Scientific American, August 1997


Plants at Risk in the U.S.

SOURCE: The Nature Conservancy in cooperation with the Natural Heritage Network and the Biota ofNorth America Program. The numbers on the map indicate the percent of native, higher plant species at risk.

Loss of plant species, even those that are rare, may lead to ecological imbalance. Furthermore, rare plants may prove of economic or medicinal value, as in the case of the meadowfoam wildflower, which contains high-grade industrial oil. It is therefore of some concern that almost a third of all plant species in the U.S. appear to be at risk, a substantially larger proportion than in the case of mammals and birds. The record of plant species extinction is incomplete but suggests that the current rate is considerably higher than historical norms. (Over the past 200 years, at least 13 plant species have gone extinct, and an additional 125 have not been seen for years and may also be lost forever.)

This assessment comes from the Nature Conservancy of Arlington, Va., and its partners in theNatural Heritage Network, organizations that have measured the risk of extinction to individual species by considering rarity, population trends and known threats. The map is based on their datafor about 16,000 species of higher plants native to the U.S. Higher plants--also called vascular plants--generally have stems, leaves and roots. They include conifers, ferns and flowering plants and span such diverse species as Douglas fir, sugar maple, sagebrush, saguaro cactus, California poppy and Kentucky bluegrass. (Nonvascular plants, which include such groups as mosses and liverworts, account for a small fraction of all plant species.)

Habitat loss or degradation is the single biggest threat to native plant species, but other, less obvious factors come into play. Introduced plants and animals, for example, have been invading natural habitats, posing serious threats to native flora.

Factors peculiar to particular states or regions also have a decisive role. In Hawaii, for example,most of the nearly 1,200 native species are endemic--found nowhere else on the earth. Extreme endemism, combined with a large number of nonindigenous plants and major habitat alteration by both Polynesians and Europeans, has made Hawaii's flora the most threatened of any state. Plant species in the upper Great Plains and much of the Midwest are the least threatened, partly because of the fairly uniform climate, topography and geology, conditions that favor species with widespread ranges. Additionally, during the period of Pleistocene glaciation, rare species tended to become extinct, whereas widespread species were more likely to survive south of the glacier and repopulate the land as the ice receded.

California harbors more native plant species than any other state and has the second highest
proportion of species at risk. The state's large size and diverse habitats provide abundant
opportunities to adapt and evolve, giving rise to numerous narrowly restricted species, which are
vulnerable to California's spectacular urban and agricultural growth. Certain other areas, such as
Oregon, the southern Rocky Mountain states, Florida and Georgia, also have high proportions of rare species because of the great diversity of their habitats. Areas of patchwork mountain and desert, which provide ample opportunities for geographical isolation, are especially rich in locally evolved plant species. Extreme examples of such habitats are the mountaintop "sky islands" in the deserts of New Mexico and Arizona, many of which support local and rare plant species.

Rodger Doyle (

Image: Roger Doyle