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Peter H. Raven


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obody knows how many species there are in the world -- or how fast they are disappearing. Fewer than 2 million have been cataloged and estimates of the total vary wildly, ranging from 7 million to as many as 80 million. The currently accepted working estimate is 13.6 million.

A quarter of the total number of species may be beetles, whose diversity is especially rich high in the rainforest canopy of the Amazon. Many more could be nematodes on the floors of the oceans. As many again could exist among single-cell microbes, whose diversity is beginning to be assessed for the first time using gene-typing [1]. [Add]

Biodiversity is a term applied to describe the complexity of life. It is generally measured at three levels: the variety of species; the genetic diversity found within members of the same species (what makes you different from your neighbor); and the diversity of the ecosystems within which species live. These three levels are intimately connected. Genetic diversity is essential to the prosperity of the species, giving it the resources to adapt. And the number of species within an ecosystem is closely tied to the health and size of the ecosystem itself [2].

However it is defined, biodiversity is the stuff of life. However far we may be removed from "wild" biodiversity in our daily lives, it remains the source of our food and most of our medicines. In addition, 15 percent of our energy is derived from burning plant materials. Even in the United States, wild species contribute around 4.5 percent of GDP3.

Some of our uses are direct. Billions of people still harvest wild or "bush" food around the world. Between a fifth and a half of all food consumed by the poor in the developing world is gathered rather than cultivated, while at global level we obtain 16 percent of our animal protein from sea fish caught in the wild. The World Health Organization (WHO) estimates that more than 60 percent of the world's population relies on traditional plant medicines for day-to-day primary health care [4], and 3 000 plant species are used in birth control alone [5]. [Add]

In the Uxpanapa region in Mexico peasant farmers use 435 wild plants and animals, eating 229 of them [6]. One Thai village was found to eat 295 different local plants and use 119 in medicines. Europe's prime treatment for prostate disorders comes directly from the bark of the African cherry tree, now severely depleted in its homeland in the Central African highlands.

But biodiversity's role extends far beyond these direct uses. We may today only eat a small proportion of the 70 000 plants known to have edible parts [7], but most food crops constantly require an infusion of "wild" genes to maintain their resistance to ever-evolving pests. These raids on nature's "genetic library" enable increases in crop productivity of about 1 percent a year, worth in excess of a billion dollars [8]. [Add]

Approximately 118 out of the top 150 prescription drugs sold in the United States are laboratory versions of chemicals found by "bioprospectors" in the wild -- mostly synthesized from plants but also from fungi, bacteria and extracts from vertebrate animals such as snakes [9]. Aspirin, for instance, derives from an acid first taken from the bark of willow trees. The promising anti-cancer drug taxol was first extracted from the wild Pacific yew tree. Globally, it has been estimated that the pharmaceuticals industry gains US$32 billion in profits a year from products derived from traditional remedies [10]. [Add]


[centers of diversity]


[genetic origins]

The emerging science of biotechnology offers new potential for using the world's genetic resources, but it is an area of some controversy, yet to be fully developed. Moreover, many of these resources are under threat from human activity. Species are being lost at a rate probably unprecedented outside times of mass extinction millions of years ago. One estimate puts the loss at 27 000 species a year [11]. The United Nations Environment Programme's Global Biodiversity Assessment estimates current extinction rates at 50 to 100 times "normal", and anticipates a tenfold or even 100-fold increase over the next quarter century, when between 2 and 25 percent of species could be lost [12]. [Add]

The primary cause of this loss is not hunting or overexploitation, though these play a part, but loss of natural habitat. Habitat loss is generally greatest where population density is highest. A study of biodiversity data from 102 countries found that in the most densely populated 51 countries (averaging 168 people per square kilometer), 5.1 percent of bird species and 3.7 percent of plant species were threatened. In the 51 less densely populated countries (averaging 22 people per square kilometer), the proportions of threatened species were only half as high at 2.7 percent and 1.8 percent respectively [13]. [Add]


Looked at on a geological timescale, the planet's biodiversity has always been faced with threats of one form or another. Mass extinctions have a history almost as long as biodiversity. There are five known cataclysmic extinctions in the Earth's history. The biggest, at the end of the Permian era 250 million years ago, eliminated between 75 and 95 percent of all species, while the best known, 65 million years ago, saw off the dinosaurs and much else. The extinctions appear to have been caused by massive climatic disruptions, some at least arising from meteor impacts.

Extinction, moreover, is an essential engine for evolutionary progress. Even mass extinctions, by killing large numbers of creatures, open up ecological "niches" to which surviving organisms swiftly adapt. Thus the demise of dinosaurs allowed the rapid evolution, within 10 million years, of bats, whales, horses and numerous other species of mammals and birds. Nonetheless, whatever such benefits to life on Earth may be in the long term, our own immediate future on the planet is jeopardized by the current human-induced mass extinctions.


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