5.1 INTRODUCTION

Giant panda, California condor, white rhinoceros, whooping crane, blue whale. These vanishing animals have captured the hearts of American school children as well as adults. We feel sympathy for the few surviving individuals of these species, see their fragility on the television screen, and try to protect them from extinction. But are these efforts really our best efforts? In order to evaluate attempts to prevent species from becoming extinct, we must understand extinction, its causes, and its impact on humans.

There are those who wonder what all the fuss is about because extinction is a natural process which has occurred since life began. We have a long, though fragmented, fossil record of species that once existed and no longer do: 600 million years ago in the Cambrian period the ocean floors were dominated by trilobites, an extinct phylum of invertebrates, and 300 million years later, dragonflies with wingspans nearly a meter across flew through primitive forests. In fact, it has been estimated that today's species account for only 2% of all those that have evolved over the millennia. Indeed Darwin recognized the extinction process in his 1869 book, On the Origin of Species: ". . . as new forms are continually and slowly being produced, unless we believe that the number of specific forms goes on perpetually and almost indefinitely increasing, numbers inevitably must become extinct."

As Darwin noted, the species that inhabit the earth are a product of a long sequence of speciation (the creation of new species) and extinction events. These events are driven by physical or biotic changes in the environment. Physical changes include climatic changes due to falling meteorites and continental drift or to more local events such as volcanic eruptions, drought, or fires. Biotic factors include such things as the invasion of new species causing extinction through competition, predation, and parasitic diseases. One factor may decrease population levels to such an extent that another seemingly harmless factor causes the final blow. Thus, when studying extinction, it is often difficult and unrealistic to look for one cause. Several causes may be acting in concert. In modern extinctions, natural events, such as a flood or a drought, may be the final blow to a species driven to low numbers by human actions. We will study these problems as we review examples of extinction and search for clues to prevent future extinctions.

Speciation is generally a slow process, happening in a single evolutionary line or from the splitting of lines. Yet as slow as this process is, it is on the average faster than the rate of extinction. How else could we have so many species on earth, probably more than have existed on earth at any time in the past? So, why is there so much concern over extinction? It is because of the rate at which extinction is presently occurring. The expansion of human populations has caused extinction rates to skyrocket; the balance now has tipped in the opposite direction; extinction rates are outstripping speciation rates.

5.2 NATURAL CAUSES OF EXTINCTION

Extinction is a natural process. Natural extinctions can be divided into two general categories: normal extinctions, those that have occurred gradually throughout evolution, and mass extinctions, those that have occurred on a global level in relatively short geological time due to catastrophic events. Normal extinction occurs as the result of localized, gradual environmental changes working on the variation among species. For example, pupfish left behind by receding waters of large lakes that once occupied Death Valley (California and Nevada) evolved into a number of separate species in isolated springs, while the original lake fishes became extinct. If the water feeding the springs should dry up, the various pupfish species would also become extinct. Mass extinction, on the other hand, is almost always seen to be caused by rapid widespread environmental change. Meteorites, glaciation, continental drift, and vulcanism are large-scale phenomena that invoke large-scale changes. However the dichotomy between normal and mass extinction is not always clear-cut. Sudden catastrophes to some scientists are considered gradual by others, depending on how the geologic record is interpreted.

Although both normal and mass extinctions are usually tied to physical changes in the environment, biotic interactions also play an important contributing role. Competition, predation, parasitism, and disease all have their effects. Thus, South America was once inhabited by many marsupial mammals, much like Australia is today. When North America and South America became joined, North American mammals invaded South America, causing widespread extinction of the marsupials, presumably through predation and competition. The rapidity with which such extinction can occur is amply demonstrated by the success of so many introduced species today, and their frequently devastating effects on native species.

5.3 MASS EXTINCTION

Paleontologists have recorded at least six episodes of mass extinctions, relatively short intervals (lasting from 1 million to 10 million years-short in geological time), in which a significant portion of the earth's taxa became extinct. Five of these episodes are predominately marine and one episode is exclusively terrestrial. The most significant mass extinction occurred 250 million years ago at the end of the Permian period. Some 77 to 96% of the species then alive are believed to have become extinct (Raup 1979; Valentine et al. 1978). In the last 250 million years there appear to have been 9 different periods when extinction rates have increased. Only two of these were drastic enough to be termed mass extinctions. As noted above, a plethora of reasons have been proposed for mass extinctions. For example, for marine mass extinctions, meteorites, massive volcanic eruptions, extraterrestrial radiation, changes in temperature, salinity, and oxygen, and the shortage of various resources or habitats have all been suggested.

The most well-known of the mass extinctions is the most recent, occurring about 65 million years ago at the end of the Cretaceous period. At this time marine reptiles, flying reptiles, and both orders of the dinosaurs died out. There have been numerous hypotheses as to why this occurred. A gradual cooling of the earth's climate may have brought an end to these creatures. The rise and dominance of flowering plants to replace the giant ferns and horsetails that comprised the dinosaurs' diet may have played a role. The appearance of mammals may have led to the dinosaurs' downfall. Yet recently these factors have been suggested to be coincident with or caused by an extra-terrestrial force, a huge meteorite. Walter Alvarez, a geologist at Berkeley, came upon this idea in a rather round-about way, not atypical for scientific endeavors. He was trying to find a tool for determining depositional rates in sedimentary rock (limestone, which is formed by the deposition of marine shells). Because meteorite dust contains a rare element, iridium, and is deposited on the earth at a fairly constant rate, Alvarez reasoned that quantifying the amount of iridium in sedimentary rocks could facilitate the determination of sedimentation rates. In testing this hypothesis on sediments in northern Italy, Alvarez found abnormally high levels of iridium, which indicated a huge influx of meteorite dust. The layer with abnormally high levels of iridium (also known as the Cretaceous-Tertiary boundary) has now been found in at least 50 other sites across the globe. This layer coincides with the end of the "Age of the Reptiles," so Alvarez hypothesized that a huge meteorite fell to the earth causing dinosaur extinctions as well as the extinction of many other members of the flora and fauna. There is evidence that numerous marine forms also became extinct during this period. More recently, support for this theory has come from discovery of a large meteor crater from the right time period near the Yucatan Peninsula in Mexico.

5.4 EARLY HUMAN-INDUCED EXTINCTION

Human-induced extinction is often viewed as a modern event, yet there is evidence that our ancestors were responsible for extinctions as well, both indirectly by changing the landscape and directly through hunting. The discovery of the use of fire had a significant impact on the course of human evolution and shaped the landscape humans inhabited. Its use has been documented for at least a quarter million years. Not only did ancient humans abandon campfires which started conflagrations, but they deliberately started fires in order to rouse and drive game during hunting. Fires opened up pasture for game, improved yields of certain plants, and later was used to clear land for agriculture. The Mediterranean and much of Europe has been substantially altered by humans.

"Denudation of the forests made such inroads upon the wood supply of Italy that by the fifth century Roman architectural technique had become modified to meet the growing scarcity and increased price of wood . . . Fires were often started, either intentionally or accidentally, by the herdsmen who ranged the mountain forests with their sheep and goats in the dry season. Burning improved the pasturage, because the ashes temporarily enriched the soil and the abundant shoots from the old roots furnished better fodder. The forests once destroyed were hard to restore." (Semple, 1931).

When the forests declined, so did forest-dependent animals. As Europe became more densely settled, many large mammals disappeared from the landscape. The lions and leopards that were still present during the rise of ancient Greek civilization were presumably hunted to extinction, as were the ancestors of modern cattle, the aurochs. The extinction large mammals in parts of Europe is analagous to the disappearance of large mammals in North America around 11, 000 B.P, coincident with the invasion of humans (see Chapter 2). Likewise the loss of giant lemurs in Madagascar and moas in New Zealand followed the invasion of humans, as did the extinction of many species of flightless birds on oceanic islands as they were colonized by Polynesians. Despite evidence that ancient humans may have caused a substantial number of extinctions is modern humans that have played a key role in the demise and decline of most species. A comparison of the earth's population curve and the extermination of mammals reveals a striking conformity (Figure 5.1). Some projections indicate that if extinction rates continue to increase at current rates, we will lose roughly 1/5 of the earth's species by the end of the century. Myers (1981) predicted that the destruction of moist tropical and temperate forests is proceeding so fast that they "may be reduced to degraded remnants by the end of the century, if they are not eliminated altogether. This will represent a biological debacle to surpass all others that have occurred since life first emerged 3.6 billion years ago." A similar, but more hidden crisis, is taking place in temperate freshwater environments, where at least 20% of the fauna is in danger of extinction worldwide (Moyle and Leidy 1992)

Figure 5.1 (a) The increase in human population over the last three hundred years. (b) The number of species of mammals (white bars) and birds (black bars) eliminated over the last three hundred years. Each bar represents a 50-year period.

Is the situation really so dire? The unfortunate answer appears to be yes, unless we change our present practices. It has been calculated that if present rates continue, the loss of tropical species will rival that of every recorded mass extinction event except for that at the end of the Permian.

5.5 WHAT MAKES SPECIES SUSCEPTIBLE TO EXTINCTION?

When humans change the landscape, many species manage to persist. Some actually become more abundant. So what makes some species so vulnerable to extinction? Although there are no steadfast answers to this question, there are key characteristics that seem to increase a species vulnerability. What immediately comes to mind, of course, is low population size, natural or human induced. Yet some species can be very abundant and still highly vulnerable to extinction. Flocks of passenger pigeons once darkened the sky, their abundance was so overwhelming, counted in the billions. Now they are but a memory. What made these exceedingly common birds so vulnerable and other once abundant species so rare, in danger of extinction?

Susceptibility to extinction is tied to many factors including trophic position, distribution, habitat and trophic specialization, and life history characteristics. For example, an animal may be rare because it is a top predator depending on a long list of successful interactions with creatures at lower trophic levels. Timber wolves, Bengal tigers, and bald eagles are examples of species that are rare partly due to their presence on the apex of the trophic pyramid. Such species may be rare even if they occupy a wide geographic range. On the other hand, a species (no matter what its trophic position) that is abundant but confined to a small area can be extremely vulnerable if the area is altered by humans or a natural catastrophe (such as a volcano blowing up). Many plants and invertebrates fit this scenario including many species in the highly diverse tropics. For example, the world's largest butterfly, the Queen Alexandra Birdwing, is confined to a tiny area in the lowlands of New Guinea. Likewise, many fish and other aquatic species are threatened because they occur in isolated lakes, streams, or springs, where the water is desired for use by humans. In California, about half the native fish species are endemic to the state and, of these, two thirds are either threatened, endangered, or have declining populations, mostly because of limited distributions.

Vulnerability to extinction can also be tied to habitat specialization. Species that specialize on certain types of patchily distributed habitats or resources that are infrequently available tend to be rare. Whether this is a symptom of habitat destruction or a cause of their sensitivity is not always apparent. Examples include the spotted owl, which nests only in old growth forests; the whooping crane, which depends on marshes for food and nesting; and the green sea turtle, which requires specific beaches for egg laying. Species that exhibit specialized feeding habits are also at risk. The black footed ferret, which survived mainly on prairie dogs and pocket gophers, is an endangered species partly because rodent control programs destroyed its prey base. The Australian koala that eats only certain types of eucalyptus leaves is threatened.

Life history characteristics that can play a role in increasing vulnerability to extinction are low reproductive rate, large size, and fixed migratory or behavioral patterns. The passenger pigeon is an example. This species was considered the most numerous of all bird species in 1800. In 1880 there were still several thousand pigeons left, and these were so scattered that it was unprofitable to hunt them. Passenger pigeons had low reproductive rates, migrated in dense flocks, and formed breeding communities of a several thousand individuals. It is believed that they needed the stimulus of a large flock to breed, which may explain why they never successfully bred in captivity. Thus when the total population fell below a size that to us seems large, they were still unable to reproduce. Their numbers had falled below the minimum viable population size (see section 5.8)

5.6 ENDANGERED SPECIES: ON THE ROAD TO EXTINCTION

The previous section emphasizes the biological characteristics that are likely to make a species prone to extinction. Indeed, most extinct species and many species in danger of extinction have these characteristics. Unfortunately, the causes of extinction increasingly have much more to do with human activity and much less to do with the characteristics of the species. Given the scope of human activity, virtually any species can be prone to becoming extinct if it happens to be in the wrong place at the wrong time. This can be seen by examining the causes of species becoming threatened or endangered, the final steps towards extinction. Major causes examined briefly here are habitat change, contamination, introduced species, and exploitation. Usually, the decline of a species, however, has multiple causes.

5.6.1 Habitat change

In the last few decades modification of habitat by humans has clearly become the most severe threat to wild organisms and ecosystems. Drainage schemes, reservoir and dam construction, urban, industrial and agricultural development, and deforestation are examples of human induced habitat modification. The view from the window of any jetliner flying over almost any country in the world will reveal the extent to which humans have altered the landscape, with our endless fields, urban spawl, and straightened rivers. Two examples in California of species endangered primarily due to habitat modification are the winter run chinook salmon and the spotted owl.

Winter run chinook salmon are unique to the Sacramento River, adapted for spawning in the cold, spring-fed water of the upper Sacramento, McCloud, and Pit rivers. When Shasta Dam was built in the 1940s, they were cut off from their historic spawning grounds, much of which were flooded by the reservoir as well. Below the dam, flows were greatly altered but increased flows in the summer duplicated the cold-water conditions the salmon needed to rear their young, so they survived the dam building. In fact, it was estimated that winter run chinook salmon populations in the Sacramento River numbered well over 100,000 fish in the mid-1960s. However, after the Red Bluff diversion dam was built in 1966, the population fell to around 2,000 fish and in subsequent years to only a few hundred fish. The new dam blocked the passage of spawning fish upstream despite the presence of salmon ladders (which were badly designed, so they had a hard time finding them). In addition, Shasta Dam was increasingly degrading the spawning habitat: dam operations caused the water to become warmer and the immense Shasta Reservoir prevented any gravel, needed for spawning, from washing downstream, to replace gravel removed by natural riverine processes. The Sacramento River below the Shasta Dam was becoming a warm stream with a solid bed of large rocks, conditions unsuitable for spawning of the few fish that made it over Red Bluff Diversion Dam. Once the salmon was listed as an endangered species, the following steps were taken: (1) the gates of Red Bluff Diversion Dam were raised to allow direct passage of the migrating fish, (2) thousands of tons of gravel were dumped into the river for spawning habitat, and (3) a multi-million dollar device was installed on Shasta Dam so cold water could be sucked from the bottom of the reservoir to lower water temperatures in the river.

Spotted owls largely depend upon large unbroken stands of old growth forest to feed and reproduce. Each pair of birds needs a large amount of this habitat to survive and reproduce, in part because they prefer to feed on a mouse that lives in big trees, the redbacked tree vole. The forest with the appropriate habitat was once widespread along the Pacific coast, from northern California to southern Alaska, where there are forests with trees up to 1,000 years old, and in the Sierra Nevada. These are uneven aged forests; ancient trees stand beside young ones. The complex ecological interaction in these old growth forests are necessary for the survival of other many species of birds and mammals as well, for which the spotted owl stood as a surrogate. Unfortunately for the owl, old growth timber is extremely valuable and has been cut rapidly in the past century. The cutting of old growth forests resulted in the loss and fragmentation of spotted owl habitat to the point where the owl became listed as an endangered species.

5.6.2 Environmental Contamination

Pollution is a pervasive and insidious problem facing humans and all other species on this planet. There is no place on Earth that is free of contaminants. Nevertheless, it is unusual to find an example of a vertebrate species that become extinct or endangered as the direct result of pollution, although many species have their ranges severely restricted by contaminants (e.g., fish that are absent from polluted waters). There is growing consensus, however, that many pollutants have subtle deleterious effects, weakening animals to make them more susceptible to disease or mimicking hormones to reduce reproduction. The best know contaminant problem in wildlife came close to eliminating some of our most spectacular birds. The osprey, peregrine falcon, bald eagle and brown pelican were all victims of the widespread use of DDT as a pesticide. As DDE, a derivative of DDT which cannot be further broken down, passes through the food chain it accumulates, reaching higher and higher concentrations at each step. Top predators thus receive the heaviest doses, which in this case almost led to their extinction. DDE accumulated to such high levels in these birds that it caused a hormonal imbalance resulting in eggshell thinning. Shells became so thin that they broke under the weight of the incubating parents. In some cases the eggs were produced with no shell at all. . The recovery of these predatory birds can be attributed to the banning in 1972 of the use of DDT in this country.

5.6.3 Introduced species

The introduction of exotic species can greatly disrupt ecosystems through predation, competition, and the spread of disease. Invasions of exotic species typically go hand in hand with habitat change because humans change habitats in ways that favor non-native species such as Norways rats, house mice, common carp, pigeons, and starlings. They can finish the job quickly that habitat alteration has started. Thus, in the Colorado River, the unique native fishes had their populations greatly reduced by habitat changes caused by major dams. It was found that these fishes are capable of living as adults in reservoirs behind the dams, but they can’t reproduce because small introduced fishes congregate in the spawning areas and eat all their eggs and young. Thus extinction of some species is likely. Introduced species can be especially devastating on islands. In the absence of natural checks that keep their populations regulated on the continent, introduced species often flourish on islands at the expense of native island fauna, many of which have evolved in the absence of major predators and consequently lack protective avoidance behavior. An example is the introduction of the predatory brown tree snake to Guam. The snakes, which probably arrived on Guam hidden in ship cargo from the Papua New Guinea area, have virtually wiped out the native forest birds of Guam. Nine species of birds, some found nowhere else, have disappeared from this island, and several others are close to extinction.

5.6.4 Exploitation

Exploitation, such as the killing of buffalo and passenger pigeons, was a major source of extinction and endangerment in North America in the 19th century, but is less of a problem here today. Unfortunately, this is not true for the rest of the world, where increasing human populations confine animals and plants to limited areas which makes them increasingly vulnerable to killing for various reasons. Improved technology and weaponry also makes even large animals like whales, tigers, and rhinoceros vulnerable to extirpation, especially as their value as dead animals increases with their rarity. A growing problem is the overexploitation of fish populations in the seas by highly mechanized fishing gear, which is changes ecosystems (as fish populations collapse) and may endanger marine mammals and birds through the loss of their food supply.

The introduction of feral burros is causing major problems for the remaining populations of desert bighorn sheep. An estimated 90% of all bighorn sheep that once inhabited the western United States have vanished. Their demise is primarily due to habitat destruction. In addition, the introduced feral burros are in direct competition with the bighorn sheep for the limited amount of water available in the remaining habitat. Burros are descendants of the African wild ass and are well adapted to arid areas. When the burros escaped or were turned lose at the end of the mining era, they quickly established large populations in California, Nevada, and Arizona.

The behavior of feral burros at water holes so greatly disturbs these sites that bighorn sheep can no longer use these areas to acquire water. The more aggressive burros often drive bighorn sheep away from prime drinking areas. Furthermore, burros congregate around springs thereby polluting the water with feces and urine, reduce the plant cover by rapidly eating all of the plants in the area, and increase the water turbidity by simply standing in it. Burros also compact the soil, which prevents plant growth and causes erosion. The resulting scarcity of forage weakens the bighorn sheep making them especially susceptible to disease spread by domestic animals.

5.6 CONSERVATION BIOLOGY: PREVENTING EXTINCTION

As it became more and more apparent that we are facing a global extinction crisis, the f ield of conservation biology developed (Chapters 1 and 2). Conservation biology has multiple origins. It draws its focus on applied, habitat-oriented solutions from fields such as wildlife management and forestry. It is based largely on theory from biology, especially ecology and population biology. It draws its energy from the environmental movement, from people concerned about the direct effects of environmental change on their own health and well being. It has a philosophical and spiritual foundation stemming from the world’s religions and from the ethical thinking of environmental philosophers. Increasingly, conservation biology also includes major components from the social sciences, recognizing that extinction trends will not be halted unless the major problems of humanity can also be solved through major changes in our social systems. Conservation biology is thus a integrative discipline that is focussed on understanding how humans are changing the world and on finding practical solutions to saving biodiversity. It is based on the assumption that conservation of biodiversity will come from recognizing that humans are part of nature and not separate from it: it is in our own best interests to protect keep ecosystems, habitats, and species from extinction.

The following sections of this chapter give you some idea of the diversity of tools and approaches used by conservation biologists to protect biodiversity. Much more comprehensive discussions of tools and approaches can be found in recent texts in conservation biology.

5.7 MINIMUM VIABLE POPULATION SIZE: AN ANALYTICAL TOOL

Biologists have long realized that the smaller the population, the more susceptible it is to extinction. This is hardly surprising. But how small can a population be? Since the National Forest Management Act of 1976, the term minimum viable population size has come into wide use. This act required the U.S. Forest Service to maintain "viable populations" of all native vertebrate species in each National Forest. But what does this mean? How long are we protecting species for? A somewhat arbitrary definition proposed by Schaffer is that, "a minimum viable population for any given species in any given habitat is the smallest isolated population having a 99% chance of remaining extant for 1000 years despite the foreseeable effects of demographic, environmental, and genetic stochasticity and natural catastrophes." Determining a minimum viable population is not an easy task. Nor can the viability of populations be pegged to one magic number. Despite these problems, the approach is a useful concept to help prevent extinctions.

Ideally when determining a minimum viable population, a biologist must weigh the requirements of the individual species and the external factors that test the ability of a species to adapt. Factors that test the capabilities of a population to adapt include natural factors, including variation in demographic, environmental, and genetic factors, as well as natural catastrophes. The more frequent a population is exposed to these events, the more likely it is to become extinct. Human factors, ranging from urbanization to noise created by vehicles, are typically an additional stress on species which increasingly are pushing species that are naturally stressed towards extinction. Thus the combination of natural and human factors has caused to coho salmon populations to reach dangerously low levels in California streams. Natural factors included long-term droughts and major floods, which caused a natural reduction in potential habitats for juveniles in fresh water, and changes in ocean conditions, which reduced survival of adults in the ocean. Human factors included activities (such as logging and road building) that degraded freshwater habitats in a massive fashion and fishing in the ocean which reduced adult populations. California coho were listed as a threatened species by the National Marine Fisheries Service in 1997, recognizing that many populations of coho were already extinct and that others were at or below the minimum viable population size.

The role chance events play in the survival and reproductive success of a population is termed "demographic stochasticity." For example, the smaller the population, the greater the probability that the offspring of the remaining females will be of one sex. Variation of physical factors such as rainfall, and biological factors such as predation, competition, parasites, and diseases play an increasingly important role the smaller a population becomes. For example, the blackfooted ferret nearly became extinct when an epidemic of canine distemper (a disease ultimately contracted from domestic dogs) swept through the last population. Similarly, the endangered red cockaded woodpecker was decimated along the eastern Atlantic seaboard when Hurricane Hugo devastated much of its remaining forest habitat.

Genetic factors also play an important role in the survival of a species. The smaller a population, the greater the risk of the loss of genetic material because not all individuals reproduce. Smaller populations have fewer genes upon which evolution can act. With less genetic material, populations may lose their vigor, have reduced fertility, and become more susceptible to genetically related problems. Numerous examples of this have been documented in zoo populations. For example, in zoos a strain of white tigers bred from a few individuals is known for being cross-eyed and having abnormal hip joints. Just how small a population must be before such problems become an irreversible problem is unclear and probably varies among species.

Sometimes populations go through a natural crash in numbers and then recover, but without the genetic variability they once had. This is referred to as a genetic "bottleneck" because small populations are rarely completely representative of the original variability in a population. Certain traits that are desirable in an evolutionary sense may be lost. When exposed to selective factors, these populations that have gone through a bottleneck are less likely respond to selective pressure and more likely become extinct. The cheetah is the classic example. The cheetah occurs naturally in the African savannah in low densities, one per every forty or fifty square miles. Cheetahs have been found to have extremely low genetic variability, indicating that all cheetahs today are most likely descended from a very small population that experienced a genetic bottleneck. They are nevertheless a successful predator, although there is concern that their low genetic diversity may make them exceptionally vulnerable to epidemic diseases or make it difficult for them to adapt to major climatic changes.

Because the factors that affect minimum population size are often intertwined and inherently difficult to quantify, assessing the relative importance of each is at best guesswork. The heath hen, once common from New England to Virginia, was reduced to a population of 100 on Martha's Vineyard island by 1900, as the result of human-caused habitat changes and hunting. A portion of the island was set aside as a refuge and, under management, the bird's population increased to 800 in just sixteen years. However, within just a few years, a fire, predation by an unusually high number of goshawks, and disease took its toll. In 1920 the population was under 100, and 12 years later the last survivor of the population, which had a high percentage of sterile male individuals, died.

The direct determination of a minimum viable population size based on multiple factors has rarely been attempted. Experiments on extinction are for obvious reasons impossible to perform! Other direct attempts require large data sets and complicated computer models. Shaffer (1981) used a simulation approach for the grizzly bear in Yellowstone National Park. He found that grizzly bears survival was most affected by demographic and environmental affects. Mortality rate, cub sex ratio, and age at first reproduction most affected survival. More to the point, his results indicated that populations of less than 30-70 bears occupying less than 2500-7400 km² have less than a 95% chance of surviving for even 100 years!

5.8 CAPTIVE BREEDING PROGRAMS: A DESPERATION APPROACH

When a species has reached numbers close to what biologists judge is the minimum viable population size, captive breeding programs may be initiated, at least for large vertebrates. The breeding of wild animals in zoos is practiced for many purposes, but recently the focus has been on breeding endangered species with the intent of reintroduction in the wild. Though most conservationists abhor the thought of placing animals in zoos or game parks, it is generally thought preferable to the total annihilation of a species. However, zoos, until fairly recently, were seen as places that used animals mainly for display before the curious public, that abused animals through neglect or bad living conditions, and that had little redeeming value as conservation tools. In addition, Captive Breeding Programs were scarce and often miserably unsuccessful. In 1972, Perry et al. found of the 162 rare or endangered mammal species in U.S. zoos, 73 had been bred and only about 30 had met with sufficient success to provide any hope of their reintroduction in the wild.

More recently zoos and game parks have been receiving attention for their successes in Captive Propagation (CP) as well as their role in educating the public in conservation. The Arabian Oryx, a small, almost pure white antelope with long, nearly straight horns, was known from biblical times to exist in the Near East. Killing an oryx, known for its endurance and strength, was considered a sign of manhood in this area. This practice did not deplete oryx populations severely when men killed oryxes by throwing spears or shooting antique rifles from camels, but the introduction of automobiles and automatic weapons led to the oryx's annilation in the wild. Fortunately, the oryx existed in zoos--there were sixty-four in three U.S. zoos in 1979. Oryx have been reintroduced into the Middle East and, with Bedouin guards (the people who were partly responsible for their demise), their populations appear to be doing quite well. Other successful reintroductions include thereturn of the European bison in Poland, the black buck in Asia, and wolves in Bavaria. But too often success is claimed upon only the first step, successful breeding in captivity. If the recently reintroduced Arabian oryx never learn their wild ancestor's trick of migrating to seasonal waterholes, have we really saved the species? This is the type of question posed by critics who oppose CP due to genetic, ecological, and behavioral considerations. Consider, for example, the Ashkania Nova zoo herd of eland, which suffers from a high level of disease due to inbreeding. Or, consider the more recent attempts to breed Europe's vanishing storks. Most of the storks raised in captivity and released into the wild do not migrate to Africa as the naturally raised individuals do. To insure their survival, the storks must be fed during the winter. In some cases captive-reared storks are displacing some of the few remaining wild pairs. This example raises a deeper philosophical question: when a species is approaching extinction, how much change in its genetic or behavioral traits are we willing to tolerate in order to save it? Frequently, if captive breeding is "successful," the species can be maintained only in an artificial environment. It is far easier to select for traits that are adaptive in captivity than maintain a semblance of the original wild species.

This phenomenon and a feeling for the intrinsic right of animals to remain "free," had led some conservationists to object to CP on any grounds. There are also those who object to the practice for fear that CP efforts will remove the impetus to preserve natural habitats. And others point out that the decision to conserve a rare animal is a decision to sacrifice a significant number of the few remaining wild specimens to a program that may fail. The California condor is a perfect example of the controversy surrounding CP. This scavenger, with a wingspan of over nine feet, had a wide range, from Florida to Texas and Northeastern Mexico, northward west of the Rockies to British Columbia. By the 1960's the breeding range of the condor had contracted to Southern California and a population of only fifty condors survived. Just 20 years later, less than half the species survived. It appeared that extinction was imminent without intervention by man. This led a group of experts chosen by the National Audubon Society and the American Ornithologists' Union to propose a "hands on" approach to save the wild condor. The U.S. Fish and Wildlife carried out the plan, which included an extensive survey of condor biology through capture and radiotagging. Later it was deemed necessary to begin a captive breeding program. Because of the success in another similiar species, it was felt that birds in captivity could be induced to lay more eggs and survive better than in the wild where they were plagued by illegal shooting, pesticides, limited habitat, and prey availability. This proposal was protested by many conservationists, including members of the Sierra Club and the Audubon Society who originally supported the study. The risks in handling and in rearing a species that was no longer equipped for a wild existence, and the loss of an impetus to protect the condor's native habitat were cited as reasons for their opposition. Kenneth Brower, a nature writer put it in a simpler, albeit poetic, fashion: "And what if nothing can bring the birds back? What if Gymnogyps, watching Los Angeles sprawl towards its last hills, has simply decided it is time to go? Perhaps feeding on ground squirrels, for a bird that once fed on mastodons, is too steep a fall from glory. If it is time for the condor to follow Teratornis, it should go unburdened by radio transmitters."

Yet the breeding of California condors in captivity has been a success and the first tentative attempts at releasing captive-reared birds in the wild are being made. Some of the released birds have died from causes as diverse as consuming antifreeze to being killed by a golden eagle defending its territory. Others have survived, however, and a particularly promising event has been the release of birds in remote canyon country in Arizona, from which they have been absent in historic times.

Many criticize captive breeding efforts for their choices in species. Nearly all the species are large, dramatic species like lions, rhinos, and birds of prey. Yet these very animals are slow breeders and costly to maintain in captivity. There are few programs for insects or rodents! Zoos are beginning to establish priorites for CP and coordinate their efforts. Coordination and trading of animals is particularly important to prevent genetic problems caused by inbreeding. Aside from considering the ecological and scientific importance of the species, CP efforts should be chosen with practical matters such as cost and potential reintroduction success. Despite the success of some CP efforts, and even with increased success, CP can never really be a panacea to extinction. This leads us to consider another means of saving species: protecting the habitats in which they live.

5.9 RESERVE DESIGN: SETTING ASIDE AREAS TO PREVENT EXTINCTION

Habitat destruction is a major cause of extinction. However, if we are to maintain certain habitats, we must consider the requirements of all species within the ecosystems we are attempting to protect. For example, in determining the boundaries of a nature reserve, we can't overlook the geographic ranges of the species within the reserve. Yellowstone National Park, for example, is large enough to preserve viable populations of butterflies and bison, but not of grizzly bears. The salmon that spawn in a large river flowing through a part are not protected if their migration is blocked by a dam below the park or if the headwaters of the river are outside the park and are logged, resulting in erosion and the smothering of eggs with silt from the land.

In a nutshell, if a variety of species are to persist, a variety of habitats must also persist. Nature reserves should be chosen in a coordinated fashion. Ideally, the international community should coordinate its efforts to choose nature reserves that, when considered jointly, represent the widest range of species possible. For example, Kirkland's warbler nests only in Michigan and winters only in the Bahamas. Preserves in both places are needed to protect the species. Though the ecology of species should be our prime consideration, political and financial realism must also play a role. It does little good to protect a species without the support of those individuals which are most likely to gain from its direct or indirect demise. In the following discussion, we will investigate the biological constraints of reserve design as well as some of the political and economic considerations.

What exactly is a reserve? Reserves come in all shapes, sizes, and are established for various purposes. Some preserves are areas where human use is strictly limited; others permit levels of human use. Partial reserves are fully protected during the breeding season of selected . Extractive reserves allow the extraction of resources in a carefully managed way to insure the maintenance of the reserve ecosystem to the benefit of the individuals who use it for their livelihood. Chico Mendez, a famous Brazilian "seguiero" (rubber tapper), brought this type of system to the limelight in his quest for the protection of Amazon forests for the extraction of rubber and other products from wild trees. It has become increasing apparent that large expanses of land relatively free from human pressures are difficult to find. In fact, on close examination, they are non-existent, even in remote places like the Antarctic. Recently, environmentalists have discovered that the establishment of extractive reserves may be a compromise that allows humans and nature to coexist as they have in some areas for centuries. What ever type of reserve is established, it must be part of a broader system of protected areas and it must be actively managed. The big question always seems to be: given limited financial resources, what is the best way todevelop a system of protected areas?

5.10 THE RESERVE CONTROVERSY: NUMEROUS SMALL OR FEW LARGE ONES?

Traditionally, particularly in the U.S., we have sought to preserve large tracts of wilderness as nature reserves. However, some investigators have questioned whether this is always the most appropriate strategy. Perhaps a series of small reserves, spread across the landscape and representing a wider variety of habitats would be better. If we have to decide between a large number of small reserves or a few large ones, what is the best strategy for maintaining species diversity? One approach to answering this question has been through the study of Island Biogeography. Nature reserves are like islands in a sea of modified habitat. Thus the trend has been to study islands in the hope that they can provide an understanding of what habitat fragmentation will mean to species survival. The theory predicts that over time, islands that have become separated from continents reach a dynamic equilibrium in the number of species. The forces that bring about this equilibrium are immigration of new species and extinction of old ones. This equilibrium depends mostly on the island's size and distance from the continent. Once an area is isolated from its source of immigrant species, the patch undergoes a reduction of species as immigration and extinction rates balance one another. Immigration rates decline causing species diversity to decline, which results in reduced extinction rates due to decreased competition.

However this theory, despite its merit, is not very good at predicting the speed of species loss. Yet logic indicates that large "islands" with larger species populations and the possibility for greater genetic variability would tend to lose species at a slower rate than would smaller islands. Recent studies indicate that this is indeed the case. Studies of land bridge islands (islands that were attached to land before the recent interglacial period began about 14,000 years ago) have supported this view as have studies of small mammal faunas that were isolated on mountain ranges rising out of the Great Basin Desert in California, Utah, and Nevada. These mammal faunas were contiguous during the cooler glacial climates which existed prior to 10,000 years ago. As temperatures rose, the populations and associate woodlands retreated to the tops of the mountain ranges. The analysis of these isolated groups has shown that the bigger the mountain top, the more species will be present.

Such studies indicate that reserves must be very large to limit the number of extinctions. In fact, even the largest reserves that exist today are probably not large enough. Soulé et al. (1979) have predicted that large nature reserves (>10,000 km²) will loose over half their species in 5000 years. However, if immigration rates to reserves can be increased by their proximity to other reserves, extinction rates will also decline. This has led many conservation biologists to propose that nature reserves should be as large and as close together as possible or connected by conservation corridors. These corridors would allow species to disperse between reserves, increasing the probability of their survival even in the face of disturbance. An alternate point of view has been that a series of small reserves would preserve more species diversity than a large reserve of equivalent area because different sets of species would survive in different reserves. Reserve sites could be chosen to protect specific ecosystems, such as a deep canyon or small lake. In addition, the species in a series of smaller reserves would be less likely to be completely annihilated by a calamity such as a fire, storm, or disease because species would be found in more than one location (provided habitat types were found in more than one reserve)..

As you can see, reserve size, number, and proximity are complicated questions that are not easily answered. Ecological and behavioral studies are necessary if we are to make informed decisions on reserve design. The answers are probably mixed depending on the type of habitat and species we wish to protect. As mentioned above, some species require huge ranges; others require limited ranges. Yet as the concerns of various conservationists are evaluated, there is an emerging consensus: We should seek to have as much area protected as possible, a few large preserves to protect the species that require large ranges and many small specialized preserves that will maintain the unique species within them. Establishing more small protected areas in a variety of habitats may save more species than establishing fewer large preserves of equal area. For example, the total number of mammals protected in three different habitats (Redwood, North Cascades, and Big Bend National Parks) exceeds the number in the single largest North American park. We have also begun to realize that even our biggest parks, like Yellowstone National Park, are not big enough. Thus a major proposal is being discussed today to protect the Greater Yellowstone Ecosystem, an immense area surrounding the part that is intimately tied to it through ecosystem processes such as animal migrations. The Greater Yellowstone Ecosystem includes many private and public lands, including towns and ranches. Thus maintaining this ecosystem requires close cooperation between the region’s human inhabitants and ecosystem managers. The difficulty of achieving such cooperation is demonstrated by the slaughter, in the winter of 1996-97, of a majority of the Park’s bison, which were migrating out of the park, in order to protect regional cattle herds from the largely hypothetical threat of contracting a disease from the bison.

Although more research is always needed to make the best decisions, the urgency of the extinction problem implores us to make decisions now. Politics and economics may not always enable us the luxury to consider all issues when designing reserves, but ecology and biology must influence reserve design whenever possible.

5. 10. WATERSHEDS : LANDSCAPE PROTECTION

Most discussions of preserves have focussed on big blocks of land or on regions defined by plant communities, especially trees and shrubs (e.g., coastal sage scrub, redwood forest). Flowing through these blocks of land, however, are streams and rivers. The streams may have their headwaters upstream of the protected area and their mouths far downstream from the protected area. Thus a logging operation in the headwaters may cause a major landslide from which sediment may be carried many miles downstream into the protected area. A dam just downstream of the protected area may prevent migratory fish, such as salmon, from reaching their historic spawning grounds. An upstream pesticide spill may kill fish far into the preserve. A non-native species planted in a downstream reach may invade and become abundant in the preserve. Not surprisingly, many parks and preserves have beautiful forests or prairies with large populations of native mammals and birds, but streams that are degraded, containing non-native species. For example, The Nature Conservancy has a beautiful preserve along the McCloud River in northern California that contains old-growth trees and a high diversity of terrestrial plants and animals. Yet the river is missing some of its most important inhabitants. Chinook salmon and steelhead, once present in large numbers, are now denied access by Shasta Dam downstream. Bull trout, once an important predator on juvenile salmon, are now extinct. Brown trout, an exotic species, however, are common. Thus, this preserve has to be regarded as having lost significant biodiversity. It is likely that more has been lost from the McCloud region than we realize, because the salmon were presumably once a major source of food for bears and other predators, which would carry the nutrients represented by the salmon into the surrounding forests.

The fact that traditional parks and preserves often do not protect aquatic environments adequately is reflected in the growing realization that aquatic species and ecosystems are often the most endangered ecosystems in a region. A recent (1996) evaluation of the status of species and ecosystems in the Sierra Nevada in California, showed that aquatic systems were in the most trouble, with a number of species of frogs, fish, and aquatic invertebrates in danger of extinction. The best way to reverse this trend to protect entire watersheds. Watersheds are the entire drainage basin of a given stream or river, from ridgetop to mouth. Because river systems are made up of small trickles feeding into brooks feeding into bigger streams feeding into rivers, watershed are nested within one another.

For example, unnamed tributaries flow into Cub Creek (a steep trout stream), which flows into Deer Creek ( a salmon spawning stream), which flows into the Sacramento River. Each has a progressively larger watershed which are increasingly difficult to protect as size increases. Cub Creek and its tributaries are completely protected from logging and other insults as a US Forest Service Research Natural Area. Deer Creek flows through the Ishi Wilderness Area and parts of Lassen National Forest and is of great interest for protection because it contains one of the last populations of wild spring-run chinook salmon. Yet about half of its watershed is owned by ranchers, timber companies, and small landowners. Protection of the watershed is being accomplished by a coalition of landowners (Deer Creek Watershed Conservancy) working with state and federal agencies. The general framework for protection is that (1) private landowners must be allowed to continue to make a living from their land, (2) the national forest lands should be operated for overall public benefit, including extractive uses, (3) the key natural elements, such as chinook salmon, must be protected and enhanced, and (4) actions taken in one part of the watershed are likely to affect all other parts of the watershed. This framework indicates that the Deer Creek watershed is not a preserve in the traditional sense, but an ecosystem that includes humans as active players in it.

Deer Creek flows into the Sacramento River, the largest and perhaps most modified river in California. Since it provides much of the water for central and southern California human activities, this river has been extensively dammed and diverted, channelized, polluted and otherwise degraded. Yet there is growing realization that even in this system, the entire watershed must ultimately be managed in an integrated fashion, especially if the remaining natural elements (such as the chinook salmon that must pass through the river to reach Deer Creek) are to be protected. A watershed management strategy may ultimately benefit the human inhabitants of the region as well, by providing a cleaner and more reliable water supply , by providing increased protection from floods, and by increasing the aesthetic and recreational values of the river and its tributaries.

The growing interest in watershed management results from the realization that (1) we all live in watersheds and (2) watersheds are a natural unit on the landscape, typically easy to define even if they cross political boundaries. In the USA, citizen watershed groups are springing up like mushrooms after a rain because people are increasingly understanding the enormous benefits to be gained from wholistic watershed management. One of those benefits is better protection of aquatic and riparian ecosystems and their associated native biota.

5.11 CHOICES: WHAT SHOULD WE PROTECT?

Each species is a unique and separate natural entity that, once lost, can never be revived. The ideal would be to save all species, even every local population of species. Yet this is obviously not a viable option. When protecting species and establishing reserves, we are faced with difficult choices. Which species do we protect? Which habitats are most critical? How do we make these decisions in light of the stresses that overpopulation and human lifestyles place on these systems?

The controversy over the Mount Graham Red Squirrel illustrates the decisions we are faced with. The University of Arizona, with the backing of an international consortium of astronomers, wishes to build a complex of telescopes atop Mount Graham in southeastern Arizona. This same area is the heart of the range for the Mount Graham red squirrel, a distinct subspecies that is found nowhere else and forms the southernmost population of the entire species. It is feared that this project will decimate the habitat of the last 100 surviving squirrels. What is a measly subpopulation of squirrels compared to a project of this magnitude and human significance? Gould (1990) illuminates the real issue at stake. The answer may lie not in the species but in the habitat itself. The Pinaleno Mountains, reaching 10,720 feet at Mount Graham, are an isolated fault block range containing unique old-growth spruce-fir habitat. The squirrel is able to exist in this southerly location because of the high elevation. In addition, these elevated bits of old growth habitat represent "sky-islands" which were islolated 10,000 years ago after the last Ice Age. Islands like these are powerful tools to those who wish to study evolutionary theory and being remnants of the past are precious habitats that probably should not be compromised.

So when considering which areas to conserve, we must weigh every aspect. Biologists tend to think of protecting species for their intrinsic appeal, but in a practical world where politics and economics often play a more important role than aesthetics, it is necessary to measure the utility of species as well as habitats. Decisions must be based on a mixture of historical, evolutionary, community, and species approaches blended with a strong dose of reality. A recent World Resources Institute publication (Reid and Miller, 1989) recommended the following three rules of thumb to evaluate the trade-offs and value judgements made in setting priorities for protecting biodiversity: 1) Distinctiveness. Numbers aren't everything. Preserving an entire species is clearly more important than saving populations of those with numerous representatives. 2) Utility. Global or local, current or future? When evaluating what to save, we clearly have to evaluate utility, often from opposing perspectives. For example, to humanity at large, tropical rainforests are extemely important not only because they contain a variety of life, but because they influence global climate. 3) Threat. Saving the most beleaguered species and ecosystems first. When establishing priorities, it is probably most important to focus on those areas that are most at risk. For example, Central America's tropical rain forests are less threatened than the remaining fragments of tropical dry forest in that region, indicating that our efforts should focus on the latter. Alternately, a triage system can be established, in which the fewest resources are put into saving (1) species and ecosystems for which the extinction is likely no matter what we do and (2) species and ecosystems that may be in decline but for which long-term persistence is likely. The most resources are therefore put into the middle category, species and ecosystems in which an infusion of energy and funds is likely to make the biggest difference in terms of conservation. The reality, of course, is that most protection decisions are made on a political basis. An example of a widespread ecosystem may be protected because it is near a large city and is familiar to local people, while an easily protectable example of a rare habitat may be ignored because it is in a remote part of a distant continent. For this reason, we need global strategies with global funding, focussing on working with local peoples on integrated strategies for conservation, such as watershed conservation strategies.

5.14 CONCLUSION

Extinction is a natural phenomenon, part of the natural selection process, yet the rates of extinction far surpass those of the most apocalyptic mass extinctions our planet has ever experienced. Because of human influence, our planet is becoming a biologically impoverished image of the world that supported humanity in past generations. Already we can no longer thrill to the sight of waves of migrating passenger pigeons, hoards of bison, and the splashing of salmon in high mountain rivers. We are a powerful biological entity. We are making choices that will influence humanity for centuries to come, not to mention the earth's biota, even after we have gone. In fifty million years, we may not exist. What does exist will largely be a result of the action we take today. We are the problem; can we be part of the solution?

LITERATURE CITED