Species extinction is the most obvious aspect of the loss of biodiversity. For example, species form the bulk of the examples in a comprehensive assessment of the state of the planet published in the early 21st century by the Millennium Ecosystem Assessment, an international effort coordinated by the United Nations Environment Programme. The subject of conservation is broader than this, however. Even a species that survives extinction can lose much of its genetic diversity as local, genetically distinct populations are lost from most of the species' original range. Furthermore, ecosystems may shrink dramatically in area and lose many of their functions, even if their constituent species manage to survive. Conservation is involved with studying all these kinds of losses, understanding the factors responsible for them, developing techniques to prevent losses, and, whenever possible, restoring biodiversity.

Conservation is a crisis discipline, one demanded by the unusual rates of loss; it is also a mission-driven one. By analogy, ecology and conservation have the same relationship as physiology and medicine. Human physiology studies the workings of the human body, whereas medicine is mission-oriented and aims to understand what goes wrong and how to treat it. The major parts of this article thus deal first with the “pathology” of extinction—why and how biodiversity is lost—and second with the “treatment” methods to prevent these losses.

Estimates of recent and future extinction rates Estimates of recent and future extinction ratesAccording to the best estimates of the world's environmental experts, human activities have driven species to extinction at rates perhaps 1,000 times the natural, or background, rate, and future rates of extinction will likely be higher. To show how the experts arrived at these conclusions, it is necessary to pose and attempt to answer a series of extremely difficult questions. How many species are there? How fast were species disappearing before human activity became pervasive? How fast are they becoming extinct at present? And finally, it is necessary to ask a further question: What does the future hold for extinctions if current trends continue?

Any absolute estimate of extinction rate, such as extinctions per year, requires knowledge of how many species there are. Unfortunately, this number is not known with any great degree of certainty, and the problems of estimating it are formidable. Taxonomists have described—that is, have given names to—about 1.5 million species. Only about 100,000 of them, comprising terrestrial vertebrates, some flowering plants, and attractive and collectible invertebrates such as butterflies and snails, are popular enough for taxonomists to know well. Birds are exceptionally well-known—there are roughly 10,000 bird species, with only one or two new species being added each year.

One estimate of how many species might still be undescribed involves a comparison of fungi (fungus) and flowering plants (angiosperms (angiosperm)). In Great Britain, where both groups are well-known, there are six times as many named species of fungi as of flowering plants. If this ratio applies worldwide, the world total of about 300,000 species of flowering plants, which are fairly well-known globally, predicts a total of about 1.8 million species of fungi, which are not. Because only about 70,000 species of fungi currently have names, the prediction would suggest that these named species constitute only ¹/₂₅of the species that exist. On the other hand, samples from poorly known parts of the world suggest that the named species constitute a much larger fraction. Although this estimate of the number of species of fungi is almost certainly too high, it nonetheless suggests that there are large numbers of unknown species.

Average species life spans and extinction rates for various groups of fossil marine speciesTo discern the effect of modern human activity on the loss of species requires determining how fast species disappeared in the absence of that activity. Studies of marine fossils show that species last about 1–10 million years (see the table (Average species life spans and extinction rates for various groups of fossil marine species), second column). Assume that all these extinctions happened independently and gradually—i.e., the “normal” way—rather than catastrophically, as they did at the end of the Cretaceous Period about 65 million years ago, when dinosaurs and many other land and marine animal species disappeared. On that basis, if one followed the fates of one million species, one would expect to observe about 0.1–1 extinction per year—in other words, one species going extinct every 1–10 years.

Species have the equivalent of siblings. They are the species' closest living relatives in the evolutionary tree (see evolution: Evolutionary trees (evolution))—something that can be determined by differences in the DNA. The closest relative of human beings is the bonobo (Pan paniscus), whereas the closest relative of the bonobo is the chimpanzee (P. troglodytes). Taxonomists call such related species sister taxa, following the analogy that they are splits from their “parent” species.

The greater the differences between the DNA of two living species, the more ancient the split from their common ancestor. Studies show that these accumulated differences result from changes whose rates are, in a certain fashion, fairly constant—hence, the concept of the molecular clock (see evolution: The molecular clock of evolution (evolution))—which allows scientists to estimate the time of the split from knowledge of the DNA differences. For example, from a comparison of their DNA, the bonobo and the chimpanzee appear to have split one million years ago, and humans split from the line containing the bonobo and chimpanzee about six million years ago.

On either side of North America's Great Plains are 35 pairs of sister taxa including western and eastern bluebirds (Sialia mexicana and S. sialis), red-shafted and yellow-shafted flickers (both considered subspecies of Colaptes auratus), and ruby-throated and black-chinned hummingbirds (Archilochus colubris and A. alexandri). According to the rapid-speciation interpretation, a single mechanism seemed to have created them all. Each pair of sister taxa had one parent species ranging across the continent. Then a major advance in glaciation during the latter part of the Pleistocene Epoch, some tens of thousands of years ago, split each population of parent species into two groups. Each pair of isolated groups evolved to become two sister taxa, one in the west and the other in the east. Finally, the ice retreated, and, as the continent became warm enough, about 10,000 years ago, the sister taxa expanded their ranges and, in some cases, met once again. (For additional discussion of this speciation mechanism, see evolution: Geographic speciation (evolution).)

There is nothing special about the islands of the Pacific that makes their animals and plants prone to extinction. The most famous recent extinction of all, the extermination of the dodo (Raphus cucullatus) by humans and their introduced animals, was one of 33 species of birds, 30 species of land snails, and 11 species of reptiles lost from Mauritius, Rodrigues, and Réunion in the Indian Ocean in the past three centuries. Over the same period, St. Helena and Madeira in the Atlantic Ocean lost 36 species of land snails as a result of human activity. These island extinctions raise an obvious question: Are extinctions predominantly on islands and not continents? Although the disappearance of island species does demonstrate a pattern of sensitivity to becoming extinct, the next example shows the answer to the question above to be “no.”

A small area at the southern tip of Africa houses an extraordinarily rich and unusual variety of plants. It has several types of vegetation, of which the fynbos (a species-rich scrubland of southwestern South Africa) contributes the most species. Of roughly 8,500 species, 36 have become extinct in the past century, and another 618 species are threatened. (In the field of conservation, the term threatened has a specific, technical meaning. It comes from the World Conservation Union's (International Union for Conservation of Nature) 【IUCN's】 Red Lists, the lists of species that are at risk of extinction. A species listed as “threatened” has a high probability of extinction in the wild within the next few decades if nothing is done to prevent it.) For the fynbos, invading alien plants—particularly various species of Australian wattle trees ( acacia)—and the conversion of natural areas to agriculture (agriculture, origins of) are the two major causes of species extinction and endangerment.

At least 60 mammals (mammal) have become extinct worldwide in the past two centuries, about a third of them from Caribbean islands. Of the remainder, 18 mammals were native to Australia, where they constituted about 6 percent of the terrestrial animal species prior to the British colonization of the continent beginning in the late 18th century. (Aboriginal peoples reached Australia at least 42,000 years earlier and, like the Polynesians across the eastern Pacific, eliminated many native species.) More than 40 other living mammal species in Australia have lost at least half their former ranges, and some survive only on protected offshore islands.

Some of the extinctions have taken place in what is now the wheat belt of the southern tip of Western Australia, which retains only 5 percent of its natural woodland vegetation cover. Others have occurred in the southern arid zone, an area of mostly desert dominated by spinifex grass—and very few people. Nonetheless, this is an area where domestic grazing animals have destroyed the natural vegetation and caused extensive soil erosion. Moreover, European rabbits (Oryctolagus cuniculus) introduced in the mid-19th century are competitors of the native mammals, and the red fox (Vulpes vulpes) introduced about the same time has likely destroyed native small-mammal populations even in remote areas. Where foxes are absent in Australia or where their numbers have been controlled, the native mammals seem to have fared better. In addition, farmers and graziers have greatly altered the natural fire regimes—the roles that fire plays in ecosystems in the absence of human intervention—with consequent changes to the vegetation and the animals that depend on it.

Estimates of recent and future extinction ratesThe same approach can be used to estimate recent extinction rates for various other groups of plants and animals. One set of such estimates for five major animal groups—the birds discussed above as well as mammals, reptiles, frogs and toads, and freshwater clams—are listed in the table (Estimates of recent and future extinction rates). The calculated extinction rates, which range from 20 to 200 extinctions per million species per year, are high compared with the benchmark background rate of 1 extinction per million species per year, and they are typical of both continents and islands, of both arid lands and rivers, and of both animals and plants.

Some threatened species are declining rapidly. For a proportion of these, eventual extinction in the wild may be so certain that conservationists may attempt to take them into captivity to breed them (see below Protective custody (conservation)). Recent examples include the California condor (Gymnogyps californianus), which has been reintroduced into the wild with some success, and the alala (or Hawaiian crow, Corvus hawaiiensis), which has not. Other species have not been as lucky. In the early 21st century an exhaustive search for the baiji (Lipotes vexillifer), a species of river dolphin found in the Yangtze River, failed to find any. The dolphin had declined in numbers for decades, and efforts to keep the species alive in captivity were unsuccessful.

Conservation justifiably prioritizes tropical moist forests (see tropical forest (tropical rainforest)) because they hold such a large fraction of the global total of species, but a comprehensive strategy must also include the conservation of other distinctive ecosystems. For example, major habitat types such as tropical dry forests, tundra, temperate grasslands, lakes, polar seas (see Arctic Ocean; Southern Ocean), and mangroves all contain characteristic species. Importantly, their biological communities house distinctive ecological and evolutionary phenomena. Some of these ecosystems are more threatened than are tropical moist forests and require immediate conservation action.

A comparison of the WWF's list of global priorities and a list of priorities based on threatened species alone uncovers many overlaps, predominantly in the tropical moist forests. It is the differences that are interesting, however. The Everglades of Florida and Brazil's Pantanal, for example, do not rank as places with high concentrations of threatened species, but they achieve prominence among the WWF's global ecoregions because they are among the few flooded grasslands (grassland) in the world, all of which are under threat. Other threatened ecosystems identified on WWF's list include the dry forests of lowland Bolivia, southern Mexico, Madagascar, and Hawaii. The temperate grasslands of North America (see Great Plains), another global priority, once sustained vast migrations of large vertebrates such as bison, pronghorn, and elk. Such species are now restricted to small pockets. The same is true in Eurasia, where comparable migrations now occur only in isolated pockets, primarily in the Daurian Steppe (Steppe, the) in China, Mongolia, and Russia and the Tibetan Plateau. And the extraordinary floral communities of the Eurasian steppes and the North American Great Plains have been largely destroyed through the land's conversion to agriculture (agriculture, origins of).

To learn what makes centres of human-caused extinctions special, one can ask what are their common features. Obviously, many of these places and their species are well known. The 17th-century Dutch artist Rembrandt van Rijn painted birds-of-paradise (bird-of-paradise) and marine cone shells gathered from the early European exploration of the Pacific, testifying to people's fascination for attractive and interesting specimens from “exotic” locales. Victorians filled their cabinets with such curiosities—birds, mammals, marine and terrestrial snail shells, and butterflies—and painted tropical flowers and grew them in their hothouses. Yet the natural history of North America and Europe are also very well known. In fact, what is special about the places with so many extinctions is that each area holds a high proportion of species restricted to it. Scientists call such species endemics.

The simplest model of extinction would be to assume that within a species group—mammals, for instance—all species had roughly the same risk of extinction. Were this to be true, then the more species that live in a region, the more would likely go extinct. This is not the case, however. For example, the Hawaiian Islands and Great Britain in their pasts held very roughly the same number of breeding land birds. Yet, the former have lost more than 100 species, as discussed in the Pacific Islands case study above, while the latter has lost only a few—and those still survive on the European continent. The difference is that almost all the Hawaiian birds (for instance, honeycreepers (Hawaiian honeycreeper) such as the apapane and iiwi) were endemic, while only one of Great Britain's birds (the Scottish crossbill) is. Thus, the number of extinctions in an area depends only very weakly on its total number of species but strongly on its total number of endemics. Areas rich in endemic species are where extinctions will concentrate, unless they are so remote that human actions do not harm them.

Third, terrestrial species with small ranges are geographically concentrated. North America, for instance, has almost no bird species with small ranges. Such species live almost exclusively in the tropics. The greatest concentrations of bird species are in the Amazon lowlands, with secondary centres in Central America and the forests along the coast of Brazil, as can be seen in the accompanying maps-->, which show distributions of songbird species in the Americas. But it is the Andes and the coastal forests that have the most bird species with small geographic ranges; i.e., these areas are the centres of endemism. And it is in these areas that threatened species are concentrated.

In the 1990s a team of researchers led by British environmental scientist Norman Myers identified 25 terrestrial “hot spots” of the world—25 areas on land where species with small geographic ranges coincide with high levels of modern human activity (see the map-->). Originally these hot spots encompassed about 17 million square km (6.6 million square miles) of the roughly 130 million square km (50 million square miles) constituting Earth's ice-free land surface. Species ranges are so concentrated geographically in these regions that, out of a total of about 300,000 flowering-plant species described worldwide, more than 133,000 occur only there. The comparable numbers for birds are roughly 2,800 of 10,000 species worldwide (of which roughly two-thirds are restricted to the land); for mammals, 1,300 of roughly 5,000 worldwide; for reptiles, roughly 3,000 of about 8,000 worldwide; and for amphibians, 2,600 of roughly 5,000 worldwide.

world's 25 hot spotsThe hot spots have been sites of unusual levels of habitat destruction. Only about one-eighth of the original habitat of these areas has survived to the beginning of the 21st century. Of this remaining habitat, only about two-fifths is protected in any way. Sixteen of the 25 hot spots are forests, most of them tropical forests. For comparison, the relatively less-disturbed forests found in the Amazon, the Congo region, and New Guinea have retained about half their original habitats. As a consequence of these high levels of habitat loss, the 25 hot spots are locations where the majority of threatened and recently extinct species are to be found. For additional details about the hot spots discussed above, see the table (world's 25 hot spots).

The land that countries protect, often as national or regional parks, frequently comprises “islands” of original habitat surrounded by a “sea” of cropland, grazing land, or cities and roads. On a real island the number of species that live there depends on its area, with a larger island housing more species than a smaller one. Many studies involving a wide range of animals and plants show that the relationship between area and species number is remarkably consistent. An island half the size of another will hold about 85 percent of the number of species.

How can these results regarding local habitat “islands” be applied to global extinctions? The mostly deciduous forests of eastern North America provide a case history. The birds of the region have been well-described beginning with the explorations of the naturalist and artist John James Audubon (Audubon, John James) in the first quarter of the 19th century, when the area was still mostly forested. Audubon shot and painted many species including four that are now extinct—the Carolina parakeet (Conuropsis carolinensis; see conure), Bachman's warbler (Vermivora bachmanii; see woodwarbler (wood warbler)), the passenger pigeon (Ectopistes migratorius), and (if not extinct, then very nearly so) the ivory-billed woodpecker (Campephilus principalis). A fifth species, the red-cockaded woodpecker (Picoides borealis), is threatened. Including the above, about 160 species of birds once lived in these eastern forests.

Species losses will likely be even greater because this calculation does not include nonendemic species—those that live both inside and outside the hot spots. For example, many of the species that live in the relatively less-disturbed tropical moist forests of the Amazon or the Congo region are the same that live in the adjacent hot spots. Human actions are clearing about 10 percent of the original area of these forests every decade, with a half of the area already gone. Species losses in these forests are still relatively few, but the rate will increase rapidly as the last remaining forests dwindle. If only the same percentage of these forests is protected as is the case for the hot spots, then they too will lose half their species.

Whereas most of the hot spots are tropical moist forests, four areas—the California Floristic Province, the Cape Floristic Province in South Africa, the Mediterranean Basin, and Southwest Australia (see the map-->)—are shrublands. They also are places where people live and grow crops; all four regions are noted for their wines, for example. Not only does this human activity convert land directly, but it also leads to the suppression of fire, especially near people's homes. This alteration of natural fire regimes by the reduction in fire frequencies leads to changes in vegetation, especially to the loss of the native fire-resistant species. Globally, huge areas of grasslands (grassland) and shrublands would become heavily canopied forests were all fires suppressed. The effects of changes in fire frequencies on species losses have not yet been calculated.

The seas cover more than two-thirds of Earth's surface, yet only 210,000 of the 1,500,000 species that have been described are marine animal and plant species. Because the oceans are still poorly explored, the count of marine species may be even more of an underestimate than that of land species. For example, the Census of Marine Life, a decade-long international program begun at the start of the 21st century, added 13,000 new marine species to the total count over the first four years of the effort. As on land, the peak of marine biodiversity lies in the tropics. coral reefs account for almost 100,000 species, yet their total area represents just 0.2 percent of the ocean surface. Between 4,000 and 5,000 species of fish—perhaps a third of the world's marine fish—live on coral reefs. The frequently cited metaphor that “coral reefs are the rainforests of the sea” underscores their importance for conservation.

pollution is a special case of habitat destruction; it is chemical destruction rather than the more obvious physical destruction. Pollution occurs in all habitats—land, sea, and fresh water—and in the atmosphere. global warming, which is discussed separately below (see Global change (conservation)), is one consequence of the increasing pollution of the atmosphere by emissions of carbon dioxide and other gases.

The case histories previously discussed often implicate introduced species as a cause of species extinctions. Humans have spread species deliberately as they colonized new areas, just one example being the Polynesians as they settled the eastern Pacific Islands. New Yorkers in the 1890s wanted all the birds in Shakespeare (Shakespeare, William)'s works to inhabit the city's Central Park, and they introduced the starling (Sturnus vulgaris) to North America as a consequence. Through the centuries hunters have demanded exotic birds and mammals to shoot, fishermen have wanted challenging fish, and gardeners have wanted beautiful flowers. Nonetheless, the consequences in some cases have been devastating. Cacti (cactus) and the shrub Lantana camara, for example, which were introduced as ornamental plants, have destroyed huge areas of grazing land worldwide.

Although not all species devastate the communities they enter, a closer look at what happened on Guam after the introduction of the tree snake Boiga illustrates just how damaging a species can be. Birds started disappearing from the central region of the island in the early 1960s. At the same time, the snakes started causing blackouts as they crawled up power poles and short-circuited the suspended lines, and many people who kept chickens noticed that snakes were killing the birds in their coops. Before Boiga made its appearance, 10 species of native birds lived in the forests of Guam—the Micronesian starling (Aplonis opaca), the Mariana crow (Corvus kubaryi), the Micronesian kingfisher (Halcyon cinnamomina), the Guam flycatcher (Myiagra freycineti), the Guam rail (Rallus owstoni), the rufous fantail (Rhipidura rufifrons), the bridled white-eye (Zosterops conspicillatus), the white-throated ground dove (Gallicolumba xanthonura), the Mariana fruit dove (Ptilinopus roseicapilla), and the cardinal honeyeater (Myzomela cardinalis). An 11th species, the island swiftlet (Aerodramus vanikorensis), nested in caves and fed over the forest. In addition, the island supported a few introduced bird species, birds that nested in wetland habitats, and some seabirds. The 10 forest birds declined following similar patterns—first on the central part of the island, where the main town and port are, and then on southern Guam by the late 1960s. By 1983 scientists could find the birds only in a small patch of forest in the north of the island, and by 1986 they were gone from there as well. The nonnative birds suffered a similar pattern of decline, and many of the waterbirds and seabirds that nested on the island also declined. The starling has survived on a small island, Cocos, off the south coast of Guam, and the swiftlet still nests on the high walls of a cave. The snake has eliminated every species for which it could easily kill the eggs and young in the nest. Of the 10 forest species, the rail and the flycatcher were endemic to the island, while the kingfisher is a distinctive subspecies. The kingfisher and the rail survive in captivity.

whaling offers an example of overharvesting that is interesting not only in itself but also for demonstrating how poorly biodiversity has been protected even when it is of economic value. The first whalers likely took their prey close to shore. right whales were the “right” whales to take because they are large and slow-moving, feed near the surface and often inshore, float to the surface when harpooned, and were of considerable commercial value for their oil and baleen (see whalebone). The southern right whale (Eubalaena australis), for example, is often seen in shallow, sheltered bays in South Africa and elsewhere. Such behaviour would make any large supply of raw materials a most tempting target. Whalers had nearly exterminated the North Atlantic species of the northern right whale (Eubalaena glacialis) and the bowhead whale (Greenland right whale; Balaena mysticetus) by 1800. They succeeded in exterminating the Atlantic population of the gray whale (Eschrichtius robustus). Whalers then moved on to species that were more difficult to kill, such as the humpback whale (Megaptera novaeangliae) and the sperm whale (Physeter macrocephalus).

Other extinctions cause changes that can be quite complicated. Species are bound together in ecological communities to form a food web (see food chain) of species interactions. Once a species is lost, those species that fed on it, were fed on by it, or were otherwise benefited or harmed by that species will all be affected, for better or worse. These species, in turn, will affect yet other species. Ecological theory suggests that the patterns of secondary extinction are quite complicated and thus may be difficult to demonstrate.

The most easily recognizable secondary extinctions should be seen in species that depend closely on each other. In the Hawaiian Islands, anecdotal evidence for secondary extinctions comes from a consideration of nectar-feeding birds. Before modern human activity on the islands, there were three nectar-feeding Hawaiian honeycreepers—the mamo (Drepanis pacifica), the black mamo (Drepanis funerea), and the iiwi (Vestiaria coccinea)—that had long decurved (downward-curving) beaks, the kind adapted to inserting into appropriately long and curved flowers. The first two birds are extinct, whereas the third is extinct on two islands, is very rare on a third, and has declined on others.

The global effects of flooding the atmosphere with carbon dioxide and other greenhouse gases (see greenhouse effect) created as by-products of human activity are many and complex. global warming, an increase in global average surface temperature, is but one of them. Both the land and the oceans are increasing in temperature at different rates in different places. The Arctic and Antarctic seem to be heating up the most, while temperature changes in the tropics are more modest. In addition, the heating is melting glaciers and ice sheets, causing sea levels to rise, and it may be increasing the frequency of the most intense hurricanes and perhaps the intensity of other storms and extreme weather, with consequences to the flow of rivers. Some areas and their ecosystems are becoming wetter, while others are becoming drier and thus likely to suffer more wildfires; some areas may even become colder. Scientific knowledge is constantly increasing and changing what is known about the way Earth works, and knowledge of global change and its effects on species is expanding particularly quickly.

Of the species that have very small ranges, many live in mountainous areas, as can be seen in the map-->. To date, human activity has had relatively small effects on such species because of the impracticalities of cultivating land in mountains, especially on steep slopes. But it is these species—living on cool mountaintops that are now becoming too warm for them—that have nowhere else to go. Such arguments lead scientists to believe that extinctions caused by rising temperatures will be additional to those caused by land-use changes such as deforestation. Rough calculations suggest that rising temperatures may threaten about a quarter of the species in hot spots.

Generally, the larger the body size of an animal, the longer it lives and the fewer offspring it produces each year. Relatively large animals also tend to have relatively low population densities; thus, a viable population of, say, elephants occupies considerably more space than an equal-sized population of rabbits. Large predators such as tigers (Panthera tigris) have lower population densities than the herbivores on which they feed. A tiger has a home range that may occupy 100 square km (40 square miles), while a rabbit may survive in 1 hectare (0.004 square mile).

A comparison of two species, a seal and a rhinoceros, serves to explore the issue in more detail. The northern elephant seal (Mirounga angustirostris) of the Pacific Coast of North America was thought to have been hunted to extinction in the late 1800s, though it later became apparent that perhaps 20–30 individuals persisted locally for a couple of decades before the population began to recover gradually under protection. The Indian rhinoceros (Rhinoceros unicornis) in the early 20th century was reduced to two isolated populations—one numbering between 12 and 100, the other between 60 and 80—before protection allowed it to make a limited recovery. Moreover, not all of the rhinoceros males in the reduced population were likely to have bred. Today the elephant seal is genetically uniform, suggesting that a high degree of inbreeding occurred during the time its population was at a minimum, whereas the rhino has probably lost little of its genetic variability. The population histories of the two species are similar, so why the differences in their genetic variability?

Determining that a species should be brought into captivity and then deciding what to do with the individuals are illustrated by the California condor. The first key decision was whether to bring the birds into protective custody or to manage their small population in the wild. There was no dispute that the condors were once widespread, ranging across the southern and western United States and northern Mexico. As long ago as the turn of the 20th century, however, they were restricted to the mountains of southern California. The widespread practice of setting out poisoned carcasses to kill livestock predators was likely the major cause of their decline. Exactly how many condors still survived was at the core of the debate. Some thought their numbers had been declining constantly, from 150 in the 1950s to 60 in 1970. Others posited constant numbers, and, according to one opinion, if there was no decline, there was need for neither explanation nor intervention—the condors, though rare, should be left alone.

This issue was at the heart of the management dilemma posed by the Florida panther (Puma concolor coryi), a distinct subspecies of puma (P. concolor) confined to a small, isolated, and inbred population in southern Florida. The specific question was whether to introduce pumas from Texas into the Florida population. Florida panthers once had been part of a continuous widespread population. In the 19th century they became isolated in southern Florida. As their numbers declined, the occurrence of genetic defects increased, including sperm and heart defects and undescended testicles in males. Conservation scientists hoped that the introduction of pumas from outside Florida would reverse the genetic damage. This proposal was highly controversial; as with the example of the California condor, some argued that the population was doing well in its limited range and did not need additional animals. Despite concerns, in the mid-1990s eight females from Texas were released, and scientists closely followed the fates of them, of their young, and of the young of cats with only Florida parents. It was found that, although hybrid cats—those with both Florida and Texas parents—do not seem to live longer than pure Florida cats and hybrid females do not produce more kittens than pure Florida females, hybrid kittens survive about twice as well. Since the introduction of the Texas females, numbers of Florida panthers have increased, and hybrid cats were expanding the known range of their habitats.

For species that are hunted or collected, direct protection may be an essential conservation tool. National laws, such as the Endangered Species Act in the United States, make collecting or killing an endangered species or threatened species illegal. An example of such a protected species in the United States is the country's national bird, the bald eagle (Haliaeetus leucocephalus). International laws protect whales of various species, and such agreements as the Convention on International Trade in Endangered Species prohibit commercial trade in designated species. Enforcing these laws and conventions is another matter, however.

The problems of implementing protection are illustrated by the conservation of the two African species of rhinoceros. The population of the black rhinoceros (Diceros bicornis) fell to about 2,400 individuals in 1995, down from a likely number of several hundred thousand at the start of the 20th century, when it ranged over most of southern Africa. The white rhinoceros (Ceratotherium simum) historically had a smaller geographic range. Today its northern subspecies occurs only in the Democratic Republic of the Congo, where it is very rare. The southern subspecies lives almost entirely in South Africa, Namibia, Zimbabwe, and Kenya. Together, they numbered under 12,000 in 2001, again likely a small fraction of their original numbers.

Estimating the economic (economics) value of biodiversity is controversial. The value of whales, for example, once would have been just the price that traders would have paid for their parts, mostly oil and baleen. Today whales have an obvious economic value that comes from what tourists spend to watch them from boats—vessels that often now sail from the same ports that once supported whaling. Less obvious as an economic factor are the deeply held beliefs that hunting whales is cruel or that whales have a right to exist. Many people throughout the world object to whaling even though they may rarely or never have seen whales. Economists incorporate these ideas into their calculations of value by asking how much the public would pay to protect whales.

Martha J. Groom, Gary K. Meffe, and C. Ronald Carroll, Principles of Conservation Biology, 3rd ed. (2006); and Richard B. Primack, Essentials of Conservation Biology, 4th ed. (2006), are introductions to the subject. Michael E. Soulé (ed.), Conservation Biology: The Science of Scarcity and Diversity (1986); and David Western and Mary C. Pearl (eds.), Conservation for the Twenty-first Century (1989), compile the ideas and experiences of several dozen leading conservation scientists.

Stuart L. Pimm, The World According to Pimm: A Scientist Audits the Earth (2001; also published as A Scientist Audits the Earth, 2004), gives an overview of the state of human activity on Earth's terrestrial, freshwater, and marine regions and discusses the effects of this activity on biodiversity. Stuart L. Pimm et al., “The Future of Biodiversity,” Science, 269(5222):347–350 (July 21, 1995), estimates past, present, and future rates of extinction. Extensive tables and figures on the distribution and status of biodiversity are provided in Brian Groombridge and Martin D. Jenkins, World Atlas of Biodiversity: Earth's Living Resources in the 21st Century (2002). N.J. Collar, M.J. Crosby, and A.J. Stattersfield, Birds to Watch 2: The World List of Threatened Birds (1994), describes the fate of the roughly 1,100 species of birds that are globally threatened; because birds are particularly well-known, the book represents the most complete discussion of species threatened with extinction and the underlying causes of the threat. Sandra Postel, Last Oasis: Facing Water Scarcity (1992); and Norman Myers, The Primary Source: Tropical Forests and Our Future, 2nd ed. (1992), analyze the fate of freshwater ecosystems and of tropical forests, respectively. The state of fish stocks within the United States and its territories is evaluated in National Marine Fisheries Service, Report to Congress: Status of Fisheries of the United States (annual). Callum M. Roberts et al., “Marine Biodiversity Hotspots and Conservation Priorities for Tropical Reefs,” Science, 295(5558):1280–84 (Feb. 15, 2002), maps out the biodiversity of coral reefs and the factors involved in their harm.

Robert L. Peters and Thomas E. Lovejoy (eds.), Global Warming and Biological Diversity (1992), explores the likely consequences of global warming on biodiversity. The damage to marine ecosystems by overfishing receives major treatment in Mark Kurlansky, Cod: A Biography of the Fish that Changed the World (1997); and Carl Safina, Song for the Blue Ocean: Encounters Along the World's Coasts and Beneath the Sea (1998). Daniel Simberloff, Don C. Schmitz, and Tom C. Brown (eds.), Strangers in Paradise: Impact and Management of Nonindigenous Species in Florida (1997); and Stuart L. Pimm, “Species that Need No Introduction,” Yearbook of Science and the Future, pp. 200–219 (1994), are reviews of introduced species and their role in reducing biodiversity.

Rosie Woodroffe and Joshua R. Ginsberg, “Edge Effects and the Extinction of Populations Inside Protected Areas,” Science, 280(5372):2126–28 (June 26, 1998), shows that the foraging behaviour of some large predators, such as African wild dogs, puts them at particular risk of local extinction. Ronald L. Westemeier et al., “Tracking the Long-Term Decline and Recovery of an Isolated Population,” Science, 282(5394):1695–98 (Nov. 27,1998), demonstrates that the decline of genetically inbred small populations can be reversed by introducing individuals from genetically diverse larger populations.

Stuart L. Pimm and John H. Lawton, “Planning for Biodiversity,” Science, 279(5359):2068–69 (March 27, 1998), reviews technical approaches to selecting the economically and ecologically most-effective areas to be protected for their biodiversity. William K. Stevens, Miracle Under the Oaks: The Revival of Nature in America (1995), describes the efforts to restore prairies in the American Midwest.

Robert Costanza et al., “The Value of the World's Ecosystem Services and Natural Capital,” Nature, 387(6630):253–260 (May 15, 1997), calculates the annual value of the ecosystem services provided by the environment. Norman Myers and Jennifer Kent, Perverse Subsidies: How Tax Dollars Can Undercut the Environment and the Economy (2001); Geoffrey Heal, Nature and the Marketplace: Capturing the Value of Ecosystem Services (2000); and Gretchen C. Daily and Katherine Ellison, The New Economy of Nature: The Quest to Make Conservation Profitable (2002), discuss the economic factors that harm biodiversity and the development of economic tools to save it. National Research Council (U.S.), Committee on Scientific Issues in the Endangered Species Act, Science and the Endangered Species Act (1995), examines the factors involved in U.S. law in protecting endangered species.Stuart L. Pimm