HOW MUCH IS THERE?
Biodiversity is the term used to refer to the variety of organisms, their genetic diversity, and the types of ecologic communities into which they are assembled. It can refer to such units as the biota of the entire earth, to the biota of some selected region, to the number and magnitude of differences among evolutionary lineages of organisms, or to the genetic variability within a species. Biodiversity is usefully treated at many different levels, because both basic scientific issues and practical problems focus on different measures of biodiversity.
A basic measure of biodiversity is number of species. Approximately 1.5 million living species and 300,000 fossil species have been described and given scientific names. Estimates of the number of living species vary widely, because they are based on a variety of sources of indirect evidence. The inventory of most species of vertebrates is nearly complete; only minor adjustments are expected in the future. The inventory of vascular plants is not as complete, but we probably know the number of species to within a factor of 2 or 3. However, the vast majority of invertebrates and microorganisms are yet to be described. Most of the insects collected by fogging the canopies of tropical trees, for example, belong to undescribed species (Erwin, 1991). The taxonomy of nematodes is in a very primitive state, and we lack even a reasonable guess of the number of species of bacteria. On the basis of various methods of estimating numbers of species, current workers believe that the total number of species is greater than 15 million, probably as high as 30 million, and possibly over 50 million (May, 1988). That is, we do not know even to within an order of magnitude the number of living species on earth today. Thus, a large fraction of the species likely to be exterminated during the next century will disappear before they have been named, let alone understood ecologically. To the extent that there is merit in the dictum
expressed by Aldo Leopold, that the first rule of intelligent tinkering is to save all the pieces, we are engaged in very dangerous tinkering.
Genetic diversity within species is a useful measure of biodiversity, because many species are divided into local populations that are uniquely adapted to the environments in which they live. The study of the causes and consequences of such local adaptations is an important part of population biology. Programs to restore populations in areas from which they have been almost eliminated must pay careful attention to the genotypes of the individuals to be introduced. Maintaining genetic diversity is an essential component of successful captive propagation efforts for rare and endangered species, and much valuable biodiversity can be lost when local populations are exterminated, even if the species survives.
Another important component of biodiversity is the distinctness of evolving lineages. Numbers of the higher taxonomic categories (phyla and classes) in the universally used hierarchic biologic classification system provide a rough measure of distinctness of lineages. By that measure, marine biodiversity is much greater than terrestrial biodiversity, even though there are far fewer marine than terrestrial species. Of the 32 extant phyla of multicellular animals, for example, 31 are marine, and 14 of the 31 are exclusively marine. From this perspective, preservation of marine biodiversity is more important than might be suggested if one simply compares the numbers of species in marine and terrestrial ecologic communities.
Organisms do not live in isolation, but are embedded in a physical environment and a complex matrix of interacting species; and the richness of ecosystems is another measure of biodiversity. Preservation of these systems is essential for the preservation of the species living in them. Although a rich array of terms is used to describe different ecologic communities, there is no generally accepted classification of ecosystems. Species can be defined objectively, but there is no objective basis for deciding how finely ecosystems should be divided. Efforts are under way to develop classification systems for ecosystems that can guide conservation efforts.
Where Is It?
Living species are not uniformly distributed over the surface of the globe. To describe the complex spatial patterns of biodiversity, ecologists and biogeographers have found it useful to divide diversity into two major components: α-diversity and ß-diversity. α-diversity refers to point diversity, that is, to the number of species found in a small homogeneous area. ß-diversity refers to the rate of change in species of composition across habitat
and landscape gradients. A high ß-diversity means that the cumulative number of species recorded increases rapidly as additional areas along some environmental gradient are subjected to a census. Species can also drop out rapidly along such gradients and cause a high rate of species turnover.
α -diversity is characterized by several widespread patterns. First, for most taxa, tropical regions have many more species than higher-latitude ecologic communities. The presence of a few well-known exceptions–such as marine algae, coniferous trees, bees, and salamanders–does not detract from the great importance of tropical regions, both marine and terrestrial, as the home of most of the world's living species.
Second, the diversity of species in most other taxa is positively correlated with the structural complexity of the ecologic community. That is, structurally simple habitats–such as the open ocean, grasslands, and cold deserts–support fewer species of organisms than structurally more complex communities, such as forests and coral reefs. The reason is believed to be fact that in most terrestrial environments plants provide the major components of physical structure within which the activities of all other organisms are carried out. Coral reefs serve the same function in marine environments. Thus, complex communities provide a greater variety of microclimates, a greater variety of resources, more ways in which to exploit those resources, and more places from which to hide from predators and the physical environment.
Third, among most ecosystems there is a positive correlation between productivity and number of species. That is due to the existence of a greater variety of resources above some critical minimum in more productive systems. There are, however, conspicuous exceptions. For example, some extremely productive ecosystems–such as salt marshes, sea-grass beds, and hot springs–are relatively species-poor. Most such systems are distributed as relatively small, fragmented patches that differ strikingly from the surrounding, dominant ecosystems. Evolutionary ecologists believe that the combination of major physical differences and isolation of the patches prevents many species of organisms from evolving adaptations to those unusual environments.
Fourth, island communities are species-poor, compared with mainland communities. In general, the number of species found on islands is inversely correlated with distance from the mainland and positively correlated with island size and topographic diversity. The low species richness on islands is usually attributed to low colonization rates, high extinction rates (because populations are usually small and subject to decimation by local catastrophes and stochastic variation), and lack of particular kinds of resources typically provided by species that are poor dispersers across ocean barriers. Also, island communities have experienced extremely high extinction rates of species during the last century, primarily because of the introduction of mammalian
predators (mammals disperse poorly across ocean barriers) and mainland diseases (against which island species have no defenses).
Patterns of ß-diversity are much less well understood. We do know that, on the average, terrestrial tropical species have smaller ranges than species of higher latitudes, but there are many exceptions. A key question is whether a given change in physical environmental conditions causes a greater species turnover in tropical environments than in higher-latitude environments. The question is difficult to answer, because ranges of tropical species are poorly known and physical conditions cannot be matched between tropical and higher-latitude environments.
An important corollary of patterns of species richness is that areas with high a-diversity inevitably have many rare species. For example, a typical tropical wet forest in Central America or South America might harbor 300 - 400 species of trees per square kilometer, and a temperate-zone forest 30-40 species of trees. Given that the number of trees per hectare is roughly the same in tropical and temperate forests, it follows that most of the tree species in tropical forests must be present at very low densities. The same argument applies to most taxa. Because risk of extinction is, in general, inversely proportional to population size, areas with high biodiversity are at greatest risk of losing many of their species.
WHAT ARE THE THREATS TO BIODIVERSITY?
Extinction is a normal evolutionary process. Many species become "extinct" because they evolve into forms that are so different from their ancestors that we assign them different names. Evolutionary biologists usually refer to these as "pseudoextinctions," because there has been no extinction of an evolving lineage. What is of central concern is true extinction, that is, the termination of an evolutionary lineage.
Six major episodes of extinctions have punctuated the history of life on earth. Paleontologists do not agree as to the causes of those episodes, but evidence suggests that there is no common cause. The extinctions at the end of the Cretaceous Period were probably caused by collision of a large meteor with earth. Other extinctions were associated with the breakup of Pangea, massive alterations of oceanic circulation, and retreat of ocean waters from large areas of the continental shelves. The last major episode, which involved primarily large vertebrates, was probably caused by humans.
After each episode, biodiversity gradually recovered to the point where overall biodiversity has tended to increase. We might be living today in the period of greatest biodiversity in the history of earth. However, recovery from
massive species loss was slow, requiring many millions of years to return to the levels present before to the extinction episode. Thus, recovery rates were lower than extinction rates. Speciation processes, although they will continue, will not save biodiversity from high rates of extinction in time frames relevant to human life and human policy decisions.
Current extinctions differ from previous ones both in their rate and in their cause. The loss of species today is due not to major physical changes on earth, but rather to the activities of a single species. Human population densities and rates of resource use are so high that massive changes in the earth's ecosystems and possibly, as a corollary, its climate are under way. About one-third of the gross primary production of the earth's terrestrial ecosystems is now being co-opted by humans (Vitousek et al., 1986), and the fraction is rising. If the human population doubles during the next 50 years–as appears extremely likely, barring major catastrophes–we will be co-opting more than half the primary terrestrial production. All other species, except human commensals, will have to survive on the fraction that remains. A much smaller fraction of marine productivity is being co-opted by humans, but co-option of coastal productivity, particularly that of the estuaries on which many marine organisms depend, is extensive.
The International Union for the Conservation of Nature and Natural Resources Invertebrate Red Data Book (1983) estimates that, among the species threatened by human activities, habitat destruction is contributing to the endangerment of about 73%, displacement by introduced species to 68%, hybridization to 38%, and overexploitation to 15%. The numbers total to more than 100% because many species are being adversely affected by more than one of these factors.
Historically, overexploitation was the major cause of human-induced species extinctions. When humans arrived on Pacific islands, New Zealand, Australia, Madagascar, and North America, they exterminated many species of large birds and mammals (Anderson, 1989, Martin and Klein, 1984). Overexploitation of terrestrial organisms continues today, but much of it is not due to local killing for subsistence, but is driven by lucrative international markets in animal products (such as ivory, rhinoceros horn, and furs), the pet trade (parrots and tropical fish), and plants (such as orchids).
Because the problem is international, it is amenable to international solution. The major tool now being used is the International Convention on Trade in Endangered Species (CITES). Many countries, including the United
States, are signatories to this convention, and substantial results have been obtained in many, but not all, cases. Overexploitation of marine organisms is also a serious problem, but its extent and magnitude are poorly understood because of the extreme difficulty of estimating population sizes of marine organisms and determining the factors most strongly influencing their population dynamics.
Humans, inadvertently or deliberately, move large numbers of species around the world. Such introduced species have caused major ecologic and economic problems (witness European rabbits in Australia and chestnut blight and Dutch elm disease in North America), and they have been responsible for many species extinctions. Especially destructive have been mammals introduced to oceanic islands that previously lacked mammals, disease organisms, and weedy plants. For example, the flora of California now includes over 1,000 species of exotic plants, and exotic species dominate many lowlands of California today.
Total elimination of international traffic in species is probably unattainable, because many small organisms travel as undetected or unwanted passengers in human baggage, in holds of ships and aircraft, and in other hideaways. However, international movements of larger organisms can be controlled to a large extent. Importation of game birds and mammals has been largely stopped, but fish introductions continue and the horticulture profession still imports and experiments with exotic plants from throughout the world. Also, the practice of biologic control of pests (most of which are introduced species) often depends on the importation of an exotic predator or disease organism from the original range of the pest. Although such introductions have caused remarkably few problems in the past, they are not riskfree.
By far the most important cause of extinction of terrestrial species today is loss of habitats. It is also the most intractable of the causes, because preservation of habitats requires allocation of large areas of land, and land usually has high-value alternative uses. The scenic national parks of the United States were established primarily around geologic marvels, and most of them were established in areas of limited alternative use (except for
mining). If we are to preserve biodiversity, more parks and reserves must be established on land with high alternative-use value. Note that biodiversity preservation contradicts a basic dictum of much of economic analysis, namely that lack of a market for a resource guarantees its preservation.
In addition, research by conservation biologists has shown that the parks and reserves we have already established and those likely to be established in the near future will inevitably be inadequate for the preservation of most of the world's species. Much of the burden will have to fall on the exploited lands that dominate the landscapes within which parks and reserves are imbedded. Therefore, forests will have to be managed for a diversity of values, rather than for maximization of wood production, and agricultural practices will have to be modified so that agricultural lands play a greater role in providing habitats for species and corridors for their movements between reserves. The social and political implications of such changes are great. Indeed, the issue cuts to the heart of how much control society will concedes to landowners over the resources that their land contains.