Conservation Biology


Enhancement Chapter: Johnson's The Living World, Third Edition

Chapter outline

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

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

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

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

Introduction

 

Among the greatest challenges facing the biosphere is the accelerating pace of species extinction---not since the Cretaceous have so many species become extinct in so short a period of time (figure 31e.1). The challenge posed by current high levels of species extinction, and the prospect of even higher levels in the future, has led to the emergence in the last decade of the new discipline of conservation biology. Conservation biology is an applied discipline that seeks to learn how to preserve species, communities, and ecosystems. It both studies the causes of declines in species richness and attempts to develop methods to prevent such declines.

Figure 31e.1
Endangered.
The Siberian tiger is in grave danger of extinction, hunted for its pelt and having its natural habitat greatly reduced. A concerted effort is being made to save it, using many of the approaches discussed in this chapter.
©Tom & Pat Leeson / Photo Researchers, Inc.;

In this chapter we first get an update on the biodiversity crisis and its importance. We then provide a basis for understanding why biodiversity is being lost by considering what factors promote biodiversity. Moving to a direct examination of extinction, we then attempt to assess the sorts of species which seem vulnerable to extinction. Using case histories, we go on to identify and study five factors that have played key roles in many extinctions. We finish with a review of recovery efforts, focusing on attempts to restore habitats, propagate endangered species in captivity, and counter habitat fragmentation.

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

OVERVIEW OF THE BIODIVERSITY CRISIS

Extinction is a fact of life, as normal and necessary as species formation is to a stable world ecosystem. Most, species, probably all, go extinct eventually. More than 99% of species known to science (most from the fossil record) are now extinct. However, current rates are alarmingly high. Based on the loss of species of well-described groups of organisms over the past 300 years, and taking into account the rapid and accelerating loss of habitat that is occurring at present, especially in the tropics, it has been calculated that as much as 20% of the world's biodiversity may be lost during the next 30 years (table 31e.1). Because we have named no more than 15% of the world's eukaryotic organisms, and a much smaller proportion of those in the tropics, it is obvious that we will not even know of the existence of many of the organisms that we are driving to extinction.

TABLE 31e.1 NUMBER AND PERCENT OF THREATENED SPECIES
TAXON NUMBER OF THREATENED SPECIES APPROXIMATE TOTAL SPECIES PERCENTAGE THREATENED
ANIMALS      
Invertebrates      
          Mollusks
254
100,000
0.4
          Crustaceans
126
4,000
3
          Insects
873
1,200,000
0.07
Vertebrates      
          Fishes
452
24,000
2
          Amphibians
59
3,000
2
          Reptiles
167
6,000
3
          Birds
1,029
9,500
11
          Mammals
505
4,500
11
Total
3,565
1,350,000
0.3
PLANTS      
          Gymnosperms
242
758
32
          Monocotyledons
4,421
52,000
9
          Monocotyledons:           palms
925
2,820
33
          Dicotyledons
17,474
190,000
9
Total
22,137
240,000
9

Source: Smith et. al., 1993

These losses will not just affect poorly known groups. As many as 50,000 species of the world's total of 250,000 species of plants, 4,000 of the world's 20,000 species of butterflies, and nearly 2,000 of the world's 9,000 species of birds could be lost during this short period of time. Considering that our species has been in existence for only 500,000 years of the world's 4.5 billion year history, and that our ancestors developed agriculture only about 10,000 years ago, this is an astonishing rate of loss.

Massive waves of extinction are nothing new to life on earth. At five times in the past, the earth has lost much of its biodiversity. Mass extinctions eliminated 50% of animal families in the Ordovician about 500 million years ago, 30% of animal families at the end of the Devonian 345 million years ago (including agnathan and placoderm fishes), 50% of animal families in the great Permian extinction 250 million years ago (including over 95% of marine species, many trees and amphibians, most bryozoans and brachiopods, and all trilobites), 35% of animal families at the end of the Triassic 180 million years ago (including many reptiles and marine mollusks), and 40% of animal families at the end of the Cretaceous 65 million years ago (including all dinosaurs and many marine forams and mollusks).

Since the last mass extinction 65 million years ago, global biodiversity has recovered nicely, and indeed reached an all time high, with insects, flowering plants, and vertebrates reaching their greatest diversity ever about 10,000 years ago. Since then, however, species richness has decreased as the world's human population has grown.



As the world's human population has grown, the rate of loss of biodiversity has accelerated. Current rates of extinction and habitat loss suggest a massive die off in the next century.


WHAT'S SO BAD ABOUT LOSING BIODIVERSITY?

What's so bad about losing species? What is the value of biodiversity? Its value can be divided into three principal components: (1) direct economic value of products we obtain from species of plants, animals, and other groups; (2) indirect economic value of benefits produced by species without our consuming them; and (3) ethical and aesthetic value.

Direct Economic Value

Many species have direct value, as sources of food, medicine, clothing, biomass (for energy and other purposes), and shelter. Most of the world's food, for example, is derived from a small number of plants that were originally domesticated from wild plants in tropical and semi-arid regions. In the future, wild strains of these species may be needed for their genetic diversity if we are to improve yields, or find a way to breed resistance to new pests. In addition, novel species of plants and animals may be found that are appropriate for domestication, or as biological control agents for pest species.

About 40% of the prescription and non-prescription drugs used today have active ingredients extracted from plants or animals. Aspirin, the world's most widely used drug, was first extracted from the leaves of the tropical willow, Salix alba. The rosy periwinkle, Catharanthus roseus, from Madagascar has yielded a potent drug for combating leukemia (figure 31e.2).

 

Figure 31e.2
The rosy periwinkle.
Two drugs extracted from the Madagascar periwinkle Catharanthus roseus, vinblastine and vincristine, effectively treat common forms of childhood leukemia, increasing chances of survival from 20% to over 95%.
From Environmental Science, 5th edition by Cunningham and Saigo © 1998 McGraw-Hill Companies, Inc. Reprinted by permission. All rights reserved.

Only in the last few decades have biologists perfected the techniques that make possible the transfer of genes from one kind of organism to another. We are just beginning to be able to use genes obtained from other species to our advantage, as explored at length in chapter 18. Pesticide and insect resistance in crops gained from genes isolated from bacteria is but one example. So-called gene prospecting of the genomes of plants and animals for useful genes has only begun. We have been able to examine only a minute proportion of the existing kinds of organisms so far, to see whether any of their genes have useful properties. By conserving biodiversity we maintain the option of finding useful benefit in the future.

Indirect Economic Value

Diverse biological communities are of vital importance to healthy ecosystems, in maintaining the chemical quality of natural water, in buffering ecosystems against floods and drought, in preserving soils and preventing loss of minerals and nutrients, in moderating local and regional climate, in absorbing pollution, and in promoting the breakdown of organic wastes and the cycling of minerals. By destroying biodiversity, we are creating conditions of instability and lessened productivity and promoting desertification, waterlogging, mineralization, and many other undesirable outcomes throughout the world.

Given the major role played by many species in maintaining healthy ecosystems, it is alarming how little we know about the details of how ecosystems and communities function. It is impossible to predict all the consequences of removing a species, or to be sure that some of them will not be catastrophic. Imagine taking a part list for an airliner, and randomly changing a digit in one of the part numbers: you might change a cushion to a roll of toilet paper–but you might as easily change a key bolt holding up the wing to a pencil. The point is, you shouldn't gamble if you cannot afford to lose, and in removing biodiversity we are gambling with the future of ecosystems upon which we depend, and we don't even know the odds.

Ethical and Aesthetic Value

Many people believe that preserving biodiversity is an ethical issue, feeling that every species is of value in its own right, even if humans are not able to exploit or benefit from it. It is clear that humans have the power to exploit and destroy other species (see table 31e.2), but it is not as ethically clear that they have the right to do so. Sometimes dismissed as "tree huggers," people who hold these views are unable to advance economic arguments–but considered ethically, their point of view has considerable power.

  TABLE 31e.2 ENDANGERED SPECIES OF MAMMALS*

MARSUPIALS

  • Leadbeater's Possum
  • Mahogany Glider
  • Western Barred Bandicoot
  • Northern Marsupial Mole
  • Black-spotted Cuscus
  • Northern Bettong
  • Goodfellow's Tree-kangaroo
  • Bridled Nailtail Wallaby
  • Prosperine Rock-wallaby
  • Long-footed Potoroo
  • Broom's Pygmy-possum

INSECTIVORA

  • Cuban Solenodon
  • Haitian Solenodon
  • Web-footed Tenrec
  • Nimba Otter-shrew
  • Giant Golden Mole
  • Dinagat Moonrat
  • Sado Mole
  • Chinese Shrew-mole

CHIROPTERA

  • Golden-capped Fruit Bat
  • Ryukyu Flying-fox
  • Marianas Flying-fox
  • Kitti's Hog-nosed Bat
  • Big Long-nosed Bat
  • Jamaican Flower Bat
  • Grey Bat
  • Indiana Bat

PERISSODACTYLA

  • Grevy's Zebra
  • Onager
  • Mountain Zebra
  • Hartmann's Mountain Zebra
  • Cape Mountain Zebra
  • Mountain Tapir
  • Great Indian Rhinoceros

PRIMATES

  • Bornean Treeshrew
  • White-collared Lemur
  • Ruffed Lemur
  • Aye-aye
  • Buffy-headed Marmoset
  • Cotton-top Tamarin
  • Night Monkey
  • Woolly Spider Monkey
  • Black Saki
  • White-collared Mangabey
  • Golden Monkey
  • Japanese Macaque
  • Lion-tailed Macaque
  • River Red Colobus
  • Temminck's Red Colobus
  • Black Gibbon
  • Gorilla
  • Western Lowland Gorilla
  • Chimpanzee

CARNIVORA

  • Northwest African Cheetah
  • Texas Jaguarundi
  • Texas Ocelot
  • North African Serval
  • Spanish Lynx
  • Indian Lion
  • North Chinese Leopard
  • Sri Lankan Leopard
  • Tiger
  • Snow Leopard
  • Marine Otter
  • Indonesian Mountain Weasel
  • Steller's Sealion
  • Grey Seal
  • Hawaiian Monk Seal
  • Saimaa Ringed Seal
  • Giant Panda
  • Lesser Panda
  • Otter-civet

CETACEA

  • Bowhead Whale
  • Northern Right Whale
  • Sei Whale
  • Blue Whale
  • Fin Whale
  • Gray Whale
  • Finless Porpoise
  • Ganges River Dolphin
  • Indus River Dolphin

ARTIODACTYLA

  • Eritrean Warthog
  • Javan Warty Pig
  • Chacoan Peccary
  • Wild Bactrian Camel
  • Philippine Spotted Deer
  • Upland Barasingha
  • Corsican Red Deer
  • Key Deer
  • Peary Caribou
  • Sonoran Pronghorn
  • Addax
  • Swayne's Hartebeest
  • Tora Hartebeest
  • Golden Takin
  • Mishmi Takin
  • Nubian Ibex
  • Red Serow
  • Aders' Duiker
  • Cuvier's Gazelle
  • Slender-horned Gazelle
  • Nilgiri Tahr
  • Arabian Tahr
  • Western Klipspringer
  • Arabian Oryx

PROBOSCIDEA

  • Indian Elephant
  • African Elephant

RODENTIA

  • Nelson's Antelope Squirrel
  • Mexican Prairie Dog
  • Woolly Flying Squirrel
  • Vancouver Island Marmot
  • Idaho Ground Squirrel
  • Michoacan Pocket Gopher
  • San Quintin Kangaroo Rat
  • Dinagat Island Cloud Rat
  • Greater Stick-nest Rat
  • Choctawhatchee Beach Mouse
  • Hastings River Mouse
  • Red-bellied Harvest Mouse
  • Poncelet's Giant Rat
  • Japanese Dormouse
  • Pacarana

XENARTHRA

  • Maned Sloth
  • Pink Fairy Armadillo
  • Giant Armadillo

LAGOMORPHA

  • Koslov's Pika
  • Riverine Rabbit
  • Hispid Hare
  • Tehuantepec Jackrabbit
  • Amami Rabbit
  • Volcano Rabbit
  • Dice's Cottontail
  • Tres Marias Rabbit
  • Lower Keys Marsh Rabbit

MACROSCELIDEA

  • Golden-rumped Elephant-shrew
  • Black-and-rufous Elephant-shrew

Almost no one would deny the aesthetic value of biodiversity, of a beautiful flower or noble elephant, but how do we place a value on beauty? Perhaps the best we can do is to appreciate the deep sense of lack we feel at its permanent loss (see figure 31e.3).


Figure 31e.3
A small sample of Earth's species that are either classified as endangered or have become extinct during the last century or so.
Declining biodiversity is a serious problem: officials at the U.S. Fish and Wildlife Service estimate that more than 500 U.S. species have gone extinct during the past 200 years. Of these, roughly 250 have gone extinct since 1980.



Biodiversity is of great value, for the products with which it provides us, for its contributions to the health of the ecosystems upon which we all depend, and for the beauty it provides us, as well as being valuable in its own right.


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

What Factors Promote High Diversity?

It is wisely said that "You cannot preserve what you do not understand." To have any hope of conserving diversity in the face of increasing human encroachment, we must attempt to understand the cause of earth's high species diversity. Some reflects separation of earth's land into five continents, with parallel evolutionary diversification on each. Three other factors also seem to play key roles: large-scale gradients in climate, dispersal limitations, and endemism.

Large-Scale Gradients of Species Diversity

Since before Darwin, biologists have recognized that there are more different kinds of animals and plants in the tropics than in temperate regions. For many species, there is a steady increase in species richness from the arctic to the tropics. Called a species diversity cline, such a biogeographic gradient in numbers of species correlated with latitude has been reported for plants and animals, including birds (figure 31e.4), mammals, reptiles. Why are there more species in the tropics? Five of the most commonly discussed suggestions:


Figure 31e.4
A latitudinal cline in species richness.
Among North and Central American birds, a marked increase in the number of species occurs as one moves towards the tropics. Fewer than 100 species are found at arctic latitudes, while more than 600 species live in southern Central America.
From Biology, 5th edition by Raven and Johnson © 1999 McGraw-Hill Companies, Inc. Reprinted by permission. All rights reserved.
  • Evolutionary age. It has often been proposed that the tropics have more species than temperate regions because the tropics have existed over long and uninterrupted periods of evolutionary time, while temperate regions have been subject to repeated glaciations. The greater age of tropical communities would have allowed complex population interactions to coevolve within them, fostering a greater variety of plants and animals in the tropics.
    However, recent work suggests that the long-term stability of tropical communities has been greatly exaggerated. An examination of pollen within undisturbed soil cores reveals that during glaciations the tropical forests contracted to a few small refuges surrounded by grassland. This suggests that the tropics have not had a continuous record of species richness over long periods of evolutionary time; unfortunately, the fossil record is too sparse to assess the past species richness of the tropics.
  • Higher productivity. A second often-advanced hypothesis is that the tropics contain more species because this part of the earth receives more solar radiation than temperate regions do. The argument is that more solar energy, coupled to a year-round growing season, greatly increases the overall photosynthetic activity of plants in the tropics. If we visualize the tropical forest as a pie (total resources) being cut into slices (species niches), we can see that a larger pie accommodates more slices. However, many field studies have indicated that species richness is highest at intermediate levels of productivity. Accordingly, increasing productivity would be expected to lead to lower, not higher, species richness. Perhaps the long column of vegetation down through which light passes in a tropical forest produces a wide range of frequencies and intensities, creating a greater variety of light environments and in this way promoting species diversity.
  • Predictability. There are no seasons in the tropics. Tropical climates are stable and predictable, one day much like the next. These unchanging environments might encourage specialization, with niches subdivided to partition resources and so avoid competition. The expected result would be a larger number of more specialized species in the tropics, which is what we see. Many field tests of this hypothesis have been carried out in the tropics, and almost all report larger numbers of narrower niches.
  • Predation. Many reports indicate that predation may be more intense in the tropics. In theory, more intense predation could reduce the importance of competition, permitting greater niche overlap and thus promoting greater species richness.
  • Spatial heterogeneity. As noted earlier, spatial heterogeneity promotes species richness. Tropical forests, by virtue of their complexity, create a variety of microhabitats and so may foster larger numbers of species.

Diversity in Homogeneous Habitats

Not all species diversity can be explained by factors such as those responsible for clines in species diversity. Over 40 years ago the great ecologist G. E. Hutchinson pointed out that freshwater lakes support hundreds of species of algae, even though theory then predicted that the number of species should not exceed the number of resources for which they compete. They couldn't be competing for hundreds of different resources in this very homogeneous habitat, so the source of diversity seemed a mystery. Ecologists have since proposed four major solutions to this paradox, each of which can easily account for the observed diversity. The challenge has been to figure out which of the four explanations is actually responsible for the high diversity often seen in homogeneous habitats in nature. The four diversity-generating mechanisms suggested by theory are:

Spatial heterogenity. A habitat that appears homogeneous to us may not in fact appear so to the species involved. Heterogeneity in microclimate or other factors not obvious to an observer could easily generate high levels of diversity.

Trophic interactions. Interactions among species of several different trophic levels (at least three) can produce high levels of diversity because of the complex forms the interactions may take. For example, plants, their herbivores, and parasites of plants and herbivores can produce many different patterns of competition and predator-prey interactions, all presenting opportunities for evolutionary diversification.

Periodic disturbance. A pattern of intermittent episodic disturbance that produce gaps in the rainforest (like when a tree falls) allow invasion of the gap by other species (figure 31e.5). Eventually the species inhabiting the gap will go through a successional sequence, one tree replacing another, until a canopy tree species comes again to occupy the gap. But if there are lots of gaps of different ages in the forest, many different species will coexist, some in young gaps, others in older ones.

 

Figure 31e.5
Maintaining high diversity in a tropical rainforest.
A small light gap created by a single fallen tree in the tropical rain forest of Panama. Such gaps play a key role in maintaining the high species diversity of the rainforest.
Reprinted with permission from SCIENCE vol. 283 Jan 22. Copyright ©1999 American Association for the Advancement of Science. "Readers may view, browse, and/or download this material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or in part, without prior written permission from AAAS"

Neighborhood recruitment limitation. A large number of species may coexist if they do not compete with each other for limiting resources. In a rainforest, a plant competes only with individuals living immediately adjacent to it, individuals near enough to cast shade on it or have roots that overlap with its roots. Because many tropical plants are distributed sparsely over wide areas, they may have low abundance at any particular site. Other plants have poor dispersal ability, and so are unlikely to reach a particular site. For these reasons, many plant species are absent from the local neighborhood when a gap occurs in the forest canopy and presents an opportunities for a colonizing individual to become established. As David Tilman, University of Minnesota ecologist, has stated, "Like a team that fails to appear at a sporting event, a species that is locally absent has forfeited any chance of competitive victory at the site. This can allow inferior competitors to win by default." When a newly-opened site is not equally available to all of the species, the site becomes colonized not by the best competitor, but rather by the best available competitor. This can lead to high levels of diversity as inferior competitors escape the impact of competition.

While all four proposed mechanisms have their proponents among ecologists, the last two are particularly popular as explanations of high species diversity in tropical rainforests, which cover large areas of relatively uniform climate and topography–but which of the two is the more likely explanation? A very significant study recently reported by ecologist Steve Hubbell and coworkers at Princeton University and the Smithsonian's Tropical Research Institute strongly implicates recruitment limitation. In a long-term study of Panama rainforest, these ecologists located more than 1200 gaps in the forest canopy, and repeatedly censused them over a 13 year period. More than 300,000 individual trees were included in the census.

Hubbell's team found that the species composition of the 1200 gaps changed very little over the 13 years of the study. This suggests that little of the succession predicted by the periodic disturbance hypothesis is occurring. When changes in species abundance did occur, they affected gaps and non-gap areas equally, again arguing against the periodic disturbance hypothesis. Gaps had a low diversity of seedling and saplings, and using seed traps, they found that the seeds of only a few species were reaching the gap sites. Overall, the study strongly supports the recruitment limitation hypothesis as a major source of diversity in homogeneous habitats.

Diversity in Discontinuous Habitats

A species found naturally in only one geographic area and no place else is said to be endemic to that area. The area over which an endemic species is found may be very large. The black cherry tree (Prunus serotina), for example, is endemic to all of temperate North America. More typically, however, endemic species occupy restricted ranges. The Komodo dragon (Varanus komodoensis) lives only on a few small islands in the Indonesian archipelago, while the Mauna Kea silversword (Argyroxiphium sandwicense) lives in a single volcano crater on the island of Hawaii.

Isolated geographical areas, such as oceanic islands, lakes, and mountain peaks, often have high percentages of endemic species, often in significant danger of extinction. The number of endemic plant species varies greatly in the United States from one state to another. Thus, 379 plant species are found in Texas and nowhere else, while New York has only one endemic plant species. California, with its varied array of habitats, including deserts, mountains, seacoast, old growth forests, grasslands, and many others, is home to more endemic species than any other state.

Worldwide, notable concentrations of endemic species occur in particular "hot spots" of high endemism. Such hot spots are found in Madagascar, in a variety of tropical rainforests, in the eastern Himalayas, in areas with Mediterranean climates like California, South Africa, and Australia, and in several other climatic areas (figure 31e.6 and table 31e.3).


Figure 31e.6
"Hot spots" of high endemism.
These areas are rich in endemic species under significant threat of imminent extinction.

TABLE 31e.3 NUMBERS OF ENDEMIC VERTEBRATE SPECIES IN SOME "HOT SPOT" AREAS
REGION
MAMMALS
REPTILES
AMPHIBIANS
Atlantic coastal Brazil
40
92
168
Colombian Chocó
8
137
111
Philippines
98
120
41
Northern Borneo
42
69
47
Southwestern Australia
10
25
22
Madagascar
86
234
142
Cape region (South Africa)
16
43
23
California Floristic Province
15
25
7
New Caledonia
2
21
0
Eastern Himalayas
--
20
25

Source: Data from Myers 1988; World Conservation and Monitoring Center 1992


The world's high level of species diversity reflects three principal factors: 1. large-scale gradients in climate and other factors from temperate zones to the tropics, 2. environments that favor spatially patchy dispersal, and 3. isolated habitats that promote evolution of endemic species.


 

Figure 31e.7
Habitat removal.
In this clearcut lumbering of National Forest land in Washington State, few if any trees have been left standing, removing as well the home of the deer, birds, and other animal inhabitants of temperate forest. Until a replacement habitat is provided by replanting, this is a truly "lost" habitat.

Predicting Which Species Are Vulnerable to Extinction

How can a biologist assess whether a particular species is vulnerable to extinction? To get some handle on this, conservation biologists look for changes in population size and habitat availability. Species whose populations are shrinking rapidly, whose habitats are being destroyed (figure 31e.7), or which are endemic to small areas can be considered to be endangered.

Population Viability Analysis

Quantifying the risk faced by a particular species is not a simple or precise enterprise. Increasingly, conservation biologists make a rough estimate of a population's risk of local extinction in terms of a minimum viable population (MVP), the estimated number or density of individuals necessary for the population to maintain or increase its numbers.

Some small populations are at high risk of extinction, while other populations equally small are at little or no risk. Conservation biologists carry out a population viability analysis (PVA) to assess how the size of a population influences its risk of becoming extinct over a specific time period, often 100 years. Many factors must be taken into account in a PVA. Two components of particular importance are demographic stochasticity (the amount of random variation in birth and death rates) and genetic stochasticity (fluctuations in a population's level of genetic variation). The smaller the population, the greater the random fluctuation is expected to be. Thus extinction is more likely in small populations when high death rates coincide with low birth rates, or when higher levels of inbreeding lead to a lowering of heterozygosity.

Many species are distributed as metapopulations, collections of small subpopulations each occupying a suitable patch of habitat in an otherwise unsuitable landscape. Each individual subpopulation may be quite small and in real threat of extinction, but the metapopulation may be quite safe from extinction so long as individuals from other populations repopulate the habitat patches vacated by extinct populations. The extent of this rescue effect is an important component of the PVA of such species.


To assess the risk of local extinction of a particular species, conservation biologists carry out a population viability analysis that takes into account demographic and genetic variation, both strongly influenced by population size.


Categories of Vulnerable Species

Studying past extinctions of species and using population viability analyses of threatened ones, conservation biologists have observed that particular categories of species are particularly vulnerable to extinction.

Local Endemic Distribution

Local endemic species typically occur at only one or a few sites in a restricted geographical range, which makes them particularly vulnerable to anything that harms the site, such as destruction of habitat by human activity. Bird species on oceanic islands, have often become extinct as humans affect the island habitats. Many endemic fish species confined to a single lake undergo similar fates.

Local endemic species often have small population sizes, placing them at particular risk of extinction because of their greater vulnerability to demographic and genetic fluctuations. Indeed, population size by itself seems to be one of the best predictors of the extinction rate of small populations.

Local endemic species often have quite specialized niche requirements. Once a habitat is altered, it may no longer be able to support a particular local endemic, while remaining satisfactory for species with less particular requirements. For example, wetlands plants that require very specific and regular changes in water level may be rapidly eliminated when human activity affects the hydrology of an area.

Declining Population Size

Species in which population size is declining are often at grave risk of extinction, particularly if the decline in numbers of individuals is severe. While there is no hard rule, population trends in nature tend to continue, so a population showing significant signs of decline should be considered at risk of extinction unless the cause of the decline is identified and corrected (figure 31e.8). Darwin makes this point very clearly in On the Origin of Species:

To admit that species generally become rare before they become extinct, to no surprise at the rarity of the species, and yet to marvel greatly when the species ceases to exist, is much the same as to admit that sickness in the individual is the forerunner of death---to feel no surprise at sickness, but when the sick man dies, to wonder and to suspect that he dies of some deed of violence.


Figure 31e.8
Population size and risk of extinction.
The more common a species, the less likely it will become endangered.
From Ecology by Molles © 1998 McGraw-Hill Companies, Inc. Reprinted by permission. All rights reserved.

While long term trends towards smaller population numbers suggest that a species may be at risk in future years, abrupt recent declines in population numbers, particularly when the population is small or locally endemic, fairly scream of risk of extinction. It is for this reason that PVA is best carried out with data on population sizes gathered over a period of time.

Lack of Genetic Variability

Species with little genetic variability are generally at significantly greater risk of extinction than more variable species, simply because they have a more limited arsenal with which to respond to the vagaries of environmental change. Genetic variability is usually measured as average heterozygosity (the probability that a randomly selected gene locus will be heterozygous) or degree of polymorphism (the proportion of loci which have more than one allele at a frequency of 1%). Species with extremely low genetic variability are more vulnerable when faced with a new disease, predator, or other environmental challenge. For example, the African cheetah (Acinonyx jubatus) has almost no genetic variability, due presumably to a "bottleneck" effect when few individuals survived a previous crisis. This lack of genetic variability is considered to be a significant contributing factor to a lack of disease resistance in the cheetah (although environmental factors also seem to have played a key role in the cheetah's decline).

Hunted or Harvested by People

Species that are hunted or harvested by people have historically been at grave risk of extinction. Overharvesting of natural populations can rapidly reduce the population size of a species, even when that species is initially very abundant. A century ago the skies of North America were darkened by huge flocks of passenger pigeons; hunted as free and tasty food, they were driven to extinction. The buffalo that used to migrate in enormous herds across the central plains of North America only narrowly escaped the same fate, a few individuals preserved from this catastrophic exercise in overhunting founding today's modest herds.

The existence of a commercial market often leads to overexploitation of a species. The international trade in furs, for example, has severely reduced the numbers of chinchilla, vicuna, otter, and many wild cat species. The harvesting of commercially valuable trees provides another telling example: almost all West Indies mahogany (Swietenia mahogani) have been logged from the Caribbean islands, and the extensive cedar forests of Lebanon, once widespread at high elevations in the Middle East, now survive in only a few isolated groves.

A particularly telling example of overharvesting of a so-called commercial species is the commercial harvesting of fish in the North Atlantic. Fishing fleets continued to harvest large amounts of cod off Newfoundland during the 1980s, even as the population numbers declined precipitously. By 1992 the cod population had dropped to less than 1% of their original numbers. The American and Canadian governments have closed the fishery, but no one can predict if the fish populations will recover. Outside the Newfoundland fishery, the Atlantic bluefin tuna has experienced a 90% population decline in the last 10 years. The swordfish has declined even further. In both cases, the drop has led to even more intense fishing of the remaining populations.

Dependent Upon Other Species

Species often become vulnerable to extinction when their web of ecological interactions becomes seriously disrupted (figure 31e.9). A recent case in point are the sea otters that live in the cold waters off Alaska and the Aleutian islands. A keystone species in the kelp forest ecosystem, the otter populations have declined sharply in recent years. In a 500 mile stretch of coastline, otter numbers had dropped to an estimated 6,000 from 53,000 in the 1970s, a plunge of 90%. Investigating this catastrophic decline, marine ecologists uncovered a chain of interactions among the species of the ocean and kelp forest ecosystems, a falling domino chain of lethal effects.


Figure 31e.9
Disruption of the kelp forest ecosystem.
Overharvesting by commercial whalers altered the balance of fish in the ocean ecosystem, inducing killer whales to feed on sea otters, a keystone species of the kelp forest ecosystem.

The first in a series of events leading to the sea otter's decline seems to have been the heavy commercial harvesting of whales (see the case history later in this chapter). Without whales to keep their numbers in check, ocean zooplankton thrived, leading in turn to proliferation of a species of fish called pollock that feed on the now-abundant zooplankton. Given this ample food supply, the pollock proved to be very successful competitors of other northern Pacific fish like herring and ocean perch, so that levels of these other fish fell steeply in the 1970s. It did not help that commercial fishermen were at the same time overfishing for cod and ocean perch, both far more nutritious than pollock.

Now the falling chain of dominos begins to accelerate. The decline in the nutritious forage fish is though to have been mainly responsible for an ensuing crash in Alaskan populations of Steller sea lions and harbor seals, for which pollock did not provide sufficient nourishment. Numbers of these pinniped species have fallen precipitously since the 1970s.

Pinnipeds are the major food of orcas, also called killer whales. Faced with a food shortage, some killer whales seem to have turned to the next best thing: sea otters. In one bay where the entrance from the sea was too narrow and shallow for orcas to enter, only 12% of the sea otters have disappeared, while in a similar bay which orcas could enter easily, two thirds of the otters disappeared in a year's time.

Without otters to eat them, the population of sea urchins in the ecosystem exploded, eating the kelp and so "deforesting" the kelp forests and denuding the ecosystem. As a result, fish species that live in the kelp forest, like sculpins and greenlings (a cod relative), will probably decline, and local populations of bald eagles, totally dependent on these fish populations, will probably drop off sharply.


Commercial whaling appears to have initiated a series of changes that have led to orcas beginning to feed on sea otters, with disastrous consequences for their kelp forest ecosystem.


Estimating Rates of Extinction

There is no precise simple way to measure rates of extinction. However, a variety of rough methods exist that allow us to estimate probable rates of extinction. The most useful involve studying species-area relationships, although much can also be learned from studying trends in habitat and species loss.

Species-Area Relationships

Biologists often use the well-established observation that larger areas support more species (figure 31e.10) to estimate the effect of reductions in habitat available to a species. The species-area relationship suggests that, on the average, a 90% loss of habitat will result in a 50% loss of species living in that habitat. Thus, deforestation in the next decades of one third of the world's tropical rainforests, which today support half of all terrestrial species, would be expected to result in the extinction of about one million species.


Figure 31e.10
Extinction and the species-area relationship.
The data present percent extinction rates as a function of habitat area for birds on a series of Finnish islands. Smaller islands experience far greater local extinction rates.

Trends in Habitat Disruption

Perhaps the greatest contribution to today's high extinction rate is the destruction or disruption of natural habitats by human activities. Natural habitats may be adversely affected by human influences in four main ways.

  1. Destruction. A proportion of the habitat available to a particular species may simply be destroyed. This is a common occurrence in the "clear-cut" harvesting of timber, in the burning of tropical forest to produce grazing land, and in urban and industrial development. Forest clearance has been, and is, by far the most pervasive form of habitat disruption (figure 31e.11). Many tropical forests are being cut or burned at a rate of 1% or more per year.
  2. Fragmentation. A habitat may become fragmented. This happens when roads and habitation intrude into forest, and in many less obvious ways. The effect is to carve up the populations living in the habitat into a series of smaller populations, often with disastrous consequences. Although detailed data are not available, fragmentation of wildlife habitat in developed temperate areas is thought to be very substantial, and probably exceeds that of the tropics.
  3. Pollution. Habitat may be degraded by pollution to the extent that some species can no longer survive there. Degradation occurs as a result of many forms of pollution, from acid rain to pesticides. Aquatic environments are particularly vulnerable.
  4. Human Disruption. Habitat may be so disturbed by human activities as to make it untenable for some species. For example, visitors to caves in Alabama and Tennessee produced significant population declines over an eight year period, some as great as 100%. When visits were fewer than one per month, less than 20% of bats were lost, but caves with more than four visits per month suffered population declines of between 86% and 95%. Outdoor recreation, ecotourism, and even ecological research played a role.


Figure 31e.11
Extinction and habitat destruction.
The rainforest covering the eastern coast of Madagascar, an island off the coast of East Africa, has been progressively destroyed as the island's human population has grown. 90% of the original forest cover is now gone. Many species have become extinct, and many others are threatened, including 16 of Madagascar's 31 primate species.

Trends in Species Loss

A great deal can be learned about current rates of extinction by studying past history, and in particular the impact of human-caused extinctions. The first major impact of human activity was the elimination of large mammals from North and South America at the time humans first reached these continents over 12,000 years ago. Shortly after humans arrived, 74%-86% of the megafauna (that is, animals weighing more than 100 pounds) became extinct. These extinctions are thought to have been caused by hunting, and indirectly by burning and clearing forests. For thousands of years, the total area of natural grassland and forest in North America, Europe, and Asia has been steadily reduced by humans to create pasture and farmland.

Historical extinction rates are best known for birds and mammals because these species are conspicuous–relatively large and well studied. Estimates of extinction rates for other species are much rougher. The data presented in Table 31e.4, based on the best available evidence, shows recorded extinctions from 1600 to the present. These estimates indicate that about 85 species of mammals and 113 species of birds have become extinct since the year 1600. That is about 2.1% of known mammal species and 1.3% of known birds. The majority of extinctions have come in the last 150 years. The extinction rate for birds and mammals was about one species every decade from 1600 to 1700, but it rose to one species every year during the period from 1850 to 1950, and four species per year between 1986 and 1990 (figure 31e.12). It is this increase in the rate of extinction that is the heart of the biodiversity crisis.

TABLE 31e.4 RECORDED EXTINCTIONS SINCE 1600 A.D. RECORDED EXTINCTIONS
TAXON MAINLAND ISLAND OCEAN TOTAL APPROXIMATE NUMBER OF SPECIES PERCENT OF TAXON EXTINCT
Mammals
30
51
4
85
4,000
2.1
Birds
21
92
0
113
9,000
1.3
Reptiles
1
20
0
21
6,300
0.3
Amphibians*
2
0
0
2
4,200
0.05
Fish
22
1
0
23
19,100
0.1
Invertebrates
49
48
1
98
1,000,000+
0.01
Flowering plants
245
139
0
384
250,000
0.2

Source: Reid and Miller 1989; data from various sources
*There has been an alarming decline in amphibian populations recently; and many species may be on the verge of extinction.


Figure 31e.12
Trends in species loss.
The graphs above present data on recorded animal extinctions since 1600. The majority of extinctions have occurred on islands, with birds and mammals particularly affected (although this may reflect to some degree our more limited knowledge of other groups).


Biologists estimate rates of extinction both by studying recorded extinction events and by analyzing trends in habitat loss and disruption.


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

What Factors Are Responsible For Extinction?

Because a species is rare does not necessarily mean that it is in danger of extinction. The habitat it utilizes may simply be in short supply, preventing population numbers from growing. In a similar way, shortage of some other resource may be limiting the size of populations. Secondary carnivores, for example, are usually rare because so little energy is available to support their populations. Nor are vulnerable species such as those categories discussed in the previous section always threatened with extinction. Many local endemics are quite stable and not at all threatened.

If it's not just size or vulnerability, what factors are responsible for extinction? Studying a wide array of recorded extinctions and many species currently threatened with extinction, conservation biologists have identified a few factors that seem to play a key role in many extinctions: overexploitation, introduced species, disruption of ecological relationships, loss of genetic variability, and habitat loss and fragmentation (figure 31e.13 and table 31e.5).


Figure 31e.13
Factors responsible for animal extinction.
These data represent known extinctions of mammals in Australasia and the Americas.
From Environmental Science, 5th edition by Cunningham and Saigo © 1998 McGraw-Hill Companies, Inc. Reprinted by permission. All rights reserved.

TABLE 31e.5 CAUSES OF EXTINCTIONS
PERCENTAGE OF SPECIES INFLUENCED BY THE GIVEN FACTOR*
GROUP HABITAT LOSS OVEREXPLOITATION SPECIES INTRODUCTION PREDATORS OTHER UNKNOWN
EXTINCTIONS            
Mammals
19
23
20
1
1
36
Birds
20
11
22
0
2
37
Reptiles
5
32
42
0
0
21
Fish
35
4
30
0
4
48
THREATENED EXTINCTIONS
Mammals
68
54
6
8
12
--
Birds
58
30
28
1
1
--
Reptiles
53
63
17
3
6
--
Amphibians
77
29
14
--
3
--
Fish
78
12
28
--
2
--

Source: Reid and Miller, 1989
Some species may be influenced by more than one factor; thus, some rows may exceed 100%


Most recorded extinctions can be attributed to one of five causes: overexploitation, introduced species, ecodisruption, loss of genetic variability, and habitat loss and fragmentation.


Case Study: Over-Exploitation---Whales

 

Figure 31e.14
A humpback whale.
Only five to ten thousand humpback whales remain, out of a world population estimated to have been one hundred thousand.
Paul Chesley / Allstock.

Whales, the largest living animals, are rare in the world's oceans today, their numbers driven down by commercial whaling. Commercial whaling began in the sixteenth century, and reached its apex in the nineteenth and early twentieth centuries. Before the advent of cheap high-grade oils manufactured from petroleum in the early twentieth century, oil made from whale blubber was an important commercial product in the worldwide marketplace. In addition, the fine lattice of bone used by baleen whales to filter-feed plankton from seawater ("whalebone") was used in undergarments. Because a whale is such a large animal, each individual captured is of significant commercial value.

Right whales were the first to bear the brunt of commercial whaling. They were called right whales because they were slow, easy to capture, and provided up to 150 barrels of blubber oil and abundant whalebone, making them the "right" whale for a commercial whaler to hunt.

As the right whale declined in the eighteenth century, whalers turned to other species, the grey, humpback (figure 31e.14), and bowhead. As their numbers declined, whalers turned to the blue, largest of all whales, and when they were decimated, to smaller whales: the fin, then the Sei, then the sperm whales. As each species of whale became the focus of commercial whaling, its numbers inevitably began a steep decline (figure 31e.15).


Figure 31e.15
A history of commercial whaling.
These data show the world catch of whales in the twentieth century. Each species in turn is hunted until its numbers fall so low that hunting it becomes commercially unprofitable.
From Environmental Science, 5th edition by Cunningham and Saigo © 1998 McGraw-Hill Companies, Inc. Reprinted by permission. All rights reserved.

Hunting of right whales was made illegal in 1935. By then, all three species had been driven to the brink of extinction, their numbers less than 5% of what they used to be. Protected since, their numbers have not recovered in either the North Atlantic or North Pacific. By 1946 several other species faced imminent extinction, and whaling nations formed the International Whaling Commission (IWC) to regulate commercial whale hunting. Like having the fox guard the hen house, the IWC for decades did little to limit whale harvests, and whale numbers continued a steep decline. Finally, in 1974, when numbers of all but the small minke whales had been driven down, the IWC banned hunting of blue, gray, and humpbacked whales, and instituted partial bans on other species. The rule was violated so often, however, that the IWC in 1986 instituted a worldwide moratorium on all commercial killing of whales. While some commercial whaling continues, often under the guise of harvesting for scientific studies, annual whale harvests have dropped dramatically in the last fifteen years.

Some species appear to be recovering, while others do not. Humpback numbers have more than doubled since the early 1960s, increasing nearly 10% annually, and Pacific gray whales have fully recovered to their previous numbers of about 20,000 animals after being hunted to less than 1000. Right, sperm, fin, and blue whales have not recovered, and no one knows whether they will.


Commercial whaling, by overharvesting, has driven most large whale species to the brink of extinction. Stopping the harvest has allowed recovery of some but not all species.


Case Study: Introduced Species---The Lake Victoria Cichlids

Lake Victoria, an immense shallow freshwater sea about the size of Switzerland in the heart of equatorial East Africa, has until 1954 been home to an incredibly diverse collection of over 300 species of cichlid fishes. These small, perchlike fishes range from 2 to 10 inches in length, with males coming in endless varieties of colors. Today, all of these cichlid species are threatened, endangered , or extinct.

What happened in 1954 to bring about the abrupt loss of so many endemic cichlid species? In 1954, the Nile perch, Lates niloticus, a commercial fish with a voracious appetite, was introduced on the Ugandan shore of Lake Victoria as a food source. Nile perch, which grow to over four feet in length, were to form the basis of a new fishing industry. For decades, the Nile perch did not seem to have a significant impact on Lake Victoria's fish–over thirty years later, in 1978, Nile perch still made up less than 2% of the fish harvested from the lake.

Then something happened to cause the Nile perch to explode and to spread rapidly through the lake, eating their way through the cichlids (figure 31e.16). By 1986, Nile perch constituted nearly 80% of the total catch of fish from the lake, and the endemic cichlid species were virtually gone. Over 70% of named species disappeared, including all open-water species. The Nile perch, in the meantime, as a result of this abrupt population explosion, became a superb source of food for people living around the lake (figure 31e.17).

Figure 31e.16
Lake Victoria cichlids.
The Nile perch, a commercial fish introduced into Lake Victoria as a potential food source, is responsible for the virtual extinction of hundreds of species of cichlid fishes.
From Biology, 5th edition by Raven and Johnson © 1999 McGraw-Hill Companies, Inc. Reprinted by permission. All rights reserved.

 

Figure 31e.17
Victor and vanquished.
A woman with a mixed catch of Nile perch (larger fishes in foreground) and cichlids (smaller fishes in tub).
Mark Chandler / New England Aquarium.

So what happened to kick-start the mass extinction of the cichlids? The trigger seems to have been eutrophication. Before 1978, Lake Victoria had high oxygen levels at all depths, down to the bottom layers exceeding 60 meters depth. However, by 1989 high inputs of nutrients from agricultural runoff and sewage from towns and villages had led to algal blooms that severely depleted oxygen levels in deeper parts of the lake. Cichlids feed on algae, and initially their population numbers are thought to have risen in response to this increase in their food supply, but unlike similar algal blooms of the past, the Nile perch was now present to take advantage of the situation. With a sudden increase in its food supply (cichlids), the numbers of Nile perch exploded, and the greater numbers of predators simply ate all available cichlids.

Since 1990 the situation has been compounded by a second factor, the introduction into Lake Victoria of a floating water weed from South America, the water hyacinth Eichornia crassipes. Extremely fecund under eutrophic conditions, thick mats of water hyacinth soon covered entire bays and inlets, choking off the coastal habitats of non-open-water cichlids.


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


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

 

 

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

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

TABLE 31e.6 ACRES OF PRAIRIE DOG HABITAT
STATE
1899-90
1998
Arizona
unknown
extinct
Colorado
7,000,000
44,000
Kansas
2,500,000
36,000
Montana
6,000,000
65,000
Nebraska
6,000,000
60,000
New Mexico
12,000,000
15,000
North Dakota
2,000,000
20,400
Oklahoma
950,000
9,500*
South Dakota
1,757,000
244,500
Texas
56,833,000
22,650
Wyoming
16,000,000
70,000 to 180,000

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

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

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

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


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


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

 

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

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

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

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

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

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


Figure 31e.20
Small populations lose much of their genetic variability.
The percentage of polymorphic genes in isolated populations of the tree Halocarpus bidwilli in the mountains of New Zealand is a sensitive function of population size.
From Conservation Biology, 2nd edition by Cox © 1997 McGraw-Hill Companies, Inc. Reprinted by permission. All rights reserved.


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


Case Study: Habitat Loss and Fragmentation---Songbirds

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

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

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

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


Figure 31e.21
The American redstart, a migratory songbird whose numbers are in serious decline.
The graph presents data on the level of 13C in redstarts arriving at summer breeding grounds. Early arrivals, with the best shot at reproductive success, have lower levels of 13C, indicating they wintered in more favorable mangrove-wetland forest habitats.
(1): Reprinted with permission from SCIENCE vol. 282 Dec. Copyright 1998 American Association for the Advancement of Science. "Readers may view, browse, and/or download this material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or in part, without prior written permission from AAAS".


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


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

At the Population and Species Level

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

Habitat Fragmentation

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


Figure 31e.22
Fragmentation of woodland habitat.
From the time of settlement of Cadiz Township, Wisconsin, the forest has been progressively reduced from a nearly continuous cover to isolated woodlots covering less than 1% of the original area.
Reprinted with permission from J. Curtis and William L. Thomas (ed), Man's Role in Changing the Face of the Earth © 1956 by The University of Chicago Press. All rights reserved.

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

 

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

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

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

Habitat Restoration

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

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

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


(a)

 


(b)

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

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

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

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

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

Captive Propagation

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

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

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


Figure 31e.25
Captive propagation.
The peregrine falcon has been reestablished in the eastern United States by releasing captive-bred birds over a period of ten years.

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

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

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

Sustaining Genetic Diversity

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

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

 

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

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

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

Preserving Keystone Species

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

 

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

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

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

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

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


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


At the Community Level

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


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


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


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


Chapter Summary

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

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

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

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

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

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

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

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

Key Concepts

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

Review Questions

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

Thought Questions

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

Internet Links

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

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

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

For Further Reading

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

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

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

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

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

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

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


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