Forest and Wildlife Management

CHAPTER 13

Botkin, D. B., & Keller, E. A. (2014). Environmental science: Earth as a living planet (9th ed.). Hoboken, NJ: John Wiley & Sons, Inc.

13.1 Traditional Single-Species

Wildlife Management

Wildlife, fisheries, and endangered species are considered together in this chapter because they have a common his- tory of exploitation, management, and conservation, and because modern attempts to manage and conserve them follow the same approaches. Although any form of life— from bacteria and fungi to flowering plants and animals— can become endangered, concern about endangered species has tended to focus on wildlife. We will maintain that fo- cus, but we ask you to remember that the general principles apply to all forms of life.

Attempts to apply science to the conservation and management of wildlife and fisheries, and therefore to endangered species, began around the turn of the 20th century, when each species was viewed as a single popula- tion in isolation.

Carrying Capacity and Sustainable Yields

The classical, early-20th-century idea of wildlife and fish- eries was formalized in the S-shaped logistic growth curve (Figure 13.2). The logistic growth curve assumes that changes in the size of a population are simply the result of the population’s size in relation to a maximum, called the carrying capacity. The logistic characterizes the popu- lation only by its total size, nothing else—not the ratio of young to old, healthy to sick, males to females, and the environment doesn’t appear at all; it is just assumed to be constant. (See the accompanying box Key Characteristics of a Logistic Population.) The carrying capacity is defined simply as the maximum population that can be sustained indefinitely. Implicit in this definition is the idea that if the population exceeds the carrying capacity, it will dam- age its environment and/or its own ability to grow and reproduce, and therefore the population will decline.

Two management goals resulted from these ideas and the logistic equation: For a species that we intend to harvest, the goal was maximum sustainable yield (MSY); for a species that we wish to conserve, the goal was to have that species reach, and remain at, its carrying capac- ity. Maximum sustainable yield is defined as the maxi- mum growth rate (measured either as a net increase in the number of individuals or in biomass over a specified time period) that the population could sustain indefi- nitely. The maximum sustainable-yield population is defined as the population size at which the maximum growth rate occurs. More simply, the population was viewed as a factory that could keep churning out exactly the same quantity of a product year after year.

Today a broader view is developing. It acknowledges that a population exists within an ecological community and within an ecosystem, and that the environment is always, or almost always, changing (including human- induced changes). Therefore, the population you are in- terested in interacts with many others, and the size and condition of those others can affect the one on which you are focusing. With this new understanding, the harvesting goal is to harvest sustainably, removing in each time pe- riod the maximum number of individuals (or maximum biomass) that can be harvested within that time period without diminishing either the population that particu- larly interests you or its ecosystem. The preservation goal for a threatened or an endangered species becomes more open, with more choices. One option is to try to keep the population near its carrying capacity. Another is to sustain a minimum viable population, which is the es- timated smallest population that can maintain itself and its genetic variability indefinitely. A third option that has been suggested, which leads to a population size some- where between the other two, is the optimum sustain- able population.

In the logistic curve, the greatest production occurs when the population is exactly one-half of the carrying capacity (see Figure 13.2). This is nifty because it makes everything seem simple—all you have to do is figure out the carrying capacity and keep the population at one-half of it. But what seems simple can easily become trouble- some. Even if the basic assumptions of the logistic curve were true, which they are not, the slightest overestimate of carrying capacity, and therefore MSY, would lead to overharvesting, a decline in production, and a decline in the abundance of the species. If a population is harvest- ed as if it were actually at one-half its carrying capacity, then unless a logistic population is actually maintained at exactly that number, its growth will decline. Since it is almost impossible to maintain a wild population at some exact number, the approach is doomed from the start.

One important mathematical result is that a logistic population is stable in terms of its carrying capacity—it will return to that number after a disturbance. If the popu- lation grows beyond its carrying capacity, deaths exceed births, and the population declines back to the carrying capacity. If the population falls below the carrying capac- ity, births exceed deaths, and the population increases. Only if the population is exactly at the carrying capacity do births exactly equal deaths, and then the population does not change.

Despite its limitations, the logistic growth curve was used for all wildlife, especially fisheries and includ- ing endangered species, throughout much of the 20th century.

The term carrying capacity as used today has three definitions. The first is the carrying capacity as defined by the logistic growth curve, the logistic carrying capacity already discussed. The second definition contains the same idea but is not dependent on that specific equation. It states that the carrying capacity is an abundance at which a population can sustain itself without any detrimental effects that would lessen the ability of that species—in the abstract, treated as separated from all others—to main- tain that abundance. The third, the optimum sustainable population, already discussed, leads to a population size between the other two.

An Example of Problems with the Logistic Curve

Suppose you are in charge of managing a deer herd for recreational hunting in one of the 50 U.S. states. Your goal is to maintain the population at its MSY level, which, as you can see from Figure 13.2, occurs at exact- ly one-half of the carrying capacity. At this abundance, the population increases by the greatest number during any time period.

To accomplish this goal, first you have to determine the logistic carrying capacity. You are immediately in trou- ble because only in a few cases has the carrying capacity ever been determined by legitimate scientific methods (see Chapter 2) and, second, we now know that the carrying capacity varies with changes in the environment. The pro- cedure in the past was to estimate the carrying capacity by nonscientific means and then attempt to maintain the population at one-half that level. This method requires ac- curate counts each year. It also requires that the environ- ment not vary, or, if it does, that it vary in a way that does not affect the population. Since these conditions cannot be met, the logistic curve has to fail as a basis for manag- ing the deer herd.

An interesting example of the staying power of the logistic growth curve can be found in the United States Marine Mammal Protection Act of 1972. This act states that its primary goal is to conserve “the health and sta- bility of marine ecosystems,” which is part of the mod- ern approach, and so the act seems to be off to a good start. But then the act states that the secondary goal is to maintain an “optimum sustainable population” of marine mammals. What is this? The wording of the act allows two interpretations. One is the logistic carrying capacity, and the other is the MSY population level of the logistic growth curve. So the act takes us back to square one, the logistic curve.

13.2 improved Approaches to Wildlife Management

In the past, the U.S. Council on Environmental Quality (an office within the executive branch of the federal gov- ernment), the World Wildlife Fund of the United States, the Ecological Society of America, the Smithsonian Insti- tution, and the International Union for the Conservation of Nature (IUCN) proposed four principles of wildlife conservation:

• A safety factor in terms of population size, to allow for limitations of knowledge and imperfections of proce- dures. An interest in harvesting a population should not allow the population to be depleted to some theoretical minimum size.

• Concern with the entire community of organisms and all the renewable resources, so that policies developed for one species are not wasteful of other resources.

• Maintenance of the ecosystem of which the wildlife are a part, minimizing risk of irreversible change and long- term adverse effects as a result of use.

• Continual monitoring, analysis, and assessment. The application of science and the pursuit of knowledge about the wildlife of interest and its ecosystem should be maintained and the results made available to the public.

These principles continue to be useful. They broad- en the scope of wildlife management from a narrow focus on a single species to inclusion of the ecological community and ecosystem. They call for a safety net in terms of population size, meaning that no population should be held at exactly the MSY level or reduced to some theoretical minimum abundance. These new prin- ciples provide a starting point for an improved approach to wildlife management.

New approaches to wildlife conservation and man- agement include: (1) historical range of abundance; (2) estimation of the probability of extinction based on historical range of abundance; (3) use of age-structure information (see Chapter 5); and (4) better use of har- vests as sources of information. These, along with an understanding of the ecosystem and landscape context for populations, are improving our ability to conserve wildlife. We discuss these approaches next.

Time Series and Historical Range of Variation

Successful scientific management and conservation of wildlife requires monitoring—an ongoing measure- ment of population size over a number of years. This set of estimates is called a time series and could provide us with a measure of the historical range of variation— the known range of abundances of a population or spe- cies over some past time interval. But shockingly, such monitoring exists for relatively few species of concern (see A Closer Look 13.1, Stories Told by the Grizzly Bear and the Bison). One of the few species with ex- cellent monitoring is the American whooping crane (Figure 13.3), America’s tallest bird, standing about 1.6 m (5 ft) tall. Because this species became so rare and because it migrated as a single flock, people be- gan counting the total population in the late 1930s. At that time, they saw only 14 whooping cranes, a popula- tion that winters in Aransas National Wildlife Refuge in Texas and summers in Wood Buffalo National Park, in Alberta, Canada. They counted all the individuals in the population each year—the total number and the number born that year. The difference between these two numbers gives the number dying each year as well. And from this time series, we can estimate the probability of extinction.

The first estimate of the probability of extinction based on the historical range of variation, made in the early 1970s, was a surprise. Although the birds were few, the probability of extinction was less than one in a bil- lion.1 How could this number be so low? Use of the his- torical range of variation carries with it the assumption that causes of variation in the future will be only those that occurred during the historical period.

Not that the whooping crane is without threats— current changes in its environment may not be simply re- peats of what happened in the past. For the whooping cranes, one catastrophe—such as a long, unprecedented drought on the wintering grounds—could cause a popu- lation decline not observed in the past. Unfortunately, at present, mathematical estimates of the probability of ex- tinction have been done for just a handful of species. The good news is that the wild whooping cranes on the main flyway have continued to do well.

Unfortunately, the original complete census meth- od, counting every bird, was abandoned in 2009, on the grounds that the cranes had become too abundant and widespread for this method to be economical. Since then, a standard aerial sampling method has been used, from which the total population is estimated. Unfor- tunately, the two methods have not been compared, so the new estimation method is not calibrated against the complete census. This decreases the reliability of the new method. However, the good news is that the population appears to be doing well, increasing from 263 in 2010 to 279 in 2011, the most recent date for population estimates.2

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Age Structure as Useful Information

An additional key to successful wildlife management is monitoring of the population’s age structure (see Chapter 5), which can provide many different kinds of in- formation. For example, the age structures of the catch of salmon from the Columbia River in Washington for two different periods, 1941–1943 and 1961–1963, were quite different. In the first period, most of the catch (60%) con- sisted of four-year-olds; the three-year-olds and five-year- olds each made up about 15% of the population. Twenty years later, in 1961 and 1962, half the catch consisted of three-year-olds, the number of five-year-olds had declined to about 8%, and the total catch had declined consider- ably. During the period 1941–1943, 1.9 million fish were caught. During the second period, 1961–1963, the total catch dropped to 849,000, just 49% of the total caught in the earlier period. The shift in catch toward younger ages, along with an overall decline in catch, suggests that the fish were being exploited to a point at which they were not reaching older ages. Such a shift in the age structure of a harvested population is an early sign of overexploitation and of a need to alter allowable catches.

A modification of measuring or estimating the entire age structure that is simpler in practice is to count the number or percentage of the females in a fish population that are at the minimum size that can reproduce. If most or all the female fish caught are at this size, the population is in trouble.3

Harvests as an Estimate of Numbers

Another way to estimate animal populations is to use the number harvested. Records of the number of buf- falo killed were neither organized nor all that well kept, but they were sufficient to give us some idea of the num- ber taken. In 1870, about 2 million buffalo were killed. In 1872, one company in Dodge City, Kansas, handled 200,000 hides. Estimates based on the sum of reports from such companies, together with guesses at how many animals were likely taken by small operators and not reported, suggest that about 1.5 million hides were shipped in 1872 and again in 1873.4 In those years, buf- falo hunting was the main economic activity in Kansas. The Indians were also killing large numbers of buffalo for their own use and for trade. According to estimates, as many as 3.5 million buffalo were killed per year, na- tionwide, during the 1870s.5 The bison numbered at least in the low millions.

Still another way harvest counts are used to estimate previous animal abundance is the catch per unit effort. This method assumes that the same effort is exerted by all hunters/harvesters per unit of time, as long as they have the same technology. So if you know the total time spent in hunting/harvesting and you know the catch per unit of effort, you can estimate the total population. This method leads to rather crude estimates with a large observational error; but where there is no other source of information, it can offer unique insights.

An interesting application of this method is the re- construction of the harvest of the bowhead whale and, from that, an estimate of the total bowhead population. Taken traditionally by Eskimos, the bowhead was the object of “Yankee,” or American, whaling from 1820 until the beginning of World War I. (See A Closer Look 13.4 later in this chapter for a general discussion of ma- rine mammals.) Every ship’s voyage was recorded, so we know essentially 100% of all ships that went out to catch bowheads. In addition, on each ship a daily log was kept, with records including sea conditions, ice conditions, visibility, number of whales caught, and their size in terms of barrels of oil. Some 20% of these logbooks still exist, and their entries have been comput- erized. Using some crude statistical techniques, it was possible to estimate the abundance of the bowhead in 1820 at 20,000, plus or minus 10,000.6 Indeed, it was possible to estimate the total catch of whales and the catch for each year—and therefore the entire history of the hunting of this species.

13.3 Fisheries

Fish are important to our diets—they provide about 16% of the world’s protein and are especially important pro- tein sources in developing countries. Fish supply 6.6% of food in North America (where people are less interested in fish than are people in most other areas), 8% in Latin

America, 9.7% in western Europe, 21% in Africa, 22% in central Asia, and 28% in the Far East.

Scientists estimate that there are 27,000 species of fish and shellfish in the oceans. People catch many of these species for food, but only a few kinds provide most of the food—anchovies, herrings, and sardines account for almost 20% (Table 13.1). Fishing is an in- ternational trade, but a few countries dominate. Japan, China, Russia, Chile, and the United States are among the major fisheries nations. And commercial fisheries are concentrated in relatively few areas of the world’s oceans (Figure 13.6). Continental shelves, which make up only 10% of the oceans, provide more than 90% of the fishery harvest. Fish are abundant where their food is abundant and, ultimately, where there is high production of algae at the base of the food chain. Algae are most abundant in areas with relatively high concentrations of the chemi- cal elements necessary for life, particularly nitrogen and phosphorus. These areas occur most commonly along the continental shelf, particularly in regions of wind-induced upwellings and sometimes quite close to shore.

The world’s total fish harvest has increased greatly since the middle of the 20th century. The total harvest was 35 million metric tons (MT) in 1960. It more than dou- bled in just 20 years (an annual increase of about 3.6%)to 76.6 MT in 1980 (Figure 13.7), then almost doubled again by 1990. The rate of increase slowed slightly in the next ten years—to 42% in 2000, then slowed to a 29% increase by 2009 and almost leveled off in 2010. The total global fish harvest doubled in 20 years because of increases in the number of boats, improvements in technology, and especially increases in aquaculture production. The recent leveling off in fish catch suggests that the harvest may be becoming sustainable. To understand whether this is the case, we need to step back and think about the species that make up the catch and what has been happening to them.

Although the total marine fisheries catch has in- creased during the past half-century, the effort required to catch a fish has increased as well. More fishing boats with better and better gear sail the oceans (Figure 13.8). That is why the total catch can increase while the total population of a fish species declines.

The Decline of Fish Populations

One indication that fish populations were declining at the end of the 20th century came from catch per unit ef- fort. In the 1990s, with marine fish caught with lines and hooks, the catch rate generally fell from 6–12 fish caught per 100 hooks—the success typical of a previously unex- ploited fish population—to 0.5–2.0 fish per 100 hooks just ten years later (Figure 13.9). Overfishing depleted fish populations quickly—about an 80% decline in 15 years. By the end of the century, many of the fish that people ate were predators and the biomass of large predatory fish on fishing grounds was only about 10% of preindustrial levels. These changes indicate that most major commer- cial fishing in the latter 20th century was mining, not sustaining, these living resources. (See A Closer Look 13.2 about similar declines in Chesapeake Bay.)

Species suffering these declines at the end of the 20th century included codfish, haddock, other flatfishes, tuna, swordfish, sharks, skates, and rays. The North Atlantic, whose Georges Bank and Grand Banks have for centu- ries provided some of the world’s largest fish harvests, is suffering. The Atlantic codfish catch was 3.7 million MT in 1957, peaked at 7.1 million MT in 1974, declined to 4.3 million MT in 2000, and climbed slightly to 4.7 in 2001.7 European scientists called for a total ban on cod fishing in the North Atlantic, and the European Union came close to accepting this call, stopping just short of a total ban and instead establishing a 65% cut in the al- lowed catch for North Sea cod for 2004 and 2005.7

Sea scallops in the western Pacific showed a typical harvest pattern, starting with a very low harvest in 1964 at 200 MT, increasing rapidly to 5,887 MT in 1975, declin- ing to 1,489 in 1974, rising to about 7,670 MT in 1993, and then declining to 2,964 in 2002.8 Catch of tuna and their relatives peaked in the early 1990s at about 730,000 MT and fell to 680,000 MT in 2000, a decline of 14% (Figure 13.9).

But during that time, changes in management practices led to remarkable recoveries for some spe- cies. For example, Atlantic sea scallops on Georges Bank increased from a few thousand in 1982 to more than 20,000 by 2002. Management changes includ- ing closing some areas to fishing for the scallops, re- ducing the number of days a vessel was allowed to fish for scallops each year, restricting the kinds of fishing gear (which limited the number of small scal- lops and finfish escaping), and limiting crew size. Ap- parently, closing off part of the sea scallop habitat led to increases in population not only within the closed area, but also in areas open to fishing.9

There is also good news for Georges Bank fish- eries, following the severe restrictions that started at the end of the 20th century (Figure 13.11). Haddock have been fished commercially on Georges Bank since the 19th century and reliable catch statistics began early, in 1904. Annual catch reached 73,200 metric tons (MT) in the 1920s and again in the 1960s, then crashed to 5,600 MT from 1985 to 2000. Once again, se- vere management restrictions similar to those for scallops have led to the start of a recovery, with the catch exceeding 20,000 MT by 2005 (Figure 13.11). But Georges Bank haddock are still considered overfished, and restrictions continue.

Ironically, the fisheries crisis in the 20th century arose for one of the living resources most subjected to science- based management. How could this have happened? First, management has been based largely on the logistic growth curve, whose problems we have discussed. Second, fisheries are an open resource, subject to the problems of Garrett Hardin’s “tragedy of the commons,” discussed in Chapter 3. In an open resource, often in international waters, the num- bers of fish that may be harvested can be limited only by international treaties, which are not tightly binding. Open resources offer ample opportunity for unregulated or illegal harvest, or harvest contrary to agreements.

Exploitation of a new fishery usually occurs before scientific assessment, so the fish are depleted by the time any reliable information about them is available. Fur- thermore, some fishing gear is destructive to the habitat. Ground-trawling equipment destroys the ocean floor, ru- ining habitat for both the target fish and its food. Long- line fishing kills sea turtles and other nontarget surface animals. Large tuna nets have killed many dolphins that were hunting the tuna.

In addition to highlighting the need for better man- agement methods, the harvest of large predators, which many commercial fish are raises questions about ocean ecological communities, especially whether these large predators play an important role in controlling the abun- dance of other species.

Human beings began as hunter-gatherers, and some hunter-gatherer cultures still exist. Wildlife on the land used to be a major source of food for hunter-gatherers. It is now a minor source of food for people of developed nations, although still a major food source for some indigenous peoples, such as the Eskimo. In contrast, even developed na- tions are still primarily hunter-gatherers in the harvesting of fish (see the discussion of aquaculture in Chapter 11).

Can Fishing Ever Be Sustainable?

We need to consider whether fishing can ever be sustainable, but to put this issue in a larger context, the question is whether fish harvest as well as fish populations can be sustainable and, don’t forget, fishermen—as a working population and as an industry—also need to be sustainable. Recovery of sea scal- lops, haddock, and other fish in the Atlantic Ocean following severe restrictions based on careful monitoring and frequent reassessment suggests that fisheries can be sustainable.

Recent events in Gloucester, Massachusetts, illustrate the larger question. Although, as a result of severe manage- ment restrictions, some of the Atlantic ground fish improved through 2006, the fishery has declined again. In January 2013, the New England Fishery Management Council rec- ommended a 77% reduction from last year’s catch for the next three years for cod in the Gulf of Maine and a 66% re- duction from last year’s catch for one year for Georges Bank. These measures appear necessary to save the cod population, as well as the long-term harvest of cod, but the short-term effect may be to put many fishermen out of business.

The discussion in Gloucester, long a port for cod fish- ermen, is whether the port should support other forms of commerce, biotech companies, and warehouse apartments. To help the fishermen, the federal government allocated $150 million, but the city’s mayor suggests that some of that money should be spent developing other industries.11

An even larger question arises, namely: Will America decide to give up any support for the last remaining major commercial harvest of wildlife—its fisheries? As a result, will we then seek much less fish in our diet? Or will we attempt to expand aquaculture to provide the same amount of fish the country now consumes? What would be the effect on agriculture? Would it put an undesirable pressure on land to produce high-protein crops or domestic animals to replace the protein from fish? Should we just eat lower on the food chain? And people concerned about cultural heritage may ask: Should the nation abandon an entire way of life that has been rich in story and song and loved by those engaged in fishing? None of these are simple questions, and the entire set is rarely discussed together.

Suppose you went into fishing as a business and expected reasonable growth in that business in the first 20 years. The world’s ocean fish catch rose from 39 million MT in 1964 to 68 million MT in 2003, an average of 3.8% per year for a total increase of 77%.12 From a business point of view, even assuming all fish caught are sold, that is not considered rapid sales growth. But it is a heavy burden on a living resource.

There is a general lesson to be learned here: Few wild biological resources can sustain a harvest large enough to meet the requirements of a growing business. Although these growth rates begin to look pretty good in times when the overall economy is poor, as was the case during the economic downturn of 2008–2012, most wild biological resources do not prove to be a good business over the long run. We also learned this lesson from the near demise of the bison, discussed earlier, and it is true for whales as well (see A Closer Look 13.3). There have been a few exceptions, such as the several hundred years of fur trading by the Hudson’s Bay Company in northern Canada. However, past experience suggests that economically beneficial sustainability is unlikely for most wild populations.

With that in mind, we note that farming fish— aquaculture (discussed in Chapter 11)—has been an im- portant source of food in China for centuries and is an increasingly important food source worldwide. But aqua- culture can create its own environmental problems.

In sum, fish are an important food and world harvests of fish are large, but the fish populations on which the har- vests depend are generally declining, easily exploited, and difficult to restore. We desperately need new approaches to forecasting acceptable harvests and establishing work- able international agreements to limit catch. This is a ma- jor environmental challenge, needing solutions within the next decade.

13.4 Endangered Species: Current Status

When we say that we want to save a species, what is it that we really want to save? There are four possible answers:

• A wild creature in a wild habitat, as a symbol to us of wilderness.

• A wild creature in a managed habitat, so that the species can feed and reproduce with little interference and so that we can see it in a naturalistic habitat.

• A population in a zoo, so that the genetic characteristics are maintained in live individuals.

• Genetic material only—frozen cells containing DNA from a species for future scientific research.

The goals we choose involve not only science but also values. As discussed, people have different reasons for wishing to save endangered species—utilitarian, eco- logical, cultural, recreational, spiritual, inspirational, aes- thetic, and moral (see A Closer Look 13.3). Policies and actions differ widely, depending on which goal is chosen.

We have managed to turn some once-large popula- tions of wildlife, including fish, into endangered species. With the expanding public interest in rare and endangered species, especially large mammals and birds, it is time for us to turn our attention to these species.

First some definitions: What does it mean to call a spe- cies “threatened” or “endangered”? The terms can have strictly biological meanings, or they can have legal mean- ings. The U.S. Endangered Species Act of 1973 defines endangered species as “any species which is in danger of extinction throughout all or a significant portion of its range other than a species of the Class Insecta determined by the Secretary to constitute a pest whose protection under the provisions of this Act would present an over- whelming and overriding risk to man.” In other words, if certain insect species are pests, we want to be rid of them. It is interesting that insect pests can be excluded from protection by this legal definition, but there is no men- tion of disease-causing bacteria or other microorganisms.

Threatened species, according to the act, “means any spe- cies which is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range.” It has been the practice since the 1960s to con- sider an animal species threatened if its population dropped below 500 individuals for a short time and endangered if it dropped below 50 individuals for a considerable length of time. But these have always been taken as informal, not precise, scientific definitions. Since species differ in so many ways, the determination has been on an individual basis, having to do with the abundance and habitat conditions, including predators, parasites, and human alternations.

Next, the basic facts: It is difficult to estimate accurately the total number of species that are threatened and endan- gered, because we are not sure how many species there are on Earth, as we learned in Chapter 9 (see Table 9.1). Esti- mates vary from about 1.5 million to 2.7 million, and some conservationists believe there are many more. Another con- sideration that makes the estimate difficult is that being rare is different from being on the verge of extinction. Some species, like the California condor and the whooping crane, seem to always have been rare. In comparison, some species that have been very abundant have gone extinct rapidly.

One of the most extreme examples is the passenger pi- geon, which was so abundant its flocks were said to darken the sky as they flew over but were brought to extinction by hunting. There may have been as many as 3 to 5 billion of these birds in Columbus’s time, but by 1990 none were known to exist in the wild. A few were saved at first, but this species was highly social and the few did not survive; the last, named “Martha,” died in the Cincinnati Zoo in 1914.13 But some species are known to be in danger of extinction, based, as noted before, not only on their abundances, but also on habitat requirements, pests, predators, and other threats.

One of the best scientific lists of threatened and en- dangered species is the 2012 International Union for the Conservation of Nature (IUCN) Red List of Threatened

Species. IUCN has assessed 63,837 species and concludes that “19,817 are threatened with extinction, including 41% of amphibians, 33% of reef-building corals, 25% of mammals, 13% of birds, and 30% of conifers.”14 The U. S. Fish and Wildlife Service provides another of the best lists of threatened and endangered species, listing more than 1,200 animals worldwide.

Past Red Lists have estimated that 33,798 species of vascular plants (the familiar kind of plants—trees, grasses, shrubs, flowering herbs), or 12.5% of those known, have re- cently become extinct or endangered13 and more than 8,000 (approximately 3%) of plant species are currently threatened.

13.5 how a Species Becomes Endangered and Extinct

Extinction is the rule of nature (see the discussion of bio- logical evolution in Chapter 9). Local extinction means that a species disappears from a part of its range but per- sists elsewhere. Global extinction means a species can no longer be found anywhere. Although extinction is the ultimate fate of all species, the rate of extinctions has varied greatly over geologic time and has accelerated since the Industrial Revolution.

From 580 million years ago until the beginning of the Industrial Revolution, about one species per year, on average, became extinct. Over much of the history of life on Earth, the rate of evolution of new species equaled or slightly exceeded the rate of extinction. The average longevity of a species has been about 10 million years.15 However, as discussed in Chapter 7, the fossil record suggests that there have been periods of catastrophic losses of species and other periods of rapid evolution of new species (Figures 13.13 and 13.14), which some refer to as “punctuated extinctions.” About 250 million years ago, a mass extinction occurred in which approximately 53% of marine animal species disappeared; about 65 million years ago, most of the dinosaurs became extinct. Interspersed with the episodes of mass extinctions, there seem to have been periods of hundreds of thousands of years with comparatively low rates of extinction.

During the past 2.5 million years, which includes our species and its near humanoid relatives, relatively few species are known to have gone extinct.16 But an intriguing example of large animal extinctions occurred about 10,000 years ago, at the end of the last great conti- nental glaciation. At that time, 33 genera of large mam- mals—those weighing 50 kg (110 lb) or more—became extinct in North America and northern Europe, whereas only 13 genera had become extinct in the preceding 1 or 2 million years (Figure 13.13a). Smaller mammals were not as affected, nor were marine mammals and plants. As early as 1876, Alfred Wallace, an English biological geographer, noted that “we live in a zoologically impov- erished world, from which all of the hugest, and fiercest, and strangest forms have recently disappeared.” It has been suggested that these sudden extinctions coincided with the arrival, on different continents at different times, of Stone Age people and therefore may have been caused by hunting.

Causes of Extinction

Causes of extinction are usually grouped into four risk categories: population risk, environmental risk, natural catastrophe, and genetic risk. Risk here means the chance that a species or population will become extinct owing to one of these causes.

Population Risk

Random variations in population rates (birth rates and death rates) can cause a species in low abundance to become extinct. This is termed population risk. For ex- ample, blue whales swim over vast areas of ocean. Because whaling once reduced their total population to only several hundred individuals, the success of individual blue whales in finding mates probably varied from year to year. If in one year most whales were unsuccessful in find- ing mates, then births could be dangerously low. Such random variation in populations, typical among many species, can occur without any change in the environ- ment. It is a risk especially to species that consist of only a single population in one habitat. Mathematical models of population growth can help calculate the population risk and determine the minimum viable population size.

Environmental Risk

Population size can be affected by changes in the en- vironment that occur from day to day, month to month, and year to year, even though the changes are not severe enough to be considered environmental catastrophes. Environmental risk involves variation in the physical or biological environment, including variations in preda- tor, prey, symbiotic species, or competitor species. Some species are so rare and isolated that such normal varia- tions can lead to their extinction.

For example, Paul and Anne Ehrlich described the local extinction of a population of butterflies in the Colorado mountains.18 These butterflies lay their eggs in the unopened buds of a single species of lupine (a mem- ber of the legume family), and the hatched caterpillars feed on the flowers. One year, however, a very late snow and freeze killed all the lupine buds, leaving the cater- pillars without food and causing local extinction of the butterflies. Had this been the only population of that butterfly, the entire species would have become extinct.

Natural Catastrophe

A sudden change in the environment that is not caused by human action is a natural catastrophe. Fires, major storms, earthquakes, and floods are natural catas- trophes on land; changes in currents and upwellings are ocean catastrophes. For example, the explosion of a volca- no on the island of Krakatoa in Indonesia in 1883 caused one of recent history’s worst natural catastrophes. Most of

the island was blown to bits, bringing about local extinc- tion of most life-forms there.

Genetic Risk

Detrimental change in genetic characteristics, not caused by external environmental changes, is called ge- netic risk. Genetic changes can occur in small popula- tions from reduced genetic variation and from genetic drift and mutation (see Chapter 9). In a small popula- tion, only some of the possible inherited characteristics will be found. The species is vulnerable to extinction be- cause it lacks variety or because a mutation can become fixed in the population.

Consider the last 20 condors in the wild in California. It stands to reason that this small number was likely to have less genetic variability than the much larger popula- tion that existed several centuries ago. This increased the condor’s vulnerability. Suppose that the last 20 condors, by chance, had inherited characteristics that made them less able to withstand lack of water. If left in the wild, these condors would have been more vulnerable to extinc- tion than a larger, more genetically varied population.

13.6 The good News: We have improved the Status of Some Species

Thanks to the efforts of many people, a number of pre- viously endangered species, such as the Aleutian goose, have recovered. Other recovered species include the following:

• The elephant seal, which had dwindled to about a dozen animals around 1900 and now numbers in the hun- dreds of thousands.

• The sea otter, reduced in the 19th century to several hundred and now numbering approximately 10,000.

Many species of birds became endangered because the in- secticide DDT caused thinning of eggshells and failure of reproduction. With the elimination of DDT in the Unit- ed States, many bird species recovered, including some that are no longer on the U.S. Endangered Species list: the bald eagle, brown pelican, white pelican, and osprey.19 Other examples include the following:

• The blue whale, thought to have been reduced to about 400 when whaling was still actively pursued by a number of nations. Today, 400 blue whales are sighted annually in the Santa Barbara Channel along the California coast, a sizable fraction of the total population.

• The gray whale, which was hunted to near- extinction but is now abundant along the California coast and in its annual migration to Alaska. (See A Closer Look 13.4.)

Since the U.S. Endangered Species Act became law in 1973, 13 species within the United States have offi- cially recovered (Table 13.3), according to the U.S. Fish and Wildlife Service. The agency has also “delisted” from protection of the act 9 species because they have gone extinct, and 17 because they were listed in error or be- cause it was decided that they were not a unique species or a genetically significant unit within a species. (The act allows listing of subspecies so genetically different from the rest of the species that they deserve protection.)

13.7 Can a Species Be Too

Abundant? if So, What to Do?

All marine mammals are protected in the United States by the Federal Marine Mammal Protection Act of 1972, which has improved the status of many marine mammals. Sometimes, however, we succeed too well in increasing the populations of a species. Case in point: Sea lions now number more than 190,000 and have become so abundant as to be local problems. (Recall also the problems with white-tailed deer discussed in the opening case study.)21,22 In San Francisco Harbor and in Santa Barbara Harbor, for example, sea lions haul out and sun themselves on boats and pollute the water with their excrement near shore. In one case, so many hauled out on a sailboat in Santa Barbara Harbor that they sank the boat, and some of the animals were trapped and drowned.

Mountain lions, too, have become locally over- abundant. In the 1990s, California voters passed an ini- tiative that protected the endangered mountain lion but contained no provisions for managing the lion if it be- came overabundant, unless it threatened human life and property. Few people thought the mountain lion could ever recover enough to become a problem, but in several cases in recent years mountain lions have attacked and even killed people. Current estimates suggest there may be as many as 4,000 to 6,000 in California.23 These at- tacks become more frequent as the mountain lion popu- lation grows and as the human population grows and people build houses in what was mountain lion habitat.

13.8 how People Cause Extinctions and Affect Biological Diversity

People have become an important factor in causing species to become threatened, endangered, and finally extinct. We do this in several ways:

• By intentional hunting or harvesting (for commercial purposes, for sport, or to control a species that is considered a pest).

• By disrupting or eliminating habitats. • By introducing exotic species, including new parasites, predators, or competitors of a native species. • By generating pollution.

People have caused extinctions over a long time, not just in recent years. The earliest people probably caused extinctions through hunting. This practice continues, especially for specific animal products considered valu- able, such as elephant ivory and rhinoceros horns. When people learned to use fire, they began to change habitats over large areas. The development of agriculture and the rise of civilization led to rapid deforestation and other habitat changes. Later, as people explored new areas, the introduction of exotic species became a greater cause of extinction (see Chapter 9), especially after Columbus’s voyage to the New World, Magellan’s circumnavigation of the globe, and the resulting spread of European civi- lization and technology. The introduction of thousands of novel chemicals into the environment made pollu- tion an increasing threat to species in the 20th century, and pollution control has proved a successful way to help species.

The IUCN estimates that 75% of the extinctions of birds and mammals since 1600 have been caused by human beings. Hunting is estimated to have caused 42% of the extinctions of birds and 33% of the extinc- tions of mammals. The current extinction rate among most groups of mammals is estimated to be 1,000 times greater than the extinction rate at the end of the Pleis- tocene epoch.14

13.9 Ecological islands and Endangered Species

Recall from our discussion in Chapter 9 that an ecologi- cal island is an area that is biologically isolated, so that a species living there cannot mix (or only rarely mixes) with any other population of the same species (Figure 13.16). Mountaintops and isolated ponds are ecological islands. Real geographic islands may also be ecological islands. In- sights gained from studies of the biogeography of islands have important implications for the conservation of en- dangered species and for the design of parks and preserves for biological conservation.

Almost every park is a biological island for some spe- cies. A small city park between buildings may be an is- land for trees and squirrels. At the other extreme, even a large national park is an ecological island. For example, the Masaai Mara Game Reserve in the Serengeti Plain, which stretches from Tanzania to Kenya in East Africa, and other great wildlife parks of eastern and southern Af- rica are becoming islands of natural landscape surrounded by human settlements. Lions and other great cats exist in these parks as isolated populations, no longer able to roam with complete freedom and to mix over large areas. Other examples are islands of uncut forests left by logging operations, and oceanic islands, where intense fishing has isolated some fish populations.

How large must an ecological island be to ensure the survival of a species? The size varies with the species but can be estimated. Some islands that seem large to us are too small for species we wish to preserve. For example, a preserve was set aside in India in an attempt to reintro- duce the Indian lion into an area where it had been elimi- nated by hunting and by changing patterns of land use. In 1957, a male and two females were introduced into a 95-km2 (36-mi2) preserve in the Chakia forest, known as the Chandraprabha Sanctuary. The introduction was carried out carefully, and the population was counted an- nually. There were four lions in 1958, five in 1960, seven in 1962, and eleven in 1965, after which they disappeared and were never seen again. 2

Why did they go? Although 95 km seems large to us, male Indian lions have territories of 130 km2 (50 mi2), within which females and young also live. A population that could persist for a long time would need a number of such territories, so an adequate pre- serve would require 640–1,300 km2 (247–500 mi2). Various other reasons were also suggested for the disap- pearance of the lions, including poisoning and shooting by villagers. But regardless of the immediate cause, a much larger area was required for long-term persistence of the lions.

13.10 using Spatial relationships to Conserve Endangered Species

The red-cockaded woodpecker (Figure 13.17a) is an en- dangered species of the American Southeast, number- ing approximately 15,000.24 The woodpecker makes its nests in old dead or dying pines, and one of its foods is the pine bark beetle (Figure 13.17b). To conserve this species of woodpecker, these pines must be preserved. But the old pines are home to the beetles, which are pests to the trees and damage them for commercial log- ging. This presents an intriguing problem: How can we maintain the woodpecker and its food (which includes the pine bark beetle) and also maintain productive forests?

The classic 20th-century way to view the relation- ship among the pines, the bark beetle, and the wood- pecker would be to show a food chain (see Chapter 6). But this alone does not solve the problem for us. A newer approach is to consider the habitat requirements of the pine bark beetle and the woodpecker. Their requirements are somewhat different, but if we overlay a map of one’s habitat requirements over a map of the other’s, we can compare the co-occurrence of habitats. Beginning with such maps, it becomes possible to design a landscape that would allow the maintenance of all three—pines, beetles, and birds.

suMMaRy

• Modern approaches to management and conservation of wildlife use a broad perspective that considers inter- actions among species as well as the ecosystem and land- scape contexts.

• To successfully manage wildlife for harvest, conserva- tion, and protection from extinction, we need certain quantitative information about the wildlife population, including measures of total abundance and of births and deaths, preferably recorded over a long period. The age structure of a population can also help. In addition, we have to characterize the habitat in quantitative terms. However, it is often difficult to obtain these data.

• A common goal of wildlife conservation today is to “restore” the abundance of a species to some previous number, usually a number thought to have existed prior to the influence of modern technological civiliza- tion. Information about long-ago abundances is rare- ly available, but sometimes we can estimate numbers indirectly—for example, by using the Lewis and Clark journals to reconstruct the 1805 population of grizzly

bears, or using logbooks from whaling ships. Adding to the complexity, wildlife abundances vary over time even in natural systems uninfluenced by modern civilization. Also, historical information often cannot be subjected to formal tests of disproof and therefore does not by itself qualify as scientific. Adequate information exists for relatively few species.

• Another approach is to seek a minimum viable popula- tion, a carrying capacity, or an optimal sustainable pop- ulation or harvest based on data that can be obtained and tested today. This approach abandons the goal of re- storing a species to some hypothetical past abundance.

• The good news is that many formerly endangered species have been restored to an abundance that sug- gests they are unlikely to become extinct. Success is achieved when the habitat is restored to conditions required by a species. The conservation and manage- ment of wildlife present great challenges but also offer great rewards of long standing and deep meaning to people.

Human beings are a primary cause of species extinctions today and also contributed to extinctions in the past. Nonindustrial societies have caused extinction by such activities as hunting and the introduction of exotic species into new habitats. With the age of exploration in the Re- naissance and with the Industrial Revolution, the rate of extinctions ac- celerated. People altered habitats more rapidly and over greater areas. Hunting efficiency increased, as did the introduction of exotic species into new habitats. As the human population grows, conflicts over habitat between people and wildlife increase. Once again, we find that the hu- man population problem underlies this environmental issue.

At the heart of issues concerning wild living resources is the question of sustainability of species and their ecosystems. One of the key questions is whether we can sustain these resources at a constant abundance. In general, it has been assumed that fish and other wildlife that are hunted for recreation, such as deer, could be maintained at some constant, highly productive level. Constant production is desirable economically because it would provide a reliable, easily forecast income each year. But despite attempts at direct management, few wild living resources have remained at constant levels. New ideas about the intrinsic variability of ecosystems and populations lead us to question the assumption that such resources can or should be maintained at constant levels.

Although the final extinction of a species takes place in one locale, the problem of biological diversity and the extinction of species is global because of the worldwide increase in the rate of extinction and because of the growth of the human population and its effects on wild living resources.

We have tended to think of wild living resources as existing outside of cities, but there is a growing recognition that urban environments will be more and more important in conserving biological diversity. This is partly because cities now occupy many sensitive habitats around the world, such as coastal and inland wetlands. It is also because appropriate- ly designed parks and backyard plantings can provide habitats for some endangered species. As the world becomes increasingly urbanized, this function of cities will become more important.

Wildlife, fish, and endangered species are popular issues. We seem to have a deep feeling of connectedness to many wild animals—we like to watch them, and we like to know that they still exist even when we cannot see them. Wild animals have always been important symbols to people, sometimes sacred ones. The conservation of wildlife of all kinds is therefore valuable to our sense of ourselves, both as individuals and as members of a civilization.

The reasons that people want to save endangered species begin with human values, including values placed on the continuation of life and on the public- service functions of ecosystems. Among the most controversial environmen- tal issues in terms of values are the conservation of biological diversity and the protection of endangered species. Science tells us what is possible with respect to conserving species, which species are likely to persist, and which are not. Ultimately, therefore, our decisions about where to focus our efforts in sustaining wild living resources depend on our values.

Botkin, D. B., & Keller, E. A. (2014). Environmental science: Earth as a living planet (9th ed.). Hoboken, NJ: John Wiley & Sons, Inc.