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 27e.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.

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 27e.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 27e.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)

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 27e.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 27e.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 27e.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.

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 27e.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.

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 27e.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 27e.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 27e.29) to jointly manage biodiversity and economic activity.

Figure 27e.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 27e.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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