last January, also suggested that there were perplexing trade-offs between studies like his and the new analysis.
"In our hot spots study, we focused exclusively on endangered species," he said. "We said, with limited resources available, focus on these areas first because here's where we're going to lose the most first. But the drawback of our approach is that you constrain your ability to focus on species that are not yet endangered but in serious decline, and on their way. This new analysis would help you be able to avoid that. On the other hand, it also ends up identifying such a significant fraction of the continent as being in serious trouble that it leaves you with the question of where to focus limited resources. It's kind of a biotic equivalent of the national health care debate."
Dr. Reed Noss, an ecologist and editor of the journal Conservation Biology, said the report's most striking finding was that "North America is probably the world's most significant region in terms of temperate-zone and Arctic biodiversity."
"But it also makes clear," he said, "that there's been a persistent decline in a lot of our ecoregions, and that we could lose a great deal in next few decades unless steps are taken to change trends."