Biodiversity

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Biodiversity

Biodiversity is the total richness of biological variation. The scope of biodiversity is usually considered to range from the genetic variation of individual organisms within and among populations of a species , to different species occurring together in ecological communities. Some definitions of biodiversity also include the spatial patterns and temporal dynamics of populations and communities on the landscape. The geographical scales at which biodiversity can be considered range from local to regional, state or provincial, national, continental, and ultimately to global.

Biodiversity at all scales is severely threatened by human activities; this is one of the most important aspects of the global environmental crisis. Humans have already caused permanent losses of biodiversity through the extinction of many species and extensive losses of distinctive, natural ecosystems. Ecologists predict that unless there are substantial changes in the ways that humans affect ecosystems, there will be much larger losses of biodiversity in the near future.

Species richness of the biosphere

About 1.7 million of Earth's species have been identified and designated with a scientific name. About 6% of the identified species live in boreal or polar latitudes, 59% in the temperate zones, and the remaining 35% in the tropics. However, the knowledge of Earth's species is highly incomplete, especially for tropical countries. According to some estimates, there could be as many as 30-50 million species on Earth , 90% of them occurring in tropical ecosystems. Tropical ecosystems are much richer in species than are those at higher latitudes.

Most of the described species on Earth are invertebrates , particularly insects , and most of the insects are beetles (order Coleoptera). A famous scientist, J.B.S. Haldane, was once asked by a theologian to briefly explain what his knowledge of biology told of God's purpose. Haldane reputedly said that God has "an inordinate fondness of beetles," reflecting the fact these insects are so much richer in species than any other group of creatures on Earth. Some biologists believe that beetles account for most of the undescribed tropical insects.

The suggestion of enormous numbers of undescribed insects in tropical forests initially emerged from the research of Terry Erwin. This entomologist performed experiments in which tropical-forest canopies in Amazonia were treated with an insecticide, and the subsequent "rain" of dead arthropods was collected using ground-level sampling devices. This innovative sampling procedure indicated that: (1) a large fraction of the insect species of tropical forests is unknown to science; (2) most insect species are confined to a single type of tropical forest, or even to a particular tree species, which may itself have only a local distribution; and (3) most species of tropical-forest insects have little ability to disperse very far. Erwin's studies of tropical rainforest found that beetles accounted for most of the insect species and that most of the beetles are narrowly endemic , that is, they have a local distribution and are found nowhere else. For example, the tree Luehea seemanii has more than 1,100 species of beetle in its canopy, of which 15% are specific to that plant . The emerging conclusion from this and other descriptive research is that there is an enormous abundance of undescribed species of insects and other invertebrates living in tropical forests.

Compared with invertebrates, the numbers of species (that is, the species richness) of other groups of tropical-forest organisms are better known. Although it is extremely difficult to do so, the numbers of species of vascular plants have been described for a few tropical forests. For example: a plot of only 0.0004 sq mi (0.1 ha) in a moist forest in Ecuador had 365 species of vascular plants; there were 98 species of large trees in 0.006 sq mi (1.5 ha) of forest in Sarawak, Malaysia; there were 90 tree species in 0.0032 sq mi (0.8 ha) of forest in Papua New Guinea; 742 woody species occurred in 0.012 sq mi (3 ha) of forest in Sarawak, with 50% of the species recorded as single individuals; and more than 300 species of woody plants were discovered on a 2-sq mi (50-ha) area of forest in Panama. These tropical forests are much richer than temperate forests, stands of which typically support fewer than 12-15 species of trees. The Great Smokey Mountains of the eastern United States have some of the richest temperate forests in the world, and they typically contain 30-35 species, far fewer than tropical rainforests.

It is extraordinarily difficult to determine the numbers of birds in tropical forest because the dense foliage and darkness of the understory make it inconvenient to see small animals, even if they are brightly colored. As a result, few studies have been made of the birds of tropical rainforest. However, one study in Peru discovered 245 resident and 74 transient species in 0.4 sq mi (97 ha) of Amazonian forest. Another study of rainforest in French Guiana recorded 239 species of birds, and another found 151 species in a forest in Sumatra. In comparison, stands of temperate forest in North America typically support only 15-30 species of birds.

Almost no systematic surveys have been made of all of the species of tropical ecosystems. In one case, a 42 sq mi (108 sq km) reserve of dry forest in Costa Rica was estimated to support about 700 plant species, 400 vertebrate species, and 13,000 species of insects, including 3,140 species of moths and butterflies .

Why is biodiversity important?

Biodiversity is valuable for the following classes of reasons:

Intrinsic Value. Biodiversity has its own intrinsic value, regardless of its worth in terms of human needs. Because of its intrinsic merit, there are ethical considerations to any degradation of biodiversity. For example, do humans have the "right" to diminish or exterminate elements of biodiversity, all of which are unique and irretrievable? Is the human existence itself diminished by losses of biodiversity? Ethical issues cannot be resolved through science, but enlightened people would mourn any loss of species, or of natural ecosystems.
Utilitarian Value. Humans have an absolute requirement for the products of other species. Because of this need, wild and domesticated species and their communities are exploited in many ways to provide food, materials, energy , and other goods and services. This fact can be illustrated in many ways. In the United States, for example, about one-quarter of prescription drugs have active ingredients obtained from higher plants, and these uses contribute about $14 billion per year to the U.S. economy, and $40 billion per year worldwide. Potentially, harvests of biodiversity can be conducted in ways that foster their renewal. Unfortunately, potentially renewable biodiversity resources are often harvested too intensively or inadequate attention is paid to regeneration, so the resource is degraded or becomes extinct.
Provision of Ecological Services. Biodiversity provides many ecological services that are directly or indirectly important to human welfare. Examples of these services include biological productivity, nutrient cycling, cleaning water and air of pollutants, control of erosion , provision of atmospheric oxygen , removal of carbon dioxide , and other functions related to the integrity of ecosystems. According to the biologist Peter Raven: "Biodiversity keeps the planet habitable and ecosystems functional."

There are many cases of the discovery, through research on previously unexploited plants and animals, of bio-products useful to humans as food, medicine, or for other purposes. Consider the case of the rosy periwinkle (Catharantus roseus), a small plant native to the tropical island of Madagascar. During an extensive screening of wild plants for anti-cancer chemicals, an extract of rosy periwinkle was observed to inhibit the growth of cancerous cells. The active biochemicals are several alkaloids in foliage of the plant, which probably serve to deter herbivores. These natural substances are now used to prepare the drugs vincristine and vinblastine, which can be successfully used to treat childhood leukemia and a lymphatic cancer known as Hodgkin's disease . In this case, a species of wild plant known only to a few botanists has proven to be of great benefit to humans by treating previously incurable diseases, in the process sustaining a large pharmaceutical economy. There is a tremendous undiscovered wealth of other biological products useful to humans in unexplored biodiversity.

Biodiversity and extinction

Extinction refers to the loss of some species or other taxonomic unit (e.g., subspecies, genus, family, etc.; each is known as a taxon), occurring over all of its range on Earth. (Extirpation refers to a more local disappearance, with the taxon still surviving elsewhere.) The extinction of any species is an irrevocable loss of part of the biological richness of Earth, the only place in the universe known to support living creatures. Extinction can be a natural occurrence caused by an unpredictable catastrophe, chronic environmental stress, or ecological interactions such as competition , disease , or predation. However, there have been dramatic increases in extinction rates since humans have become Earth's dominant large animal and the cause of global environmental change.

Extinction has always occurred naturally. Almost all species that have ever lived on Earth have become extinct. Perhaps they could not cope with changes occurring in their environment, such as climate changes, the intensity of predation, or disease. Alternatively, many extinctions may have occurred simultaneously as a result of unpredictable catastrophes. From the geological record it is known that species, families, and even phyla have appeared and disappeared over time . For example, numerous phyla of invertebrates proliferated during an evolutionary radiation occurring at the beginning of the Cambrian era about 570 million years ago, but most of these animals are now extinct. The 15-20 extinct phyla from that period are known from the Burgess Shale of British Columbia, and they represent unique experiments in invertebrate form and function. Similarly, entire divisions of plants have appeared, radiated, and disappeared, such as the seed ferns Pteridospermales, the cycad-like Cycadeoidea, and woody plants known as Cordaites. Of the 12 orders within the class Reptilia, only three survive today: crocodilians, turtles , and snakes/lizards. Clearly, the fossil record displays a great deal of evidence of natural extinctions.

Overall, the geological record suggests that there have been long periods of time characterized by uniform rates of extinction, but punctuated by about nine catastrophic episodes of mass extinction . The most intense extinction event occurred at the end of the Permian period some 245 million years ago, when 54% of marine families, 84% of genera, and 96% of species are estimated to have become extinct.

Another famous, apparently synchronous, extinction of vertebrate animals occurred about 65 million years ago at the end of the Cretaceous period. The most renowned extinctions were of the last of the reptilian dinosaurs and pterosaurs, but many plants and invertebrates also became extinct at that time. In total, perhaps 76% of species and 47% of genera became extinct in the end-of-Cretaceous crisis. One hypothesis to explain the cause of this mass extinction involves a meteorite impacting the Earth, causing huge quantities of fine dust to be spewed into the atmosphere, and resulting in a climatic deterioration that most large animals could not tolerate. However, some scientists believe that the extinctions of the last dinosaurs were more gradual.

However, humans have been responsible for almost all of Earth's recent extinctions. These extinctions are occurring so quickly that they represent a modern mass extinction of similar intensity to those documented in the geological record. Examples of recent extinctions caused by humans include such well-known cases as the dodo, passenger pigeon, and great auk. Many other high-profile species have been taken to the brink of extinction, including the plains bison , whooping crane, ivory-billed woodpecker, right whale, and other marine mammals . These losses have been caused by insatiable overhunting and intense disturbance or conversion of natural habitats.

Beyond these well-known and tragic cases involving large animals, Earth's biodiversity is experiencing an even larger loss. This ruin is mostly being caused by extensive conversions of tropical ecosystems, particularly rainforest, into agricultural habitats that sustain few of the original species. As was described previously, tropical ecosystems sustain extremely large numbers of species, most of which have restricted distributions. The conversion of tropical forest into other habitats inevitably causes the loss of most of the locally endemic biota. This is a great tragedy, and the lost species will never occur again.

The most important human influences causing the extinction or endangerment of species are: (1) excessive exploitation, (2) effects of introduced predators, competitors, and diseases, and (3) habitat disturbance and conversion. These stressors can result in small and fragmented populations that experience the deleterious effects of inbreeding and population instability and then decline further, ultimately to extirpation or extinction.

The increased rate of extinction and endangerment of biodiversity during the past several centuries is best documented for vertebrates because, as noted previously, most invertebrate species, particularly insects, have not yet been described by scientists. During the last four centuries there have been more than 700 known extinctions globally, including about 100 species of mammals and 160 species of birds, all because of human influences.

A much larger number of species is facing imminent extinction; they are endangered. For example, more than 1,000 species of birds are considered to be threatened with extinction. Of this total, 46% live only on oceanic islands, a situation in which species are especially vulnerable to extinction caused by stresses associated with human activity. Birds of tropical forests account for 43% of the threatened bird species, wetland species for 21%, grassland and savanna species 19%, and other habitats 17%. Only 1.5% of the threatened species are North American, 4.2% are European or Russian, 33% Central and South American, 18% African, 30% Asian, and 14% from Australia and the Pacific.

Protection of endangered biodiversity

Biodiversity can be protected in ecological reserves. These are protected areas established for the conservation of natural values, usually the known habitat of endangered species , threatened ecosystems, or representative examples of widespread communities. In the early 1990s there were about 7,000 protected areas globally, with an area of 2.5 million sq mi (651 million ha.) Of this total, about 2,400 sites comprising 1.5 million sq mi (379 million ha) were fully protected and could be considered to be true ecological reserves.

Ideally, the design of a national system of ecological reserves would provide for the longer-term protection of all native species and their natural communities, including terrestrial, freshwater , and marine ecosystems. So far, however, no country has implemented a comprehensive system of ecological reserves to fully protect its natural biodiversity. Moreover, in many cases the existing reserves are relatively small and threatened by eenvironmental change and other stressors, such as illegal poaching of animals and plants and sometimes excessive tourism.

The World Conservation Union, World Resources Institute, and United Nations Environment Program are three important agencies whose mandates center on the conservation of the world's biodiversity. These agencies have developed the Global Biodiversity Strategy, an international program to help protect biodiversity. The broad objectives are to: (1) preserve biodiversity; (2) maintain Earth's ecological processes and life-support systems; and (3) ensure that biodiversity resources are used in a sustainable manner. As such, the Global Biodiversity Strategy is a mechanism by which countries and peoples can initiate meaningful actions to protect biodiversity for the benefit of present and future generations of people, and also for its intrinsic value. Because it only began in the late 1970s, it is too early to evaluate the success of this program. However, the existence of this comprehensive international effort is encouraging, as is the participation of most of Earth's countries, representing all stages of socioeconomic development.

Another important international effort is the Convention on Biological Diversity, negotiated under the auspices of the United Nations Environment Program and signed by many countries at a major conference at Rio De Janeiro in 1992. This international treaty requires signatory nations to take measures to systematically catalogue their indigenous biodiversity, and to take action to ensure that it is conserved.

Numerous other agencies are working to preserve biodiversity. In the United States, the World Wildlife Fund and Nature Conservancy are important organizations at the national level. There are numerous other national and local groups. The lead federal agency is the U.S. Fish and Wildlife Service, and all states have similar agencies.

Important progress is being made, and the progressive worldwide development of activities intended to identify, conserve, and preserve biodiversity will hopefully come to be regarded as an ecological "success story."

Resources

books

Becher, A. Biodiversity: A Reference Handbook. Abo-Clio Pub, 1998.

Dobson, A.P. Conservation and Biodiversity. W.H. Freeman and Co., 1998.

Freedman, B. Environmental Ecology. 2nd edition. San Diego: Academic Press, 1994.

Gaston, K.J., and J.I. Spicer. Biodiversity: An Introduction. Blackwell Science Inc., 1998.

Hamblin, W.K., and E.H. Christiansen. Earth's Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001. Levin, Simon A., ed. Encyclopedia of Biodiversity. San Diego, CA: Academic Press, 2000.

Myers, Judith, and Dawn Bazely. Ecology and Control of Introduced Plants. Cambridge: Cambridge University Press, 2003.

Nebel, Bernard J., and Richard T. Wright. Environmental Science: Toward a Sustainable Future. 8th ed. Englewood Cliffs, NJ: Prentice Hall, 2002.

Schneiderman, Jill S. The Earth Around Us: Maintaining aLivable Planet. New York: W.H. Freeman & Co., 2000.

Wilson, E.O. The Diversity of Life. Cambridge: Harvard University Press, 1992.

Wilson, E.O., ed. BioDiversity. Washington, DC: National Academy Press, 1988.

periodicals

Caballero A., and M.A. Toro. "Interrelations Between Effective Population Size and Other Pedigree Tools for the Management of Conserved Populations." Genet Res 75(no. 3) (June 2000): 331–43.

Bill Freedman

KEY TERMS

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Binomial: —The scientific name of organisms consists of two words, and is therefore a binomial. Humans, for example, are named Homo sapiens, which designates our genus as Homo, and our species within that genus as sapiens. Binomials are always latinized words, and are written in italics, or underlined.
Endangerment: —Refers to a situation in which a species is vulnerable to extinction or extirpation.
Endemic: —Refers to species with a relatively local distribution, sometimes occurring as small populations confined to a single place, such as an oceanic island. Endemic species are more vulnerable to extinction than are more widespread species.
Extinction: —The condition in which all members of a group of organisms have ceased to exist. Extirpation means that a species no longer occurs in a place or country, although it survives elsewhere.
Species diversity: —An indicator of biodiversity at the community level, which accommodates both the number of species present (i.e., richness) and their relative abundance. Ecologists consider species diversity to be a good indicator of biodiversity within communities, because it accommodates differences amongst species in rarity and commonness.
Species richness: —The number of species occurring in a community, a landscape, or some other defined area.