Lake Victoria's diverse collection of cichlid species is being driven to extinction by an introduced species, the Nile perch. A normal increase in cichlid numbers due to algal blooms led to an explosive increase in perch, which then ate their way through the cichlids.

Case Study: Disruption of Ecological Relationships---Black-Footed Ferrets

Figure 31e.18 Teetering on the brink. The black-footed ferret is a predator of prairie dogs, and loss of prairie dog habitat as agriculture came to dominate the plains states in this century has led to a drastic decline in prairie dogs, and an even more drastic decline in the black-footed ferrets that feed on them. Attempts are now being made to reestablish natural populations of these ferrets, which have been extinct in the wild since 1986. Tim W. Clark.

The black-footed ferret (Mustela nigripes) is one of the most attractive weasels of North America. A highly specialized predator, black-footed ferrets prey on prairie dogs, which live in large underground colonies connected by a maze of tunnels. These ferrets have experienced a dramatic decline in their North American range during this century, as agricultural development has destroyed their prairie habitat, and particularly the prairie dogs on which they feed (figure 31e.18). Prairie dogs once roamed freely over 100 million acres of the Great Plains states, but are now confined to under 700,000 acres (table 31e.6). Their ecological niche devastated, populations of the black-footed ferret collapsed. Increasingly rare in the second half of the century, the black-footed ferret was thought to have gone extinct in the late 1970s, when the only known wild population–a small colony in South Dakota–died out.

TABLE 31e.6 ACRES OF PRAIRIE DOG HABITAT

STATE	1899-90	1998
Arizona	unknown	extinct
Colorado	7,000,000	44,000
Kansas	2,500,000	36,000
Montana	6,000,000	65,000
Nebraska	6,000,000	60,000
New Mexico	12,000,000	15,000
North Dakota	2,000,000	20,400
Oklahoma	950,000	9,500*
South Dakota	1,757,000	244,500
Texas	56,833,000	22,650
Wyoming	16,000,000	70,000 to 180,000

Source: National Wildlife Federation and U.S. Fish and Wildlife report, 1998
*1990

In 1981, a colony of 128 animals was located in Meeteese, Wyoming. Left undisturbed for four years, the number of ferrets dropped by 50%, and the entire population seemed in immediate danger of extinction. A decision was made to capture some animals for a captive breeding program. The first six black-footed ferrets captured died of canine distemper, a disease present in the colony and probably responsible for its rapid decline.

At this point, drastic measures seemed called for. In the next year, a concerted effort was made to capture all the remaining ferrets in the Meeteese colony. A captive population of 18 individuals was established before the Meeteese colony died out. The breeding program proved a great success, the population jumping to 311 individuals by 1991.

In 1991, biologists began to attempt to reintroduce black-footed ferrets to the wild, releasing 49 animals in Wyoming. An additional 159 were released over the next two years. Six litters were born that year in the wild, and the reintroduction seemed a success. However, the released animals then underwent a drastic decline, and only ten individuals were still alive in the wild five years later in 1998. The reason for the decline is not completely understood, but predators such as coyotes appear to have played a large role. Current attempts at reintroduction involve killing the local coyotes. It is important that these attempts succeed, as numbers of offspring in the captive breeding colony are declining, probably as a result of the intensive inbreeding. The black-footed ferret still teeters at the brink of extinction.

Loss of its natural prey has eliminated black-footed ferrets from the wild; attempts to reintroduce them have not yet proven successful.

Case Study: Loss of Genetic Variation---Prairie Chickens

Figure 31e.19 A male prairie chicken performing a mating ritual. He inflates bright orange air sacs, part of his esophagus, into balloons on each side of his head. As air is drawn into the sacs, it creates a three-syllable "boom-boom-boom" that can be heard for miles. © Wm J. Weber / Visuals Unlimited.

The greater prairie chicken Tympanuchus cupido pinnatus is a showy two-pound wild bird renowned for its flamboyant mating rituals (figure 31e.19). Abundant in many Midwestern states, the prairie chickens in Illinois have in the last six decades undergone a population collapse. Once, enormous numbers of birds covered the state, but with the introduction of the steel plow in 1837, the first that could slice through the deep dense root systems of prairie grasses, the Illinois prairie began to be replaced by farmland, and by the turn of the century the prairie had vanished. By 1931, the subspecies known as the heath hen (Tympanuchus cupido cupido) became extinct in Illinois. The greater prairie chicken fared little better in Illinois, its numbers falling to 25,000 statewide in 1933, then to 2,000 in 1962. In surrounding states with less intensive agriculture, it continued to prosper.

In 1962, a sanctuary was established in an attempt to preserve the prairie chicken, and another in 1967. But privately owned grasslands kept disappearing, with their prairie chickens, and by the 1980s the birds were extinct in Illinois except for the two preserves. And they were not doing well. Their numbers kept falling. By 1990, the egg hatching rate, which had averaged between 91 and 100 percent, had dropped to an extremely low 38 percent. By the mid-1990s, the count of males dropped to as low as six in each sanctuary.

What was wrong with the sanctuary populations? One reasonable suggestion was that because of very small population sizes and a mating ritual where one male may dominate a flock, the Illinois prairie chickens had lost so much genetic variability as to create serious inbreeding problems. To test this idea, biologists at the University of Illinois compared DNA from frozen tissue samples of birds that died in Illinois between 1974 and 1993 and found that in recent years, Illinois birds had indeed become genetically less diverse. Extracting DNA from tissue in the roots of feathers from stuffed birds collected in the 1930s from the same population, the researchers found little genetic difference between the Illinois birds of the 1930s and present-day prairie chickens of other states. However, present-day Illinois birds had lost fully one-third of the genetic diversity of birds living in the same place before the population collapse of the 1970s.

Now the stage was set to halt the Illinois prairie chicken's race toward extinction. Wildlife managers began to transplant birds from genetically diverse populations of Minnesota, Kansas, and Nebraska to Illinois. Between 1992 and 1996, a total of 518 out-of-state prairie chickens were brought in to interbreed with the Illinois birds, and hatching rates were back up to 94 percent by 1998. It looks like the Illinois prairie chickens have been saved from extinction.

The key lesson to be learned is the importance of not allowing things to go too far, not to drop down to a single isolated population (figure 31e.20). Without the outlying genetically-different populations, the prairie chickens in Illinois could not have been saved. The black-footed ferrets discussed on the previous page are particularly endangered because they are a single isolated

When their numbers fell, Illinois prairie chickens lost much of their genetic variability, resulting in reproductive failure and the threat of immediate extinction. Breeding with genetically more variable birds appears to have reversed the decline.

Case Study: Habitat Loss and Fragmentation---Songbirds

Every year since 1966, the U.S. Fish and Wildlife Service has organized thousands of amateur ornithologists and bird watchers in an annual bird count called the Breeding Bird Survey. In recent years, a shocking trend has emerged. While year-round residents that prosper around humans, like robins, starlings, and blackbirds, have increased their numbers and distribution over the last thirty years, forest songbirds have declined severely. The decline has been greatest among long-distance migrants such as thrushes, orioles, tanagers, catbirds, vireos, buntings, and warblers. These birds nest in northern forests in the summer, but spend their winters in South or Central America or the Caribbean Islands.

In many areas of the eastern United States, more than three-quarters of the Neotropical migrant bird species have declined significantly. Rock Creek Park in Washington, D.C., for example, has lost 90 percent of its long distance migrants in the last 20 years. Nationwide, American redstarts declined about 50% in the single decade of the 1970s. Studies of radar images from National Weather Service stations in Texas and Louisiana indicate that only about half as many birds fly over the Gulf of Mexico each spring compared to numbers in the 1960s. This suggests a loss of about half a billion birds in total, a devastating loss.

The culprit responsible for this widespread decline appears to be habitat fragmentation and loss. Fragmentation of breeding habitat and nesting failures in the summer nesting grounds of the United States and Canada have had a major negative impact on the breeding of woodland songbirds. Many of the most threatened species are adapted to deep woods and need an area of 25 acres or more per pair to breed and raise their young. As woodlands are broken up by roads and developments, it is becoming increasingly difficult to find enough contiguous woods to nest successfully.

A second and perhaps even more important factor seems to be the availability of critical winter habitat in Central and South America. Living in densely crowded limited areas, the availability of high-quality food is critical. Studies of the American redstart clearly indicate that birds with better winter habitat have a superior chance of successfully migrating back to their breeding grounds in the spring. Peter Marra and Richard Holmes of Dartmouth College and Keith Hobson of the Canadian Wildlife Service captured birds, took blood samples, and measured the levels of the stable carbon isotope ¹³C. Plants growing in the best over-wintering habitats in Jamaica and Honduras (mangroves and wetland forests) have low levels of ¹³C, and so do the redstarts that feed on them. 65% of the wet forest birds maintained or gained weight over the winter. Plants growing in substandard dry scrub, by contrast, have high levels of ¹³C, and so do the redstarts that feed on them. Scrub-dwelling birds lost up to 11% of their body mass over the winter. Now here's the key: birds that winter in the substandard scrub leave later in the spring on the long flight to northern breeding grounds, arrive later when courting and nesting real estate has already been picked over, and have fewer young. You can see this clearly in the redstart study (figure 31e.21): the proportion of ¹³C carbon in birds arriving in New Hampshire breeding grounds increases as spring wears on and scrub-overwintering stragglers belatedly arrive. Thus, loss of mangrove habitat in the Neotropics is having a real negative impact. The Caribbean lost about 10% of its mangroves in the 1980s, and continues to lose about 1% a year. This loss of key habitat appears to be a driving force in the looming extinction of songbirds.

Fragmentation of summer breeding grounds and loss of high-quality overwintering habitat seem both to be contributing to a marked decline in migratory songbird species.

31e.4 Successful recovery plans will need to be multidimensional.

At the Population and Species Level

Once you understand the reasons why a particular species is endangered, it becomes possible to think of designing a recovery plan. If the cause is commercial overharvesting, regulations can be designed to lessen the impact and protect the threatened species. If the cause is habitat loss, plans can be instituted to restore lost habitat. Loss of genetic variability in isolated subpopulations can be countered by transplanting individuals from genetically different populations. Populations in immediate danger of extinction can be captured, introduced into a captive breeding program, and later reintroduced to other suitable habitat. At the population and species level, efforts are focused on dealing with habitat fragmentation, restoring degraded habitat, and interventions to save particular highly-threatened species.

Habitat Fragmentation

Loss of habitat by a species frequently results not only in a lowering of population numbers, but also in fragmentation of the population into unconnected patches (figure 31e.22)---what a conservation biologist would describe as a metapopulation of subpopulations. Further fragmentation often then results in a decrease in the average size of the fragments, an increase in the distance between them, and an increase in the proportion of "edge habitat."

Edge effects can significantly degrade a population's chances of survival. Changes in microclimate (temperature, wind, humidity, etc.) near the edge may reduce appropriate habitat for many species more than the physical fragmentation suggests. Also, increasing habitat edges opens up opportunities for parasites and predators, both more effective at edges. The majority of studies of the effects of habitat fragmentation on birds reveal that enemies such as hawks take a greater toll near the edges. Habitat fragmentation is thought to have been responsible for local extinctions in a wide range of species.

Figure 31e.23 A study of habitat fragmentation. Biodiversity was monitored in the isolated patches of rainforest in Manaus, Brazil before and after logging. Fragmentation led to significant species loss within patches. © 1990 R.O. Bierregaard.

The impact of habitat fragmentation can be seen clearly in a major study done in Manaus, Brazil as the rainforest was commercially logged. Landowners agreed to preserve patches of rainforest of various sizes, and censuses of these patches were taken before the logging started, while they were still part of a continuous forest. After logging, species began to disappear from the now-isolated patches (figure 31e.23). First to go were the monkeys, which have large home ranges. Birds that prey on ant colonies followed, disappearing from patches too small to maintain enough ant colonies to support them.

Several studies have shown that species typically occur only in patches exceeding a threshold size, each species having its own characteristic threshold. Because some species like the monkeys in the Manaus study require large patches, this means that large fragments are indispensable if we wish to preserve high levels of biodiversity. The take-home lesson is that preservation programs will need to provide suitably large habitat fragments to avoid this impact.

Habitat Restoration

Conservation biology typically concerns itself with preserving populations and species in danger of decline or extinction. Conservation, however, requires that there be something left to preserve. In many situations, however, conservation is no longer an option. Species, and in some cases whole communities, have disappeared or have been irretrievably modified. The clearcutting of the temperate forests of Washington State leaves little behind to conserve; nor does converting a piece of land into a wheat field or an asphalt parking lot. Redeeming these situations requires restoration rather than conservation.

Four quite different sorts of habitat restoration programs might be undertaken, depending very much on the cause of the habitat loss.

Pristine Restoration. In situations where all species have been effectively removed, one might attempt to restore the plants and animals that are believed to be the natural inhabitants of the area, when such information is available. In doing this, patches of undisturbed habitat are invaluable. Ecological information is often a limiting factor. When abandoned farmland is to be restored to prairie (figure 31e.24), or other sites to habitats for particular species, it is very important (but often difficult) to acquire the necessary information. While it is in principle possible to reestablish each of the original species in their original proportions, rebuilding a community requires that you know the identity of all of the original inhabitants, and the ecologies of each of the species. We rarely ever have this much information, so no restoration is truly pristine.

(a)		(b)
Figure 31e.24 The University of Wisconsin-Madison Arboretum has pioneered restoration ecology. (a) The restoration of the prairie was at an early stage in November, 1935. (b) The prairie as it looks today. This picture was taken at approximately the same location as the 1935 photograph. Courtesy of University of Wisconsin - Madison Arboretum.

Removing Introduced Species. Sometimes the habitat of a species has been destroyed by a single introduced species. In such a case, habitat restoration involves removal of the introduced species. Introduction of viruses in Australia to remove introduced rabbits is a well-known example. Restoration of the once-diverse cichlid fishes to Lake Victoria will require more than breeding and restocking the endangered species. Eutrophication will have to be reversed, and the introduced water hyacinth and Nile perch populations brought under control or removed.

It is important to move quickly if an invading species is to be stopped. When aggressive African bees (sometimes called "killer bees") were inadvertently released in Brazil in 1957, they remained in the local area only one season. By 1965 they had invaded two-thirds of Brazil; by 1980 they had reached Central America, by 1986 Mexico, and by 1990 Texas. In 33 years they had conquered 5 million square miles (and killed some 1,500 people and over 100,000 cows!). There is no practical way now of removing this introduced species.

Rehabilitation. When a habitat has been totally destroyed, say by paving it over with concrete or asphalt, restoration of the original habitat may not be realistic. Establishment of species similar but not identical to the original inhabitants ("rehabilitation") may be the only practical approach, and in some cases entirely different communities may be preferable ("replacement").

Cleanup. Habitats seriously degraded by chemical pollution cannot be restored until the pollution is cleaned up. The successful restoration of the Nashua River in New England, once polluted and now nearly pristine, was largely a matter of identifying pollutants and then cleaning them up and keeping them out. Textile and paper factories were built on the Nashua River in the 1800s, seriously polluting the river. Outflow from textile mills in Fitchburg, MA, for example, was carried downstream past smaller towns in Massachusetts that added their own contribution of pollution to the river's flow. By the 1960s, the Nashua River was so clogged with wastes that it was declared ecologically dead. Now, forty years later, the Nashua River has been successfully restored. A successful citizen's campaign led to the Massachusetts Clean Water Act of 1966, which mandated cleanup of the Nashua River watershed, and the set up of a system of regulations that prevented its re-pollution.

Captive Propagation

Recovery programs, particularly those focused on one or a few species, often must involve direct intervention in natural populations to avoid an immediate threat of extinction. Earlier we learned how introducing wild-caught individuals into captive breeding programs is being used in an attempt to save ferret and prairie chicken populations in immediate danger of disappearing. Several other such captive propagation programs have had significant success.

Case History: The Peregrine Falcon. American populations of birds of prey such as the Peregrine falcon (Falco peregrinus) began an abrupt decline shortly after World War II. Of the approximately 350 breeding pairs east of the Mississippi River in 1942, all had disappeared by 1960. The culprit proved to be the chemical pesticide DDT (dichlorodiphenyltrichloroethane) and related organochlorine pesticides. Birds of prey are particularly vulnerable to DDT because they feed at the top of the food chain, where DDT becomes concentrated. DDT interferes with the deposition of calcium in the bird's eggshells, causing most of the eggs to break before they hatch.

The use of DDT was banned by federal law in 1972, causing levels in the eastern United States to fall quickly. There were no peregrine falcons left in the eastern United States to reestablish a natural population, however. Falcons from other parts of the country were used to establish a captive breeding program at Cornell University in 1970, with the intent of reestablishing the peregrine falcon in the eastern United States by releasing offspring of these birds By the end of 1986, over 850 birds had been released in 13 eastern states, producing an astonishingly strong recovery (Figure 31e.25).

Case History: The California Condor. Numbers of the California condor (Gymnogyps californianus), a large vulture-like bird with a wingspan of nearly 3 meters, have been declining gradually for the last 200 years. By 1985 condor numbers had dropped so low the bird was on the verge of extinction. Six of the remaining 15 wild birds disappeared that year alone. The entire breeding population of the species consisted of the 6 birds remaining in the wild, and an additional 21 birds in captivity. In a last-ditch attempt to save the condor from extinction, the remaining birds were captured and placed in a captive breeding population. The breeding program was set up in zoos, with release on a large 5300-ha ranch in prime condor habitat. Birds were isolated from human contact as much as possible, and closely related individuals were prevented from breeding. However, as all of the current condor population stems from only 14 founder lines, further inbreeding is unavoidable. By 1996 the captive population of California condors had reached over 120 individuals. Seventeen captive-reared condors have been released in California at two sites in the mountains north of Los Angeles, after extensive pre-release training to avoid power poles and people, all of the released birds seem to be doing well. Six additional birds released into the Grand Canyon have adapted well. Biologists are waiting to see if the released condors will breed in the wild and successfully raise a new generation of wild condors.

Case History: Yellowstone Wolves. The ultimate goal of captive breeding programs is not simply to preserve interesting species, but rather to restore ecosystems to a balanced functional state. Yellowstone Park has been an ecosystem out of balance, due in large part to the systematic extermination of the gray wolf (Canis lupus) in the park early in this century. Without these predators to keep their numbers in check, herds of elk and deer expanded rapidly, damaging vegetation so that the elk themselves starve in times of scarcity. In an attempt to restore the park's natural balance, two complete wolf packs from Canada were released into the park in 1995 and 1996. The wolves adapted well, breeding so successfully that by 1998 the park contained 9 free-ranging packs, a total of 90 wolves.

While ranchers near the park have been unhappy about the return of the wolves, little damage to livestock has been noted, and the ecological equilibrium of Yellowstone Park seems well on the way to recovery. Elk are congregating in larger herds, and their populations are not growing as rapidly as in years past. Importantly, wolves are killing coyotes and their pups, driving them out of some areas. Coyotes, the top predators in the absence of wolves, are known to attack cattle on surrounding ranches, so reintroduction of wolves to the park may actually benefit the cattle ranchers that are opposed to it.

Sustaining Genetic Diversity

One of the chief obstacles to a successful species recovery program is that a species is generally in serious trouble by the time a recovery program is instituted. When populations become very small, much of their genetic diversity is lost (see figure 31e.20), as we have seen clearly in our examination of the case histories of prairie chickens and black-footed ferrets. If a program is to have any chance of success, every effort must be made to sustain as much genetic diversity as possible.

Case History: The Black Rhino. All five species of rhinoceros are critically endangered. The three Asian species live in forest habitat that is rapidly being destroyed, while the two African species are illegally killed for their horns. Fewer than 11,000 individuals of all five species survive today. The problem is intensified by the fact that many of the remaining animals live in very small, isolated populations. The 2400 wild-living individuals of the black rhino Diceros bicornis live in approximately 75 small widely separated groups (figure 31e.26) consisting of six subspecies adapted to local conditions throughout the species' range. All of these subspecies appear to have low genetic variability; in three of the subspecies, only a few dozen animals remain. Analysis of mitochondrial DNA suggests that in these populations most individuals are very closely related.

Figure 31e.26 Sustaining genetic diversity. The black rhino is highly endangered, living in 75 small, widely separated populations. Only about 2400 individuals survive in the wild. Elizabeth N. Orians.

This lack of genetic variability represents the greatest challenge to the future of the species. Much of the range of the black rhino is still open and not yet subject to human encroachment. To have any significant chance of success, a species recovery program will have to find a way to sustain the genetic diversity that remains in this species. Heterozygosity could be best maintained by bringing all black rhinos together in a single breeding population, but this is not a practical possibility. A more feasible solution would be to move individuals between populations. Managing the black rhino populations for genetic diversity could fully restore the species to its original numbers and much of its range.

Placing black rhinos from a number of different locations together in a sanctuary to increase genetic diversity raises a potential problem: local subspecies are adapted in different ways to their immediate habitats–what if these local adaptations are crucial to their survival? Homogenizing the black rhino populations by pooling their genes would destroy such local adaptations, perhaps at great cost to survival.

Preserving Keystone Species

Keystone species are species that exert a particularly strong influence on the structure and functioning of a particular ecosystem. The sea otters of Figure 31e.12 are a keystone species of the kelp forest ecosystem, and their removal can have disastrous consequences. There is no hard and fast line that allows us to clearly identify keystone species. It is rather a qualitative concept, a statement that a species plays a particularly important role in its community. Keystone species are usually characterized by measuring the strength of their impact on their community. Community importance measures the change in some quantitative aspect of the ecosystem (species richness, productivity, nutrient cycling) per unit of change in the abundance of a species.

Figure 31e.27 Preserving keystone species. The flying fox is a keystone species in many Old World tropical islands. It pollinates many of the plants, and is a key disperser of seeds. Its elimination by hunting and habitat loss is having a devastating effect on the ecosystems of many south Pacific islands. Merlin D. Tuttle, Bat Conservation International.

Case History: Flying Foxes. The severe decline of many species of pteropodid bats, or "flying foxes," in the Old World tropics is an example of how the loss of a keystone species can have dramatic effects on the other species living within an ecosystem, sometimes even leading to a cascade of further extinctions (figure 31e.27). These bats have very close relationships with important plant species on the islands of the Pacific and Indian Oceans. The family Pteropodidae contains nearly 200 species, approximately a quarter of them in the genus Pteropus widespread in the islands of the South Pacific, where they are the most important---and often the only---pollinators and seed dispersers. A study in Samoa found that 80%-100% of the seeds landing on the ground during the dry season were deposited by flying foxes. Many species are entirely dependent on these bats for pollination. Some have evolved features like night-blooming flowers that prevent any other potential pollinators from taking over the role of the fruitbats.

In Guam, where the two local species of flying fox have recently been driven extinct or nearly so, the impact on the ecosystem appears to be substantial. Botanists have found some plant species are not fruiting, or are doing so only marginally, with fewer fruits than normal. Fruits are not being dispersed away from parent plants, so offspring shoots are being crowded out by the adults.

Flying foxes are being driven to extinction by human hunting. They are hunted for food, for sport, and by orchard farmers, who consider them pests. Flying foxes are particularly vulnerable because they live in large, easily seen groups of up to a million individuals. Because they move in regular predictable patterns and can be easily tracked to their home roost, hunters can easily bag thousands at a time.

Species preservation programs aimed at preserving particular species of flying foxes are only just beginning. One particularly successful example is the program to save the Rodrigues fruit bat Pteropus rodricensis, which occurs only on Rodrigues Island in the Mascarene Islands. The population dropped from about 1000 individuals in 1955 to fewer than 100 by 1974, the drop reflecting largely the loss of the fruit bat's forest habitat to farming. Since 1974 the species has been legally protected, and the forest area of the island is being increased through a tree-planting program. Eleven captive breeding colonies have been established, and the bat population is now increasing rapidly. The combination of legal protection, habitat restoration, and captive breeding has in this instance produced a very effective preservation program.

Recovery programs at the species level must deal with habitat loss and fragmentation, and often with a marked reduction in genetic diversity. Captive breeding programs that stabilize genetic diversity and careful attention to habitat preservation and restoration are typically involved in successful recoveries.

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At the Community Level

Habitat fragmentation is one of the most pervasive enemies of biodiversity conservation efforts. As we have seen, some species simply require large patches of habitat to thrive, and conservation efforts that cannot provide suitable habitat of such a size are doomed to failure. As it has become clear that isolated patches of habitat lose species far more rapidly than large preserves do, conservation biologists have promoted the creation, particularly in the tropics, of so-called megareserves, large areas of land containing a core of one or more undisturbed habitats (figure 31e.28). The key to devoting such large tracts of land to reserves successfully over a long period of time is to operate the reserve in a way compatible with local land use. Thus, while no economic activity is allowed in the core regions of the megareserve, the remainder of the reserve may be used for nondestructive harvesting of resources. Linking preserved areas to carefully managed land zones creates a much larger total "patch" of habitat than would otherwise be economically practical, and thus addresses the key problem created by habitat fragmentation. Pioneering these efforts, a series of eight such megareserves have been created in Costa Rica (figure 31e.29) to jointly manage biodiversity and economic activity.

Figure 31e.28
Design of a megareserve. A megareserve, or biosphere reserve, recognizes the need for people to have access to resources. Critical ecosystems are preserved in the core zone. Research and tourism is allowed in the buffer zone. Sustainable resource harvesting and permanent habitation is allowed in the multiple-use areas surrounding the buffer.
From Conservation Biology, 2nd edition by Cox © 1997 McGraw-Hill Companies, Inc. Reprinted by permission. All rights reserved.

Figure 31e.29
Biopreserves in Costa Rica. Costa Rica has placed about 12% of its land into national parks and eight megareserves.
From The Quetzal & the Macaw: The Story of Costa Rica's National Parks by David Rains Wallace. Copyright © 1992 by David Rains Wallace. Reprinted with permission of Sierra Club Books.

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Efforts are being undertaken worldwide to preserve biodiversity in megareserves designed to counter the influences of habitat fragmentation.

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Chapter Summary

31e.1 The new science of conservation biology is focusing on conserving biodiversity.

• At the current rate of biodiversity loss, many organisms will become extinct before we have had the chance to learn of their existence.
• Some species have direct economic value and are needed for food, medicines and drugs, and sources of variability for genetic engineering.
• Many other species play a role in maintaining their particular ecosystem, which indirectly, but critically, affects our economy and the world's ecosystem as a whole.
• All species have the ethical right to exist, and, indeed, the beauty and uniqueness of many species warrants their preservation.

31e.2 A great deal is being learned about the dynamics of extinction.

• The stability, spatial heterogeneity, and intense predation in the tropics may all contribute to its high species diversity.
• In various unique places around the world, biologists have described "hot spot" areas that have high concentrations of endemic species.
• As suggested by the species-area relationship, a reduced habitat will support fewer numbers of species.
• This reduction in habitat can occur in four different ways: a habitat can be completely removed or destroyed, a habitat can become fragmented and disjunct, a habitat can be degraded or altered, or a habitat can become too frequently used by humans so as to disturb the species there.
• Conservation biologists often use standard estimates, such as the minimum viable population (MVP) and a population viability analysis (MVA), to determine a particular population's risk of extinction.

31e.3 Causes of endangerment usually reflect human activities.

• Overexploiting a particular species for its commercial products can lead the species to the brink of extinction, as has occurred in whales.
• Introduced species often upset the delicate balance of particular ecosystem in ways that cause the population sizes of local species to decrease drastically.
• As in the case of the prairie habitat, prairie dogs, and black-footed ferrets, the disruption of one component of an ecological relationship can have a profound impact on the other components.
• A loss of genetic variation has occurred in Illinois prairie chickens and may account for their decline during the latter half of the past century.
• The fragmentation of breeding habitat and loss of wintering habitat has led to the drastic decline of many songbird species in the United States.

31e.4 Successful recovery plans will need to be multidimensional.

• Further fragmentation of the habitat must be carefully monitored, as different species have different patch size requirements and reactions to edge effects.
• Pristine restoration of a habitat may be attempted, but removing introduced species, rehabilitating the habitat, and cleaning up the habitat may be more feasible.
• Captive propagation, sustaining genetic variability, and preserving keystone species have been effective in preserving biodiversity.
• Megareserves have been successfully designed in many parts of the world to contain core areas of undisturbed habitat surrounded by managed land.

Key Concepts

1. Conservation biology.  Conservation biology is the study of biodiversity, its rate of loss, and methods for sustaining it.
2. Species-area relationship.  The observation that greater sizes of habitat will support greater numbers of species is called the species-area relationship. Its converse, smaller areas will support smaller numbers of species, has been used to estimate the effects if habitat loss.
3. Endemism.  Endemic species have evolved in, and are restricted to, only one geographic area.
4. Minimum viable population (MVP).  The approximate number or density of individuals necessary for the continuation and growth of a population, or the minimum viable population, is often estimated to determine a population's risk of extinction.
5. Population viability analysis (PVA).  Conservation biologists conduct a population viability analysis to determine how the population size is influenced by random variations in birth rates, death rates, and genetic variability.

Review Questions

1. Even though five mass extinctions have occurred in the past, the current wave of extinction is causing alarm. Why?
2. Describe some of the indirect economic values of biodiversity.
3. Describe the four diversity-generating mechanisms that may explain the high diversity that has been observed in some homogeneous habitats.
4. What factors contribute to the extinction rate on a particular piece of land?
5. Compare and contrast some of the trends in species loss for mammals, birds, and reptiles during the last 400 years.
6. Why do lakes, mountain tops, and oceanic islands tend to have more endemic species?
7. What are some reasons that endemic populations tend to be particularly vulnerable to extinction?
8. How does a low genetic variability contribute to a species' greater risk of extinction?
9. What are some examples of edge effects that put a population at risk?

Thought Questions

1. If the wolves being reintroduced into Yellowstone Park begin to hunt outside the park boundaries and prey on cattle of neighboring ranches, as the coyotes have done in the past when coyotes were the top predators in the park, what do you think should be done?
2. Careful observation suggests that most of the sea otters being eaten by orcas are killed by only a few individual killer whales. Do you think a program of killing orcas seen in the vicinity of sea otters would solve the problem? If not, why not?
3. Many millions of dollars have been spent on the successful peregrine falcon and California condor captive breeding and release programs. It is clearly not going to be possible to devote similar funds to saving a large number of other species. If only a few similar species survival plans could be attempted, how would you recommend choosing which species to attempt saving?

Internet Links

What We Have to Lose
A wonderful collection of links to conservation organizations worldwide, and a rich resource of information about particular species. Contains, for example, the Smithsonian's MAMMALIAN SPECIES OF THE WORLD, a taxonomic hierarchy of all 4,629 currently recognized mammal species. See also http://www.wcmc.org.uk/.

Our Biological Resources
The USGS database of biological resources, maintained by Cornell University. Lots of good links on this NATIONAL BIOLOGICAL INFORMATION INFRASTRUCTURE homepage.

Measuring Biodiversity
A site devoted to assessing the distribution of highly valued terrestrial biodiversity and assigning conservation priorities.

For Further Reading

Gilbert, L. E.: "Food Web Organization and the Conservation of Neotropical Diversity," in M. E. Soule and B. A. Wilcox (eds.), Conservation Biology: An Evolutionary-Ecological Perspective, Sinauer Associates, Sunderland, MA, 1980, pages 11-34. A fine discussion of why species interactions must be considered in efforts to preserve biodiversity, by a researcher deeply involved in studying diversity in the field.

McPeek, M.: "The Consequences of Changing the Top Predator in a Food Web: A Comparative Experimental Approach," Ecological Monograph, 1998, vol. 68, pages 1-23. An excellent example of how experiments can enlighten us about the key role certain species play in the stability of biological communities.

Morell, V.: "Biodiversity: The Fragile Web," National Geographic, February 1999, vol. 195, pages 6-88. Five articles in an issue devoted to front-line efforts to save Earth's biological treasures.

Ruggiero, L. F., G. D. Hayward, and J. R. Squires: "Viability Analysis in Biological Evaluations: Concepts of Population Viability Analysis, Biological Population, and Ecological Scale," Conservation Biology, June 1994, vol. 8, pages 364-372. A good introduction to how conservation biologists employ PVA analysis in practice.

Saunders, D. A., R. J. Hobbs, and C. R. Margules: "Biological Consequences of Ecosystem Fragmentation: a Review," Conservation Biology, March 1991, vol. 5, pages 18-32. One of the most important threats to biodiversity is habitat fragmentation.

Soule, M. E.: "What Is Conservation Biology?," Bioscience, December 1985, vol. 35, pages 727-34. A classic short article describing the new discipline of conservation biology, by one of its key founders.

Stiassny, L. J., and A. Meyer: "Cichlids of the Rift Lakes," Scientific American, February 1999, pages 64-69. The problems experienced by fishes in Lake Victoria are mirrored in the other great African lakes.

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