These losses will not just affect poorly known groups. As many as 50,000 species of the world's total of 250,000 species of plants, 4,000 of the world's 20,000 species of butterflies, and nearly 2,000 of the world's 9,000 species of birds could be lost during this short period of time. Considering that our species has been in existence for only 500,000 years of the world's 4.5 billion year history, and that our ancestors developed agriculture only about 10,000 years ago, this is an astonishing rate of loss.

Massive waves of extinction are nothing new to life on earth. At five times in the past, the earth has lost much of its biodiversity. Mass extinctions eliminated 50% of animal families in the Ordovician about 500 million years ago, 30% of animal families at the end of the Devonian 345 million years ago (including agnathan and placoderm fishes), 50% of animal families in the great Permian extinction 250 million years ago (including over 95% of marine species, many trees and amphibians, most bryozoans and brachiopods, and all trilobites), 35% of animal families at the end of the Triassic 180 million years ago (including many reptiles and marine mollusks), and 40% of animal families at the end of the Cretaceous 65 million years ago (including all dinosaurs and many marine forams and mollusks).

WHAT'S SO BAD ABOUT LOSING BIODIVERSITY?

Direct Economic Value

Many species have direct value, as sources of food, medicine, clothing, biomass (for energy and other purposes), and shelter. Most of the world's food, for example, is derived from a small number of plants that were originally domesticated from wild plants in tropical and semi-arid regions. In the future, wild strains of these species may be needed for their genetic diversity if we are to improve yields, or find a way to breed resistance to new pests. In addition, novel species of plants and animals may be found that are appropriate for domestication, or as biological control agents for pest species.

About 40% of the prescription and non-prescription drugs used today have active ingredients extracted from plants or animals. Aspirin, the world's most widely used drug, was first extracted from the leaves of the tropical willow, Salix alba. The rosy periwinkle, Catharanthus roseus, from Madagascar has yielded a potent drug for combating leukemia (figure 27e.2).

Only in the last few decades have biologists perfected the techniques that make possible the transfer of genes from one kind of organism to another. We are just beginning to be able to use genes obtained from other species to our advantage, as explored at length in chapter 18. Pesticide and insect resistance in crops gained from genes isolated from bacteria is but one example. So-called gene prospecting of the genomes of plants and animals for useful genes has only begun. We have been able to examine only a minute proportion of the existing kinds of organisms so far, to see whether any of their genes have useful properties. By conserving biodiversity we maintain the option of finding useful benefit in the future.

Indirect Economic Value

Diverse biological communities are of vital importance to healthy ecosystems, in maintaining the chemical quality of natural water, in buffering ecosystems against floods and drought, in preserving soils and preventing loss of minerals and nutrients, in moderating local and regional climate, in absorbing pollution, and in promoting the breakdown of organic wastes and the cycling of minerals. By destroying biodiversity, we are creating conditions of instability and lessened productivity and promoting desertification, waterlogging, mineralization, and many other undesirable outcomes throughout the world.

Given the major role played by many species in maintaining healthy ecosystems, it is alarming how little we know about the details of how ecosystems and communities function. It is impossible to predict all the consequences of removing a species, or to be sure that some of them will not be catastrophic. Imagine taking a part list for an airliner, and randomly changing a digit in one of the part numbers: you might change a cushion to a roll of toilet paper–but you might as easily change a key bolt holding up the wing to a pencil. The point is, you shouldn't gamble if you cannot afford to lose, and in removing biodiversity we are gambling with the future of ecosystems upon which we depend, and we don't even know the odds.

Ethical and Aesthetic Value

Many people believe that preserving biodiversity is an ethical issue, feeling that every species is of value in its own right, even if humans are not able to exploit or benefit from it. It is clear that humans have the power to exploit and destroy other species (see table 27e.2), but it is not as ethically clear that they have the right to do so. Sometimes dismissed as "tree huggers," people who hold these views are unable to advance economic arguments–but considered ethically, their point of view has considerable power.

  TABLE 27e.2 ENDANGERED SPECIES OF MAMMALS*

MARSUPIALS Leadbeater's Possum Mahogany Glider Western Barred Bandicoot Northern Marsupial Mole Black-spotted Cuscus Northern Bettong Goodfellow's Tree-kangaroo Bridled Nailtail Wallaby Prosperine Rock-wallaby Long-footed Potoroo Broom's Pygmy-possum INSECTIVORA Cuban Solenodon Haitian Solenodon Web-footed Tenrec Nimba Otter-shrew Giant Golden Mole Dinagat Moonrat Sado Mole Chinese Shrew-mole CHIROPTERA Golden-capped Fruit Bat Ryukyu Flying-fox Marianas Flying-fox Kitti's Hog-nosed Bat Big Long-nosed Bat Jamaican Flower Bat Grey Bat Indiana Bat PERISSODACTYLA Grevy's Zebra Onager Mountain Zebra Hartmann's Mountain Zebra Cape Mountain Zebra Mountain Tapir Great Indian Rhinoceros

PRIMATES Bornean Treeshrew White-collared Lemur Ruffed Lemur Aye-aye Buffy-headed Marmoset Cotton-top Tamarin Night Monkey Woolly Spider Monkey Black Saki White-collared Mangabey Golden Monkey Japanese Macaque Lion-tailed Macaque River Red Colobus Temminck's Red Colobus Black Gibbon Gorilla Western Lowland Gorilla Chimpanzee CARNIVORA Northwest African Cheetah Texas Jaguarundi Texas Ocelot North African Serval Spanish Lynx Indian Lion North Chinese Leopard Sri Lankan Leopard Tiger Snow Leopard Marine Otter Indonesian Mountain Weasel Steller's Sealion Grey Seal Hawaiian Monk Seal Saimaa Ringed Seal Giant Panda Lesser Panda Otter-civet

CETACEA Bowhead Whale Northern Right Whale Sei Whale Blue Whale Fin Whale Gray Whale Finless Porpoise Ganges River Dolphin Indus River Dolphin ARTIODACTYLA Eritrean Warthog Javan Warty Pig Chacoan Peccary Wild Bactrian Camel Philippine Spotted Deer Upland Barasingha Corsican Red Deer Key Deer Peary Caribou Sonoran Pronghorn Addax Swayne's Hartebeest Tora Hartebeest Golden Takin Mishmi Takin Nubian Ibex Red Serow Aders' Duiker Cuvier's Gazelle Slender-horned Gazelle Nilgiri Tahr Arabian Tahr Western Klipspringer Arabian Oryx PROBOSCIDEA Indian Elephant African Elephant

RODENTIA Nelson's Antelope Squirrel Mexican Prairie Dog Woolly Flying Squirrel Vancouver Island Marmot Idaho Ground Squirrel Michoacan Pocket Gopher San Quintin Kangaroo Rat Dinagat Island Cloud Rat Greater Stick-nest Rat Choctawhatchee Beach Mouse Hastings River Mouse Red-bellied Harvest Mouse Poncelet's Giant Rat Japanese Dormouse Pacarana XENARTHRA Maned Sloth Pink Fairy Armadillo Giant Armadillo LAGOMORPHA Koslov's Pika Riverine Rabbit Hispid Hare Tehuantepec Jackrabbit Amami Rabbit Volcano Rabbit Dice's Cottontail Tres Marias Rabbit Lower Keys Marsh Rabbit MACROSCELIDEA Golden-rumped Elephant-shrew Black-and-rufous Elephant-shrew

Almost no one would deny the aesthetic value of biodiversity, of a beautiful flower or noble elephant, but how do we place a value on beauty? Perhaps the best we can do is to appreciate the deep sense of lack we feel at its permanent loss (see figure 27e.3).