Leading Threats to Biodiversity: What’s Imperiling U.S. Species

On April 28, 1987, a biologist hiking through the remote Alakai swamp on the island of Kauai paused to listen to the sweet, flutelike song of a distant bird. He recognized the song as belonging to a Kauai ’o’o (Moho braccatus), a sleek chocolate-brown bird native to these woods. He was surely aware of the significance of this particular song, for during the past four years this particular ’o’o, the very last of its kind, had been the object of much attention among scientists and conservationists. But he could not have known that he was about to become the last person ever to hear it. The next time biologists visited the Alakai swamp, the ’o’o was gone, and yet another American species had moved from the realm of the living to the realm of the dead. The causes of the Kauai ’o’o’s extinction are reasonably clear, although the precise role each factor played in the species’ demise is debatable. Much of the bird’s forested habitat was destroyed for agriculture, leaving only a relatively few safe havens on steep slopes or in wet, inaccessible places. Most of these places, in turn, were eventually overrun with alien species, including feral pigs that destroyed the native vegetation, as well as plants and songbirds transported to Hawaii from around the world. The introduction of mosquitoes to Hawaii, which occurred in 1826 when the crew of a sailing ship dumped the mosquito larvae—infested dregs from their water barrels, created additional problems for Hawaii’s beleaguered birds. The mosquitoes became a vector for the spread of avian malaria and avian pox, diseases that were probably carried by the introduced birds. The native avifauna, presumably including the ’o’o, lacked resistance to these diseases, and many species quickly succumbed. Soon, only the forests at higher elevations, where cold temperatures kept the mosquitoes at bay, offered a disease free environment for the native birds. Eventually, however, the mosque toes reached even these forests, including the Alakai swamp, abetted by feral pig wallows, which created pools of stagnant water ideal for breeding mosquitoes. Thus a combination of factors, including habitat destruction, alien species, and diseases, contributed to the demise of the Kauai ’o’o.

Vanishing Assets: Conservation Status of U.S. Species

“The air was literally filled with pigeons; the light of noonday was obscured as by an eclipse.” So observed John James Audubon, the eminent naturalist and bird artist, of a mass migration of passenger pigeons (Ectopistes migratorius) passing through Kentucky in 1813. For three days the pigeons poured out of the Northeast in search of forests bearing nuts and acorns. By Audubon’s estimate, the flock that passed overhead contained more than I billion birds, a number consistent with calculations made by other ornithologists. As the pigeons approached their roost, Audubon noted that the noise they made “reminded me of a hard gale at sea passing through the rigging of a close-reefed vessel.” Indeed, they were so numerous that by some accounts every other bird on the North American continent was probably a passenger pigeon at the time of European colonization (Schorger 1955). Yet despite this extraordinary abundance, barely 100 years later the last passenger pigeon, a female bird named Martha, died in the Cincinnati Zoo. The vast flocks of passenger pigeons moved around eastern North America, feeding mostly on the fruits of forest trees such as beechnuts and acorns. Two factors conspired to seal their fate. Because of their huge numbers, the birds were easy to hunt, especially at their roosting sites. Hunters were ingenious in developing increasingly efficient ways to slaughter the birds. Armed with sticks, guns, nets, or sulfur fires, hunters swept through the enormous roosting colonies, carting away what they could carry and feeding the remaining carcasses to their pigs. One of these methods, in which a decoy pigeon with its eyes sewn shut was attached to a perch, or stool, gave rise to the term stool pigeon. As the railroads expanded west, enormous numbers could be sent to major urban markets like New York, where pigeons became the cheapest meat available. They were so cheap and abundant that live birds were used as targets in shooting galleries. At the same time that this frontal assault on the pigeons was under way, human settlers were expanding into the interior of the country, clearing large areas of the forests on which the flocks depended for food.

A Remarkable Array: Species Diversity in the United States

The Carolina hemlock (Tsuga caroliniana) survives in just a few rocky streambeds along the lower slopes of the Blue Ridge Mountains. Other species of hemlock abound across the United States, but none bear a close resemblance to this particular tree. The closest relatives of the Carolina hemlock, in fact, survive in only one other forest on Earth, some 7,000 miles away in Hubei province of eastern China. The forests of eastern Asia and eastern North America are so similar that if you were suddenly transported from one to the other, you would be hard-pressed to tell them apart. In the swift mountain streams rushing past these seemingly displaced hemlocks live a number of small, colorful fish known as darters. Darters are found only in North America and have evolved into a prolific variety of fishes. Up to 175 species inhabit U.S. waters, including the famous snail darter (Percina tanasi), which brought endangered species issues to the fore when it held up construction of the Tellico Dam on the Little Tennessee River. How is it that these two organisms, hemlock and darter, one with its closest relatives on the other side of the globe and the other found nowhere else in the world, came to be living side by side? Just how many plants and animals share the piece of Earth that we know as the United States of America? Why these and not others? These are central questions for understanding the diversity of the nation’s living resources. The United States encompasses an enormous piece of geography. With more than 3.5 million square miles of land and 12,000 miles of coastline, it is the fourth largest country on Earth, surpassed only by Russia, Canada, and China. The nation spans nearly a third of the globe, extending more than 120 degrees of longitude from eastern Maine to the tip of the Aleutian chain, and 50 degrees in latitude from Point Barrow above the Arctic Circle to the southern tip of Hawaii below the tropic of Cancer. This expanse of terrain includes an exceptional variety of topographic features, from Death Valley at 282 feet below sea level to Mt. McKinley at 20,320 feet above sea level.

State of the States: Geographic Patterns of Diversity, Rarity, and Endemism

The natural geography of the 50 states varies tremendously, supporting an equally varied suite of wild species—from flocks of tropical birds in southern Florida to caribou migrations across the Alaskan tundra. The geography of risk, too, varies across the nation, reflecting the interaction between natural and human history. Similarly, present-day land and water uses will largely determine the future diversity and condition of the flora and fauna. We can learn much, though, from looking at the current condition of a state’s biota, since this both reflects the past and helps illuminate the future. A state’s ecological complexion and the evolutionary history of its biota are the primary determinants of its biological diversity. These environmental factors have encouraged spectacular diversification in many regions: for instance, the freshwater fish fauna in the Southeast, the magnificent conifers along the Pacific cordillera, and the small mammal assemblages of the arid Southwest. Conversely, geological events such as the expansion and contraction of the ice sheets have left other areas of the country with a more modest array of species. States, however, are artificial constructs laid out on the landscape’s natural ecological patterns. While some state lines follow natural boundaries, such as shorelines or major rivers, most cut across the land with no sensitivity to natural features or topography. Nonetheless, urban and rural dwellers alike identify with the major ecological regions within which they live, and this is often the source of considerable pride. Montana is “big sky country,” referring to the vast open plains that sweep up against the eastern phalanx of the Rocky Mountains. California’s moniker “the golden state” now refers more to its tawny hills of summer—unfortunately at present composed mostly of alien species—than to the nuggets first found at Sutter’s Creek. Maryland, home of the Chesapeake Bay, offers the tasty blue crab (Callinectes sapidus) as its unofficial invertebrate mascot. The list could go on, evidenced by the growing number of states that offer vanity license plates celebrating their natural environment. Natural features have always played a dominant role in determining patterns of settlement and land use.

Biodiversity: Our Precious Heritage

Unusually heavy rains in the winter of 1969 transformed California’s normally dry Owens Valley, causing an explosion of grasses and reeds along the edge of the Owens River. Lying in the eastern rain shadow of the Sierra Nevada, not far from Death Valley, the river flows south down the valley before disappearing into a dry lake bed. By summer the heavy vegetation along the river and its adjacent spring-fed marshes was sucking up moisture and releasing it into the hot, dry air. At the same time, the flow from one of these springs suddenly and mysteriously dropped, and parts of a wetland called Fish Slough began to dry up fast. The disappearance of the small pools that make up Fish Slough would have gone unnoticed in a world not reshaped by human hands. Desert springs and marshes can be verdant one year, parched the next. Human activity, however, had made Fish Slough a vital place. The need for water to support Los Angeles and other cities has led to all manner of water projects, including dams, reservoirs, canals, and aqueducts. One of those projects, the Los Angeles Aqueduct, diverted nearly all the water from the Owens River beginning in 1913, greatly reducing the flows that once created seasonally flooded shallows along the river’s edge. Those shallow, warm waters provided ideal habitat for a unique species offish, the Owens pupfish (Cyprinodon radiosus). The loss of habitat, along with the introduction of exotic species like largemouth bass, gradually eliminated the pupfish from most of its relatively limited range, until the species remained only in Fish Slough. If the marsh disappeared, so would the Owens pupfish. Alerted to the potential disaster, Phil Pister, a fishery biologist working nearby with the California Department of Fish and Game, and two colleagues grabbed nets, buckets, and aerators and raced for the pond (Pister 1993). They removed the last 800 of the two-inch-long pupfish to wire mesh cages in the main channel of the slough. As his colleagues drove off, thinking the pupfish at least temporarily secure, Pister realized that the cages were in eddies out of the main current and that the water in the eddies was not carrying enough dissolved oxygen.

Owning Up to Our Responsibilities: Who Owns Lands Important for Biodiversity?

Standing watch over San Francisco Bay, Ring Mountain lies just a short stretch north of the Golden Gate Bridge. The mountain rises from the Tiburon Peninsula, lined with exclusive residential communities and one of San Francisco’s most desirable suburbs. The slopes of Ring Mountain are exclusive in another sense as well: Here lives the world’s sole population of the Tiburon mariposa lily (Calochortus tiburonemis). Found in the open grasslands near the summit of the mountain, this attractive flower grows only on an unusual rock type known as serpentine for its slick, scalelike, blue-green appearance. A product of California’s restless geology, these rock formations also support several other rare plant species, including the Tiburon paintbrush (Castilleja affinis ssp. Neglecta) and Tiburon jewelflower (Streptanthus niger). While serpentine occurs elsewhere in California and beyond, the Tiburon mariposa lily does not. The view from atop Ring Mountain, a spectacular vista of water, mountains, and skyscrapers, would fetch top dollar for residential development. Bulldozers may well have overrun this unique piece of real estate if The Nature Conservancy had not purchased and managed it as a nature preserve. In buying the land on Ring Mountain, the Conservancy also bought the earth’s entire population of the Tiburon mariposa lily. This created a weighty responsibility, but it also freed the organization to take any action necessary to ensure the survival of this flower and Ring Mountain’s other rare species. Ownership offers the most direct and absolute way to offer conservation to those plants and animals inhabiting a piece of land. On the other hand, land ownership also conveys rights that allow management of property in ways not nearly so beneficial to our native species. Of the four basic strategies for biodiversity conservation discussed in the previous chapter, three relate directly to land: owning and managing land; regulating land use; and influencing land use through nonregulatory means. If we are to understand not only the current condition of species and ecosystems in the United States but also the opportunities and challenges for their long-term protection, we must also understand how the underlying natural patterns relate to the patterns of land ownership and management that have been laid atop them.

More than the Sum of the Parts: Diversity and Status of Ecological Systems

On July 5, 1803, Captain Meriwether Lewis of the First Infantry left Washington, D.C., and headed west. His destination was St. Louis, Missouri, where he was to take command, with his good friend William Clark, of the aptly named Corps of Discovery. President Thomas Jefferson had long dreamed of exploring the West, and on the day before Lewis set out from the capital, Jefferson doubled the size of the country, purchasing 820,000 square miles from France for 3 cents an acre. Jefferson planned the expedition partly to expand commerce in the young nation—he sought the “Northwest Passage,” a water route from coast to coast—but, just as important, to further scientific understanding. Lewis shared with his commander in chief a deep curiosity about the natural world, and the expedition set out with a presidential charge to discover the flora and fauna of the United States. Jefferson, as talented a scientist as has ever held the office of president, introduced Lewis to the leading natural scientists of the day, and they trained him to collect samples of plants and animals. Jefferson instructed the two commanders to record everything they could about the countryside—“the soil and face of the country, its growth and vegetable productions . . . the animals of the country . . . the remains and accounts of any which may be deemed rare or extinct,” he said. And so they did, plainly but accurately. Jefferson’s personal library, one of the largest collections in the country and later the nucleus of the Library of Congress, included copies of works by Linnaeus and John Bartram, along with many other scientific texts. Meriwether Lewis served as Jefferson’s private secretary for two years before leading the expedition west, and Jefferson undoubtedly introduced his protégé to those works. The Corps of Discovery, like the Bartrams and Peter Kalm, played an important role in the ongoing effort to document the natural heritage of the United States.

The Geography of Imperilment: Targeting Conservation toward Critical Biodiversity Areas

The Lake Wales Ridge stretches out along Florida’s central spine, pointing southward like an arrow toward Lake Okeechobee and the Everglades beyond. The “river of grass,” as the Everglades are known, attracts visitors from around the world to experience this unique ecosystem and view its immense wildlife concentrations. Compared to its famous neighbor to the south, the Lake Wales Ridge is virtually unknown to the public. From a biological perspective, though, these low, scrub-covered sand hills are of perhaps greater interest than the immense wetlands of the Everglades, because the ancient sand dunes that form this ridge are home to some of the most distinctive and highly localized species in the world. Yet most of the scrub vegetation that supports these species has been destroyed, replaced by agriculture and housing developments: Only about 15% of this unique habitat remains (Menges 1997). Among the rarest of the ridge’s inhabitants is the Lake Placid scrub mint (Dicerandra frutescens), known from just a handful of localities. This mint produces chemicals that have a powerful deterrent effect on insects and that could provide the key to developing new forms of insect repellents useful to people (Eisner et al. 1990). Although these chemicals protect the mint from being devoured by insects, the plant has little protection against the development pressures that threaten it. Another resident of the ridge is the yellow scrub balm (Dicerandra christmanii), a closely related mint that has an even more restricted distribution. Both of these plants are regarded as critically imperiled (GI), and both are listed by the federal government as endangered. Sharing the Lake Wales Ridge with these rare plants is the Florida scrub jay (Aphelocoma coerulescens), a bird that is mostly restricted to the scrub along Florida’s central ridge but occurs in scattered locations along Florida’s Gulf and Atlantic Coasts as well. Florida scrub jays have the unusual characteristic of living in family groups. To survive in a particular location, these birds need a large enough area of suitable habitat to support a number of these family groupings. This species is regarded as vulnerable (G3) and listed as threatened under the Endangered Species Act.

Safeguarding Our Precious Heritage

Nestled amid the Appalachian Mountains of southwestern Virginia and northeastern Tennessee lies the Clinch Valley, the nation’s leading hot spot for imperiled aquatic organisms. The Clinch River is the only undimmed headwater of the Tennessee River basin, which in turn is the nation’s most biologically diverse drainage system. The surface waters of the Clinch run rich indeed: They are home to at least 29 rare mussels and 19 rare fish. Underground, the region’s limestone bedrock is honeycombed by more than a thousand caves and uncounted underground springs and streams. This little-known world is filled with a menagerie of rare beetles, isopods, and other subterranean insects. These underground realms have yielded more than 30 species new to science in just the past few years. The Clinch Valley is largely rural and sparsely populated. Most residents make their living directly from the land, either mining coal, harvesting timber, grazing cattle, or planting crops. These rural lifestyles have maintained much of the region in a relatively natural state, and more than two-thirds of the Clinch Valley remains forested. The forested hills mask a history of ecologically unsound land use practices, however, that have degraded the legendary quality of the region’s waterways. Virtually anything released in the valley flows downhill into the streams and rivers. Among the greatest threats to the valley’s extraordinary aquatic life are heavy metals leaching from abandoned coal mines, sediment eroding from cutover slopes, and nutrients released by streamside-grazing cattle. These and other threats have already taken a toll on the region’s extraordinary biological richness. Where once there were 60 kinds of freshwater mussels, only about 40 remain. Coastal southern California, in contrast, is one of the most densely populated regions in the nation. It, too, is a hot spot for imperiled species. Its dry Mediterranean climate and varied topography have favored the evolution of a host of unique plants and animals. Altogether, some 86 imperiled species are found along the coast and in the mountains of this nationally significant center of biodiversity. Certain areas stand out even by California standards as having a truly extraordinary diversity of rare species.

Strategies for Biodiversity Protection

After a half century of ditching, diking, and draining the swamplands of southern Florida, a major effort to undo some of the ecological damage of those activities is now under way. In what is perhaps the largest ecological restoration effort of its kind anywhere, the federal and state governments are buying up large parcels of private land, changing dramatically the timing and quantity of freshwater flows to the huge “river of grass” that comprises the Florida Everglades, and even restoring the meanders and backwaters to the same Kissimmee River that an earlier generation of engineers “improved” by straightening and channelizing so as to eliminate its meanders and backwaters. Hundreds of millions of public dollars will be spent in this effort. If it succeeds, the steady degradation of one of the most biologically diverse and distinctive environments of the United States will be halted, and its recovery will have begun. The wood stork (Mycteria americana), snail kite (Rostrhamus sociabilis plumbeus), and Florida panther (Felis concolor coryi) are among the endangered species that this effort may ultimately benefit. Several hundred miles to the north, in the sandhills of North Carolina, a more modest but no less noteworthy conservation effort is under way. There, private owners of woodlots, horse farms, resorts, and even residential property are actively managing their longleaf pines to encourage the presence on their own land of another endangered species, the red-cockaded woodpecker (Picoides borealis). After a quarter century in which many private landowners came to fear the presence of endangered species on their land, sandhills landowners are now inviting them. The state and federal governments are spending few public dollars in this effort, and its scale is much smaller than that of the Everglades restoration. What drives the novel effort in North Carolina is a creative and flexible use of the provisions of the Endangered Species Act to encourage the sort of positive land stewardship that many landowners are willing to embrace. As the Florida and North Carolina examples illustrate, the challenge of effectively conserving the natural biological diversity of the nation requires the use of a flexible and diverse array of strategies.