Conservation and Ecology

Britannica Educational Publishing, John P Rafferty

• Publisher: Britannica Educational Publishing
• Year: 2010
• eBook ISBN: 9781615303908
• Topic: Biological Sciences
• Subtopic: Ecology
• Index: Biological Sciences

The Earth's capacity to sustain life increasingly depends on the actions of humans and our commitment to preserving the planet's incredible biodiversity. Because its ecosystems are vulnerable to pollution, overuse, and numerous other damaging activities, plants and animals that rely on these regions to subsist are significantly threatened. This insightful volume examines various conservation measures, such as habitat management, that help control the extinction rates of numerous species. Tireless, incredible individuals whose work has been crucial to protecting the planet for future generations are highlighted.

CHAPTER 1
THE PATHOLOGY OF EXTINCTION

Conservation is the study of the loss of Earth’s biological diversity and the ways this loss can be prevented. Biological diversity, or biodiversity, is the variety of life either in a particular place or on the entire Earth, including its ecosystems, species, populations, and genes. Conservation thus seeks to protect life’s variety at all levels of biological organization.

Species extinction is the most obvious aspect of the loss of biodiversity. For example, species form the bulk of the examples in a comprehensive assessment of the state of the planet published in the early 21st century by the Millennium Ecosystem Assessment, an international effort coordinated by the United Nations Environment Programme. The subject of conservation is broader than this, however. Even a species that survives extinction can lose much of its genetic diversity as local, genetically distinct populations are lost from most of the species’ original range. Furthermore, ecosystems may shrink dramatically in area and lose many of their functions, even if their constituent species manage to survive. Conservation is involved with studying all these kinds of losses, understanding the factors responsible for them, developing techniques to prevent losses, and, whenever possible, restoring biodiversity.

Conservation is a crisis discipline, one demanded by the unusual rates of loss. It is also a mission-driven discipline. By analogy, ecology and conservation have the same relationship as physiology and medicine. Human physiology studies the workings of the human body, whereas medicine is mission-oriented and aims to understand what goes wrong and how to treat it.

Conservation is often considered a purely biological topic, as exemplified by major scientific journals with titles such as Conservation Biology and Animal Conservation as well as college textbooks with such titles as Principles of Conservation Biology and Essentials of Conservation Biology. However, because the underlying cause of the loss of biodiversity is increasing human activity, conservation must inevitably involve human interactions. Many of the techniques to prevent the loss of biodiversity involve issues of economics, law, social sciences, and religion—all of which are covered by the journals and textbooks previously cited.

According to the best estimates of the world’s environmental experts, human activities have driven species to extinction at rates perhaps 1,000 times the natural, or background, rate, and future rates of extinction will likely be higher. To show how the experts arrived at these conclusions, it is necessary to pose and attempt to answer a series of extremely difficult questions. How many species are there? How fast were species disappearing before human activity became pervasive? How fast are they becoming extinct at present? And finally, it is necessary to ask a further question: What does the future hold for extinctions if current trends continue?

HOW MANY SPECIES ARE THERE?

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Any absolute estimate of extinction rate, such as extinctions per year, requires knowledge of how many species there are. Unfortunately, this number is not known with any great degree of certainty, and the problems of estimating it are formidable. Taxonomists have described—that is, have given names to—about 1.5 million species. Only about 100,000 of them, comprising terrestrial vertebrates, some flowering plants, and attractive and collectible invertebrates such as butterflies and snails, are popular enough for taxonomists to know well. Birds are exceptionally well-known—there are roughly 10,000 bird species, with only one or two new species being added each year.

Club fungus (Claveria) growing in soil. Ken Brate/Photo Researchers

Those who describe species cannot always be certain that the specimen in hand has not been given a name by someone else in a different country and sometimes even in a different century. Consequently, some taxonomic groups may have more names assigned to them than constituent species, which would result in erroneously high species estimates. Potentially much more serious as a source of error is the fact that some species groups have relatively few named members compared with the numbers that experts think exist in those groups. For example, taxonomists have only sparsely sampled some potentially rich communities, such as the bottom of the deep ocean and the canopies of rainforests.

One estimate of how many species might still be undescribed involves a comparison of fungi and flowering plants (angiosperms). In Great Britain, where both groups are well-known, there are six times as many named species of fungi as of flowering plants. If this ratio applies worldwide, the world total of about 300,000 species of flowering plants, which are fairly well-known globally, predicts a total of about 1.8 million species of fungi, which are not. Because only about 70,000 species of fungi currently have names, the prediction would suggest that these named species constitute only

of the species that exist. Conversely, samples from poorly known parts of the world suggest that the named species constitute a much larger fraction. Although this estimate of the number of species of fungi is almost certainly too high, it nonetheless suggests that there are large numbers of unknown species.

Dandelion (Taraxacum officinale). Alan Punton/A to Z Botanical Collection

For insects, there are about 1 million described species, yet estimates of how many insect species exist range from 10 to 100 times this number. As an example of how part of one such estimate was made, an entomologist collected a large sample of canopy-dwelling beetles from one species of tropical tree. The tree species had 163 beetle species specific to it. There are about 50,000 tree species in the world’s tropical forests, so simple multiplication predicts roughly 8 million species of tropical forest-canopy beetles. Because 40 percent of described insects are beetles, the total number of tropical forest-canopy insects could be 20 million. Adding half that number for insect species found on the ground beneath the trees gives an estimated grand total of 30 million species of tropical forest insects alone.

This example is critically dependent on the first number—the 163 species found only on the one species of tree. In addition, other calculations suggest that this estimate is too high. The number of insect species in tropical forests is more likely to be between 7 and 15 million, which still means that most of Earth’s species would be tropical forest insects. Given the considerable uncertainties, many scientists consider that the total of all species is very roughly 10 million, a number that likely translates to “somewhere between 5 and 20 million with arguments for numbers both lower and higher.”

An obvious concern follows regarding the usefulness of such calculations as a basis for assessing the loss of species. Any absolute estimate of species extinctions must be extrapolated from the 100,000 well-known species of living plants and animals, to the roughly 1.5 million described species, to the likely grand total of very roughly 10 million. Because of uncertainties about the total number of living species, published statements regarding the total number of species that become extinct per year or per day can vary a hundredfold.

Another approach to assessing species loss is to derive relative estimates—estimates of the proportion of well-known species that become extinct in a given interval. Estimating such proportions is the basis for the remainder of the discussion on rates of extinction, but it raises a critical concern of its own: are these proportions actually typical of most species that are still undescribed? They are likely to be so if extinction rates in widely different species groups and regions turn out to be broadly similar.

There also is another way in which estimates of extinctions can be made relative. Extinctions have always been a part of Earth’s history. It is possible to make any estimates of massive future extinction relative to that history.

CALCULATING BACKGROUND EXTINCTION RATES

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To discern the effect of modern human activity on the loss of species requires determining how fast species disappeared in the absence of that activity. Studies of marine fossils show that species last about 1–10 million years. Assume that all these extinctions happened independently and gradually (i.e., the “normal” way) rather than catastrophically, as they did at the end of the Cretaceous Period about 65 million years ago, when dinosaurs and many other land and marine animal species disappeared. On that basis, if one followed the fates of one million species, one would expect to observe about 0.1–1 extinction per year—in other words, one species going extinct every 1–10 years.

Human life spans provide a useful analogy to the foregoing. If humans live for about 80 years on average, all things being equal one would expect that 1 in 80 individuals should die each year under normal circumstances. If, however, many more than 1 in 80 were dying each year, something would be abnormal. There might be an epidemic, for instance.

To make comparisons of present-day extinction rates conservative, assume that the normal rate is just one extinction per million species per year. This then is the benchmark—the background rate against which one can compare modern rates. For example, given a sample of 10,000 living described species (roughly the number of modern bird species), one should see one extinction every 100 years. Comparing this to the actual number of extinctions within the past century provides a measure of relative extinction rates.

The estimates of the background extinction rate described above derive from the abundant and widespread species that dominate the fossil record. By contrast, the species most likely to become extinct today are rare and local. Thus, the fossil data might underestimate background extinction rates. Importantly, however, these estimates can be supplemented from knowledge of speciation rates—the rates that new species come into being—of those species that often are rare and local. These rates cannot be much less than the extinction rates, or there would be no species left.

To explore the idea of speciation rates, one can refer again to the analogy of human life spans and ask: How old are my living siblings? The answer might be anything from that of a newborn to that of a retiree living out his or her last days. The average age will be midway between them—that is, about half a lifetime. Ask the same question for a mouse, and the answer will be a few months; of long-living trees such as redwoods, perhaps a millennium or more. The age of one’s siblings is a clue to how long one will live.

Representative apes (superfamily Hominoidea). Encyclopædia Britannica, Inc.

Species have the equivalent of siblings. They are the species’ closest living relatives in the evolutionary tree—something that can be determined by differences in the DNA. The closest relative of human beings is the bonobo (Pan paniscus), whereas the closest relative of the bonobo is the chimpanzee (P. troglodytes). Taxonomists call such related species sister taxa, following the analogy that they are splits from their “parent” species.

The greater the differences between the DNA of two living species, the more ancient the split from their common ancestor. Studies show that these accumulated differences result from changes whose rates are, in a certain fashion, fairly constant—hence, the concept of the molecular clock—which allows scientists to estimate the time of the split from knowledge of the DNA differences. For example, from a comparison of their DNA, the bonobo and the chimpanzee appear to have split one million years ago, and humans split from the line containing the bonobo and chimpanzee about six million years ago.

Encyclopædia Britannica, Inc.

The advantage of using the molecular clock to determine speciation rates is that it works well for all species, whether common or rare. It works for birds and, in the previous example, for forest-living apes, for which only scant fossils have been recovered. In the preceding example, the bonobo and chimpanzee split a million years ago, suggesting such species’ life spans are, like those of the abundant and widespread marine species discussed earlier, on million-year timescales, at least in the absence of modern human actions that threaten them. This is just one example, however. Is there evidence that speciation can be much more rapid?

Until recently, there seemed to be an obvious example of a high rate of speciation—a “baby boom” of bird species. Its existence allowed for the possibility that the high rates of bird extinction that are observed today might be just a natural pruning of this evolutionary exuberance.

Western bluebird (Sialia mexicana). Herbert Clarke

On either side of North America’s Great Plains are 35 pairs of sister taxa including western and eastern bluebirds (Sialia mexicana and S. sialis), red-shafted and yellow-shafted flickers (both considered subspecies of Colaptes auratus), and ruby-throated and black-chinned hummingbirds (Archilochus colubris and A. alexandri). According to the rapid-speciation interpretation, a single mechanism seemed to have created them all. Each pair of sister taxa had one parent species ranging across the continent. Then a major advance in glaciation during the latter part of the Pleistocene Epoch (2.6 million to 11,700 years ago) split each population of parent species into two groups. Each pair of isolated groups evolved to become two sister taxa, one in the west and the other in the east. Finally, the ice retreated, and, as the continent became warm enough, about 10,000 years ago, the sister taxa expanded their ranges and, in some cases, met once again.

The story, while compelling, is now known to be wrong. Molecular data show that, on average, the sister taxa split 2.45 million years ago. This means that the average species life span for these taxa is not only very much older than the rapid-speciation explanation for them requires but is also considerably older than the one-million-year estimate for the extinction rate previously suggested as a conservative benchmark.

Molecular-based studies find that many sister species were created a few million years ago, which suggests that species should last “a few million” years, too. Indeed, they suggest that the background rate of one extinction among a million species per year may be too high. Nevertheless, this rate remains a convenient benchmark against which to compare modern extinctions.

RECENT EXTINCTION RATES

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To what extent has modern human activity increased extinction rates above the background rate? This discussion presents five well-known case histories of recent extinctions. From them, some general features can be deduced about recent extinctions that also provide clues to the future.

PACIFIC ISLAND BIRDS

Polynesians reached such remote Pacific islands as the Hawaiian Islands, New Zealand, and Easter Island—Earth’s last habi...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Introduction
• Chapter 1: The Pathology of Extinction
• Chapter 2: The Causes of Extinction
• Chapter 3: Preventing the Loss of Biodiversity
• Chapter 4: Ecology
• Chapter 5: Community Ecology
• Chapter 6: Dormancy
• Chapter 7: Notable Figures in Conservation
• Appendix: Related Concepts in Conservation and Ecology
• Glossary
• Bibliography
• Index