Biodiversity

views updated

Biodiversity

Biodiversity is an ecological notion that refers to the richness of biological types at a range of hierarchical levels, including: (1) genetic diversity within species , (2) the richness of species within communities, and (3) the richness of communities on landscapes. In the context of environmental studies, however, biodiversity usually refers to the richness of species in some geographic area, and how that richness may be endangered by human activities, especially through local or global extinction .

Extinction represents an irrevocable and highly regrettable loss of a portion of the biodiversity of Earth. Extinction can be a natural process, caused by: 1) random catastrophic events; 2) biological interactions such as competition , disease, and predation; 3) chronic stresses; or 4) frequent disturbance. However, with the recent ascendance of human activities as a dominant force behind environmental changes, there has been a dramatic increase in rates of extinction at local, regional, and even global levels.

The recent wave of anthropogenic extinctions includes such well-known cases as the dodo , passenger pigeon , great auk, and others. There are many other high-profile species that humans have brought to the brink of extinction, including the plains buffalo, whooping crane , eskimo curlew, ivory-billed woodpecker , and various marine mammals. Most of these instances were caused by an insatiable over-exploitation of species that were unable to sustain a high rate of mortality , often coupled with an intense disturbance of their habitat .

Beyond these tragic cases of extinction or endangerment of large, charismatic vertebrates, the earth's biota is experiencing an even more substantial loss of biodiversity caused by the loss of habitat. In part, this loss is due to the conversion of large areas of tropical ecosystems, particularly moist forest, to agricultural or otherwise ecologically degraded habitats. A large fraction of the biodiversity of tropical biomes is comprised of rare, endemic (i.e., with a local distribution) species. Consequently the conversion of tropical rain forest to habitats unsuitable for these specialized species inevitably causes the extinction of most of the locally endemic biota. Remarkably, the biodiversity of tropical forests is so large, particularly in insects, that most of it has not yet been identified taxonomically. We are therefore faced with the prospect of a mass extinction of perhaps millions of species before they have been recognized by science.

To date, about 1.7 million organisms have been identified and designated with a scientific name. About 6% of identified species live in boreal or polar latitudes, 59% in the temperate zones, and the remaining 35% in the tropics. The knowledge of the global richness of species is very incomplete, particularly in the tropics. If a conservative estimate is made of the number of unidentified tropical species, the fraction of global species that live in the tropics would increase to at least 86%.

Invertebrates comprise the largest number of described species, with insects making up the bulk of that total and beetles (Coleoptera ) comprising most of the insects. Biologists believe that there still is a tremendous number of undescribed species of insects in the tropics, possibly as many as another 30 million species. This remarkable conclusion has emerged from experiments conducted by the entomologist Terry Erwin, in which scientists "fogged" tropical forest canopies and then collected the "rain" of dead arthropods. This research suggests that: (1) a large fraction of the insect biodiversity of tropical forests is undescribed; (2) most insect species are confined to a single type of forest, or even to particular plant species, both of which are restricted in distribution; and (3) most tropical forest insects have a very limited dispersal ability.

The biodiversity and endemism of other tropical forest biota are better known than that of arthropods. For example, a plot of only 0.2 acres (0.1 ha) in an Ecuadorian forest had 365 species of vascular plants. The richness of woody plants in tropical rain forest can approach 300 species per hectare, compared with fewer than 12–15 tree species in a typical temperate forest, and thirty to thirty-five species in the Great Smokies of the United States, the richest temperate forest in the world.

There have been few systematic studies of all of the biota of particular tropical communities. In one case, D. H. Janzen studied a savanna-like, 67-mi² (108-km²) reserve of dry tropical forest in Costa Rica for several years. He estimated that the site had at least 700 plant species, 400 vertebrate species, and a remarkable 13,000 species of insect, including 3,140 species of moths and butterflies.

Why should one worry about the likelihood of extinction of so many rare species of tropical insects, or of many other rare species of plants and animals? There are three classes of reasons why extinctions are regrettable:

There are important concerns in terms of the ethics of extinction. Central questions are whether humans have the "right" to act as the exterminator of unique and irrevocable species of wildlife and whether the human existence is somehow impoverished by the tragedy of extinction. These are philosophical issues that cannot be scientifically resolved, but it is certain that few people would applaud the extinction of unique species.
There are utilitarian reasons. Humans must take advantage of other organisms in myriad ways for sustenance, medicine, shelter, and other purposes. If species become extinct, their unique services, be they biological, ecological, or otherwise, are no longer available for exploitation.
The third class of reasons is ecological and involves the roles of species in maintaining the stability and integrity of ecosystems, i.e., in terms of preventing erosion and controlling nutrient cycling, productivity, trophic dynamics, and other aspects of ecosystem structure and function. Because we rarely have sufficient knowledge to evaluate the ecological "importance" of particular species, it is likely that an extraordinary number of species will disappear before their ecological roles are understood.

There are many cases where research on previously unexploited species of plants and animals has revealed the existence of products of great utility to humans, such as food or medicinals. One example is the rosy periwinkle (Catharantus roseus ), a plant native to Madagascar . During a screening of many plants for possible anti-cancer properties, an extract of rosy periwinkle was found to counteract the reproduction of cancer cells. Research identified the active ingredients as several alkaloids, which are now used to prepare the important anti-cancer drugs vincristine and vinblastine. This once obscure plant now allows treatment of several previously incurable cancers and is the basis of a multi-million-dollar economy.

Undoubtedly, there is a tremendous, undiscovered wealth of other biological products that are of potential use to humans. Many of these natural products are present in the biodiversity of tropical species that has not yet been "discovered" by taxonomists.

It is well known that extinction can be a natural process. In fact, most of the species that have ever lived on Earth are now extinct, having disappeared "naturally" for some reason or other. Perhaps they could not cope with changes in their inorganic or biotic environment , or they may have succumbed to some catastrophic event, such as a meteorite impact.

The rate of extinction has not been uniform over geological time. Long periods characterized by a slow and uniform rate of extinction have been punctuated by about nine catastrophic events of mass extinction. The most intense mass extinction occurred some 250 million years ago, when about 96% of marine species became extinct. Another example occurred 65 million years ago, when there were extinctions of many vertebrate species, including the reptilian orders Dinosauria and Pterosauria, but also of many plants and invertebrates, including about one half of the global fauna that existed then.

In modern times, however, humans are the dominant force causing extinction, mostly because of: (1) overharvesting; (2) effects of introduced predators, competitors, and diseases; and (3) habitat destruction. During the last 200 years, a global total of perhaps 100 species of mammals, 160 birds, and many other taxa are known to have become extinct through some human influence, in addition to untold numbers of undescribed, tropical species.

Even pre-industrial human societies caused extinctions. Stone-age humans are believed to have caused the extinctions of large-animal fauna in various places, by the unsustainable and insatiable hunting of vulnerable species in newly discovered islands and continents. Such events of mass extinction of large animals, co-incident with human colonization events, have occurred at various times during the last 10–50,000 years in Madagascar, New Zealand, Australia , Tasmania, Hawaii, North and South America, and elsewhere.

In more recent times, overhunting has caused the extinction of other large, vulnerable species, for example the flightless dodo (Raphus cucullatus )of Mauritius. Some North American examples include Labrador duck (Camptorhynchus labradorium ), passenger pigeon (Ectopistes migratorius ), Carolina parakeet (Conuropsis carolinensis ), great auk (Pinguinus impennis ), and Steller's sea cow (Hydrodamalis stelleri ). Many other species have been brought to the brink of extinction by overhunting and loss of habitat. Some North American examples include eskimo curlew (Numenius borealis ), plains bison (Bison bison ), and a variety of marine mammals, including manatee (Trichechus manatus ), right whales (Eubalaena glacialis ), bowhead whale (Balaena mysticetus ), and blue whale (Balaenoptera musculus ).

Island biotas are especially prone to both natural and anthropogenic extinction. This syndrome can be illustrated by the case of the Hawaiian Islands ,an ancient volcanic archipelago in the Pacific Ocean, about 994 mi (1,600 km) from the nearest island group and 2,484 mi (4,000 km) from the nearest continental landmass. At the time of colonization by Polynesians, there were at least 68 endemic species of Hawaiian birds, out of a total richness of land birds of 86 species. Of the initial 68 endemics, 24 are now extinct and 29 are perilously endangered. Especially hard hit has been an endemic family, the Hawaiian honeycreepers (Drepanididae), of which 13 species are believed extinct, and 12 endangered. More than 50 alien species of birds have been introduced to the Hawaiian Islands, but this gain hardly compensates for the loss and endangerment of specifically evolved endemics. Similarly, the native flora of the islands is estimated to have been comprised of 1,765–2,000 taxa of angiosperm plants, of which at least 94% were endemic. During the last two centuries, more than 100 native plants have become extinct, and the survival of at least an additional 500 taxa is threatened or endangered, some now being represented by only single individuals. The most important causes of extinction of Hawaiian biota have been the conversion of natural ecosystems to agricultural and urban landscapes, the introduction of alien predators, competitors, herbivores, and diseases, and to some extent, aboriginal over-hunting of some species of bird.

Overhunting has been an important cause of extinction, but in modern times habitat destruction is the most important reason for the event of mass extinction that Earth's biodiversity is now experiencing. As was noted previously, most of the global biodiversity is comprised of millions of as yet undescribed taxa of tropical insects and other organisms. Because of the extreme endemism of most tropical biota, it is likely that many species will become extinct as a result of the clearing of natural tropical habitats, especially forest, and its conversion to other types of habitat.

The amount and rate of deforestation in the tropics are increasing rapidly, in contrast to the situation at higher latitudes where forest cover is relatively stable. Between the mid 1960s and the mid 1980s there was little change (less than 2%) in the forest area of North America, but in Central America forest cover decreased by 17%, and in South America by 7% (but by a larger%age in equatorial countries of South America). The global rate of clearing of tropical rain forest in the mid 1980s was equivalent to 6–8% of that biome per year, a rate that if projected into the future would predict a biome half-life of only nine to 12 years. Some of the cleared forest will regenerate through secondary succession , which would ultimately produce another mature forest. Little is known, however, about the rate and biological character of succession in tropical rainforests, or how long it would take to restore a fully biodiverse ecosystem after disturbance.

The present rate of disturbance and conversion of tropical forest predicts grave consequences for global biodiversity. Because of a widespread awareness and concern about this important problem, much research and other activity has recently been directed towards the conservation and protection of tropical forests. As of 1985, several thousand sites, comprising more than 640,000 mi² (1 million km), had received some sort of "protection" in low-latitude countries. Of course the operational effectiveness of the protected status varies greatly, depending on the commitment of governments to these issues. Important factors include: (1) political stability; (2) political priorities; (3) finances available to mount effective programs to control poaching of animals and lumber and to prevent other disturbances; (4) the support of local peoples and communities for biodiversity programs; (5) the willingness of relatively wealthy nations to provide a measure of debt relief to impoverished tropical countries and thereby reduce their short term need to liquidate natural resources in order to raise capital and provide employment; and (6) local population growth , which also generates extreme pressures to over exploit natural resources.

The biodiversity crisis is a very real and very important aspect of the global environmental crisis. All nations have a responsibility to maintain biodiversity within their own jurisdictions and to aid nations with less economic and scientific capability to maintain their biodiversity on behalf of the entire planet. The modern biodiversity crisis focuses on species-rich tropical ecosystems, but the developed nations of temperate latitudes also have a large stake in the outcome and will have to substantially subsidize global conservation activities if these are to be successful. Much needs to be done, but an encouraging level of activity in the conservation and protection of biodiversity is beginning in many countries, including an emerging commitment by many nations to the conservation of threatened ecosystems in the tropics.

[Bill Freedman Ph.D. ]