The diversity of life in the United States is extraordinary. It is estimated that more than 200,000 formally recognized, native species are now known from the United States; however, this figure may represent fewer than half of the plants, animals, and microbes yet to be discovered and given a formal Latin name. In addition, it is estimated that around 3,500 species of non-native, non-cultivated plants and at least 2,300 nonindigenous animals occur in the United States, although the total number is not precisely known. Many of these exotic species are a threat to U.S. biodiversity. (Also see Chapter 2. Economy and the Environment.)

U.S. biotic diversity is not only a national treasure, but is significant on a global scale as well. In their recently published report, Precious Heritage: The Status of Biodiversity in the United States, the Nature Conservancy and Association for Biodiversity Information compare the number of U.S. species with those found worldwide (Text Table 4.1). The nation's freshwater biota is particularly significant. Several species groups, including freshwater mussels, freshwater snails, crayfishes, and several freshwater insect orders, have their highest levels of diversity in the United States and the U.S. freshwater fish fauna represents the world's most diverse temperate fish assemblage. The United States is also a world leader in the number of salamanders and freshwater turtles. Among the plants, the number of conifers and other gymnosperms in the United States ranks second only to that in China and the number of flowering plants makes the United states one of the leading temperate zone nations.

This chapter examines national patterns and trends in U.S. biotic diversity.

TRENDS

The Association for Biodiversity Information manages The Nature Conservancy and Natural Heritage Network database -- NatureServe -- that contains biological information on many of the known plants and animals occurring in the United States (and Canada). Of the 50,093 records in NatureServe, the conservation status is known for 16,775 species and 8,170 subspecies and varieties of plants (excluding hybrids and plants present only in cultivation), 2,667 species and 1,450 subspecies of vertebrates (excluding marine fishes), and 3,638 species and 852 subspecies of invertebrates. Based on their global rarity, one third of these native species (including subspecies) are at risk of extinction or elimination. Another 500 native species (1 percent) may be already extinct. Of these, at least 100 plants and animals have disappeared forever and the rest are feared lost. Half of the species analyzed (52 percent) are not vulnerable in most of their range, but there is concern for some. Many species (14 percent) have yet to be ranked (Figure 4.1).

Freshwater habitats and their inhabitants -- mussels, crayfish, fishes, and amphibians -- are particularly under stress nationwide. In many waterways, freshwater populations have suffered badly from habitat loss as a result of dam construction, channelization, dredging operations, water pollution, and competition with exotic species. Nearly 60 percent of native mussel populations, 70 percent of freshwater snails, 52 percent of crayfish, and 38 percent of amphibians are vulnerable to extinction or are already extinct. Thirty-nine species (and subspecies) of freshwater fish are potentially extinct and nearly 500 more (42 percent) are at risk.

NatureServe also includes information for 3,837 natural ecological communities, primarily in the contiguous United States and Hawaii. However, because inventory and classification work is currently incomplete, a definitive interpretation of ecological status is difficult.

Endangered and Threatened Species

The number of U.S. species listed as threatened and endangered under the Endangered Species Act (ESA) has risen steadily since 1980 (Figure 4.2). By the end of 1998, 922 U.S. species were listed as endangered, including 567 plant species, and another 230 U.S. species (135 plants) were considered threatened. Among the listed species, 21 have been reclassified since 1975 -- 14 from endangered to threatened and 7 from threatened to endangered. Another 24 species have been delisted -- 3 because they have recovered, 7 because they are presumed to be extinct, and the remaining because of taxonomic revision or new information discovered.

The protection provided by the Endangered Species Act has benefited many species. Among the listed species that are recovering, raptor populations -- bald eagles, peregrine falcons, and ospreys, for example -- have increased in number over the past two decades. The bald eagle population increased from a low of 400 nesting pairs in 1963 to just over 5,748 nesting pairs in 1998 within the contiguous United States (there are an estimated 20,000-25,000 pairs within Alaska) (Figure 4.3). The 1972 ban on the pesticide DDT, protection from illegal shooting, and habitat protection are significant factors in this recovery. Because the bald eagle is doing so well, the Fish and Wildlife Service has proposed to remove it from the endangered list.

Recovery of peregrine falcons is another success story. One of nature's swiftest and most beautiful birds of prey, peregrine falcons have never been very abundant, totaling only about 500 breeding pairs in the eastern United States and 1,000 pairs in the West and Mexico in the 1930s and 1940s. These numbers dropped precipitously, however, beginning in the late 1940s, primarily due to pesticide poisoning. By the mid-1960s, the species was almost totally eliminated from the eastern United States and by the mid-1970s, was reduced by 80 to 90 percent in the West. Listed as endangered in 1970, peregrine falcons have made a dramatic recovery, being rescued from extinction by the ban on DDT and the reintroduction of captive-bred peregrine chicks. By 1998, populations of peregrine falcons had grown to an estimated 1,650 breeding pairs in the United States and Canada, with additional birds in Mexico, and the species was proposed for delisting.

Terrestrial Bird Species

Many U.S. resident and neotropical migrant bird species are declining in numbers, some drastically so. Forty-six percent of the 421 U.S. bird species monitored by the FWS Breeding Bird Survey have decreased in numbers over the past 32 years. Some species, such as northern bobwhite (Figure 4.4) and loggerhead shrike (Figure 4.5) are so consistently declining that few question the trend.

In general, though, species populations tend to be cyclical, decreasing in some time periods (or locations) and increasing in other periods (or locations). For example, resident populations and birds that migrant short distances to winter in the United States and Canada can be adversely affected by unusually harsh winter weather. Examples include the Carolina wren (Figure 4.6) and eastern bluebird (Figure 4.7) whose populations dramatically declined during the harsh winters of 1976-1977 and 1977-1978 but generally recovered since then.

Analysis of population trends based on groups of species that share similar attributes, such as breeding habitat, nest type, migration status, and nest location, help identify major patterns of population change. For example, 43 percent of all grassland-breeding bird species declined significantly over the 1966-1998 period (Figure 4.8). But this approach can hide detail within groups, such as the steep decline in forest-dwelling birds within the Neotropical migrant group. Declines are quite severe for some forest-dwelling species -- the wood thrush (Figure 4.9) and the cerulean warbler (Figure 4.10) are declining at rates of 2 to 4 percent per year over the period -- and for forest-dwelling birds, as a whole, in certain regions -- the Adirondacks and the Great Smoky Mountains. Possible causes for these declines include changes in land use that reduce or alter summer breeding habitat and loss of winter habitat.

Waterfowl

The estimated 1998 breeding population of 10 North American duck species surveyed by the FWS was 39.1 million birds, an 8 percent decrease from the 1997 census, but 20 percent higher than the long-term average (Figure 4.11). Nesting conditions throughout survey area were variable in 1998: fair-to-good in Alaska; fair-to-poor in western Canada and Montana; and good-to-excellent in northern portions of Saskatchewan and Manitoba, and the Dakotas. Estimates for 6 of the 10 principal species were similar to their respective estimates for 1997. The number of northern pintails, green-winged teal, and northern shovelers decreased from 1997 counts, while abundances of mallards, gadwell, American widgeon, blue-winged teal, canvasbacks, and redheads were similar to last year. The estimate of abundance of scaup reached a record low, which was 16 percent below the 1997 count and 36 percent below the long-term average. Approximately 61 percent of the ducks were found in the prairie-pothole region of the United States and Canada, a percentage similar to that which occurred during the 1970s (60 percent) when wetland and upland conditions in this ecologically important region were considered good.

Mid-winter counts of the American black duck in the Atlantic and Mississippi flyways were similar to last year, but 18 percent below the most recent 10-year average. Many factors may play a role in the long-term decline, including blood parasites, lead poisoning, red tide, oil spills, habitat losses, shooting, pesticide residues, predators, weather conditions, heavy metals, and hybridization with mallards.

Most goose and swan populations in North America remain numerically sound. Nine of the 28 populations of geese and swans surveyed appear to have decreased since last year, seven appear to have increased, seven appear to have changed little, and no comparisons were possible for the remaining five.

Marine Species

Knowledge about marine species and ecosystems lags far behind that of terrestrial systems. The best-known groups are commercially exploited fish species, marine mammals, sea turtles and fishes protected under the ESA or the Marine Mammal Protection Act, and certain commercially significant and accessible coastal ecosystems such as wetlands and coral reefs.

Marine Fisheries. Based measures of current fisheries abundance relative to maximum sustainable yield -- the long-term average catch that can be achieved from the resource -- it appears that many fishery stocks are imperiled. The National Marine Fisheries Service estimates that 73 marine stocks under NMFS purview and 14 nearshore stocks under purview of coastal states (for a combined total of 31 percent of stocks) are below maximum sustainable levels, and another 62 marine stocks and 26 nearshore species (totaling 31 percent of stocks) are near sustainable levels. No nearshore stocks are abundant enough to be above the sustainable level, while 23 marine stocks are. The status of 45 marine groups and 40 nearshore stocks is currently unknown (Figure 4.12).

Similar compilations based on the degree of utilization indicate that 22 percent of known stocks are overutilized, 39 percent are fully utilized, and only 13 percent are underutrilized. The utilization status of 27 percent is unknown.

The list of stocks that are overutilized or below sustainable levels includes some of the nation's most valuable fishery resources, such as New England groundfish, Atlantic sea scallops, several pelagic highly migratory fish stocks (including Atlantic bluefin tuna and swordfish), some Pacific salmon stocks, some rockfish off Alaska, and Alaska king crab. Many nearshore stocks (including several oyster populations, bay scallops, abalones, and Pacific striped bass) are also overutilized.

Examples of resource overutilization can be found in all regions, but the worse cases occur in the Northeast. Cod, haddock, and yellowtail flounder, historically the most important groundfish species on Georges Bank off New England, are presently among the most depleted stocks in U.S. waters. By 1994, the Georges Bank haddock and yellowtail flounder had collapsed, and Georges Bank cod was in danger of collapse. Restrictive controls on fishing effort, and closure of large areas on Georges Bank, have been implemented to reduce fishing mortaility, and haddock and yellowtail stocks are now improving.

The situation for Pacific salmon is also particularly troubling. Declines in chinook and coho salmon, particularly since the mid-1970s, are linked to changing ocean conditions and freshwater habitat loss and degradation, although intense fishing pressure from competing user groups has exacerbated the problem. Even with remedies already in place, such as harvest reductions, closures of ocean fisheries, and ESA listings, the future viability of these commercial fisheries is uncertain.

Marine Mammals. There are sufficient long-term population data to assign trends for only 18 stocks (12 percent) of mariner mammals in U.S. waters, with the remaining stocks undetermined. Where reliable data are available, 3 stocks are declining, 6 are stable, and 9 are increasing. Some of these trends are summarized below:

• Gray Whale. Recent stock assessments show continued incremental improvement for Alaska stock of North Pacific gray whale while the eastern stock of North Pacific gray whale has fully recovered and is at or above its initial unexploited stock size. This eastern stock has the distinction of being the first marine mammal removed from ESA listing.

• Bowhead Whale. All stocks of bowhead whales were severely depleted during intense commercial whaling prior to the 20^th century, declining from an estimated 10,400-23,000 in the western Arctic to less than 3,000 when commercial whaling on this stock ceased at the end of the 19^th century. Since 1978, the western Arctic stock has increased at a rate of 3.1 percent per year. Native subsistence harvest in Alaska authorized by the International Whaling Commission removes approximately 0.1-0.5 percent of the population per year.

• Hawaiian Monk Seal. The Hawaiian monk seal remains critically endangered throughout its range. Average beach counts at the main reproductive population sites declined 60 percent from 1958 to 1993 but have since remained relatively stable. Human-induced mortality caused major historic declines -- sealers, surviving sailors of wrecked ships, and guano and feather hunters during the 1800s; human disturbances of pupping habitat between 1958 and 1978; and expansion of fisheries in the north west Hawaiian Islands beginning in the 1970s -- and may continue to be an important factor impeding recovery. Since 1979, human disturbance on land has generally declined and is currently a matter of concern only on Midway Island, where ecotourism could cause disturbance.

• Northern Right Whale. The northern right whale is critically endangered throughout its North Atlantic range and no sustained growth in the population is apparent despite six decades of protection. The eastern stock appears to be essentially extinct and the western population numbers fewer than 300 animals. The northern right whale appears to suffer from anthropogenic mortalities more than any other. Entanglement in fishing gear and ship strikes are contributing to the apparent failure to recover.

• Steller Sea Lion. An unprecedented and continuing decline in the western Pacific stock of Steller sea lion has caused a recent change in ESA listing from threatened to endangered. The first reported trend counts of Steller sea lions in Alaska recorded at least 140,000 sea lions in the Gulf of Alaska and Aleutian Islands during 1956-1960 surveys. Since then, the population size has dropped to 39,500 in 1996. Many theories have been implicated causing the decline, including human-caused mortality, interactions with human activities around rookeries, environmental change, El Niño, and disease. In contrast, the eastern Pacific stock increased nearly 6 percent from 1989 (22,600) to 1994 (23,533), an increase that is apparently continuing.

• Florida Manatee. Scientists are concerned by the increasing trend in number and proportion of Florida manatee deaths caused by collisions with watercraft, despite the establishment of manatee speed zones. The number of manatee deaths has exceeded 10 percent of the estimated total population each year since 1974. In 1998, the Florida population was estimated to be more than 2,600 animals.

Sea Turtles. Sea turtles in the Pacific and Atlantic -- the loggerhead, green, Kemp's ridley, leatherback, olive ridley, and hawksbill -- are in perilous condition and are all listed as either endangered or threatened. While trends are particularly difficult to assess, many of the populations appear to be declining in number. The most serious threats are coastal development, commercial fisheries interactions, pollution, and harvest of eggs, juveniles, and adults.

• Loggerhead. The loggerhead turtle was listed as threatened throughout its range in 1970, and its status has not changed. Most recent evidence suggests that the number of nesting females in South Carolina and Georgia may be declining, while the number of nesting females in Florida appears to be stable. The status of loggerhead populations in most areas of the Pacific is unknown due to a lack of historical data.

• Hawksbill. The hawksbill populations in the Atlantic have been severely depleted during the 20^th century as a result of overharvest for trade in products made from its shell and their status has not changed since it was listed as endangered in 1970. The status of the hawksbill throughout the Pacific is unknown, although the Hawaiian population appears to be stable.
• Green. The green sea turtle was listed as endangered/threatened in 1978. The breeding populations off Florida and the Pacific coast of Mexico are listed as endangered while all others are threatened. Population trends are particularly difficult to assess because of wide year-to-year fluctuations in numbers of nesting females. In recent years, the number of nests deposited annually on Atlantic beaches has ranged from less than 450 to over 3,800. The Hawaiian population appears to be increasing.
• Kemp's Ridley. It is estimated that there were once more than 40,000 females nesting annually. Though now numbering only about 954 nesting females, the critically endangered Kemp's ridley nesting population has shown steady increases in the number of nests from approximately 700 in the mid-1980s to nearly 3,500 in 1998. Today, under strict protection, the population appears to be in the earliest stages of recovery.
• Olive Ridley. Large assemblages are found along the Pacific Coast of Mexico and Central America. Although the trend in nesting population appears to be increasing, there continues to be significant pressure on the population from harvest of eggs and incidental capture in trawl and longline fisheries.
• Leatherback. Nesting data for assemblages of leatherbacks in the U.S. Virgin Islands, Puerto Rico, and Florida collected since the early 1980s indicate that the annual number of nests is likely stable. Information regarding the status of the entire leatherback population in the Atlantic is lacking. In the Pacific, leatherbacks are seriously declining at all major nesting beaches, particularly along the Pacific Coast of Mexico and Costa Rica, due to overharvest of eggs, direct harvest of adults, and incidental mortality from fishing.

References

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Pimental, D. L. Lach, R. Zuniga, and D. Morrison, Environmental and Economic Costs Associated with Non-Indigenous Species in the United States (College of Agriculture and Life Sciences, Cornell University, Ithaca, NY, 1999). (http://www.news.cornell.edu/releases/Jan99/species_costs.html)

Sauer, J.R., J.E. Hines, I. Thomas, J. Fallon, and G. Gough, The North American Breeding Bird Survey Results and Analysis,1966-1999, Version 98.1 (U.S. Department of the Interior, Patuxent Wildlife Research Center, Laurel, MD, 2000). (http://www.mbr-pwrc.usgs.gov/bbs/bbs.html)

The Nature Conservancy, Rivers of Life: Critical Watersheds for Protecting Freshwater Biodiversity (TNC, Arlington, VA, 1998).

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U.S. Department of the Interior, U.S. Geological Survey, Biological Resources Division, Status and Trends of the Nation's Biological Resources (GPO, Washington, DC, 1998). (http://biology.usgs.gov/s+t/SNT/index.htm)

Wilkins, K.A., M.C. Otto, and G.W. Smith, Trends in Duck Breeding Populations, 1955-2000 (U.S. Department of the Interior, Fish and Wildlife Service, Office of Migratory Bird Management, Laurel, MD, 2000). (http://migratorybirds.fws.gov/reports/reports.html)

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