Biodiversity - biology.

Biodiversity - biology. I INTRODUCTION Biodiversity or Biological Diversity, sum of all the different species of animals, plants, fungi, and microbial organisms living on Earth and the variety of habitats in which they live. Scientists estimate that upwards of 10 million--and some suggest more than 100 million--different species inhabit the Earth. Each species is adapted to its unique niche in the environment, from the peaks of mountains to the depths of deep-sea hydrothermal vents, and from polar ice caps to tropical rain forests. Biodiversity underlies everything from food production to medical research. Humans the world over use at least 40,000 species of plants and animals on a daily basis. Many people around the world still depend on wild species for some or all of their food, shelter, and clothing. All of our domesticated plants and animals came from wildliving ancestral species. Close to 40 percent of the pharmaceuticals used in the United States are either based on or synthesized from natural compounds found in plants, animals, or microorganisms. The array of living organisms found in a particular environment together with the physical and environmental factors that affect them is called an ecosystem. Healthy ecosystems are vital to life: They regulate many of the chemical and climatic systems that make available clean air and water and plentiful oxygen. Forests, for example, regulate the amount of carbon dioxide in the air, produce oxygen as a byproduct of photosynthesis (the process by which plants convert energy from sunlight into carbohydrate energy), and control rainfall and soil erosion. Ecosystems, in turn, depend on the continued health and vitality of the individual organisms that compose them. Removing just one species from an ecosystem can prevent the ecosystem from operating optimally. Perhaps the greatest value of biodiversity is yet unknown. Scientists have discovered and named only 1.75 million species--less than 20 percent of those estimated to exist. And of those identified, only a fraction have been examined for potential medicinal, agricultural, or industrial value. Much of the Earth's great biodiversity is rapidly disappearing, even before we know what is missing. Most biologists agree that life on Earth is now faced with the most severe extinction episode since the event that drove the dinosaurs to extinction 65 million years ago. Species of plants, animals, fungi, and microscopic organisms such as bacteria are being lost at alarming rates--so many, in fact, that biologists estimate that three species go extinct every hour. Scientists around the world are cataloging and studying global biodiversity in hopes that they might better understand it, or at least slow the rate of loss. II INTERCONNECTEDNESS OF THE LIVING WORLD Everywhere there is life, there is more than one distinct type of organism. Even a drop of seawater offers a multitude of different microscopic plants, animals, and less complex life forms. The rich diversity of the living world is connected in two distinct ways. First, different types of organisms live side by side in complex ecological networks of interdependency, each relying on the others that share its habitat for nutrients and energy. Second, all life on Earth is connected in an evolutionary tree of life. At the bottom of the tree is the common ancestor from which all living things descended--a single-celled microbe that lived more than 3.5 billion years ago--and in its uppermost branches are gorillas, chimpanzees, orangutans, and our own species, Homo sapiens. A Ecological Diversity Ecological diversity is the intricate network of different species present in local ecosystems and the dynamic interplay between them. An ecosystem consists of organisms from many different species living together in a region that are connected by the flow of energy, nutrients, and matter that occurs as the organisms of different species interact with one another. The ultimate source of energy in nearly all ecosystems is the Sun. The Sun's radiant energy is converted to chemical energy by plants. This energy flows through the systems when animals eat the plants and then are eaten, in turn, by other animals. Fungi derive energy by decomposing organisms, releasing nutrients back into the soil as they do so. An ecosystem, then, is a collection of living components--microbes, plants, animals, and fungi--and nonliving components--climate and chemicals--that are connected by energy flow. Removing just one species from an ecosystem damages the flow of energy of that system. For instance, in the late 19th and early 20th centuries, sea otters were hunted to near extinction in many kelp forests off the coast of the Pacific Northwest of the United States and western Canada, causing the entire ecosystem to suffer. Otters eat sea urchins, small, spiny organisms that share their habitat. When the otters disappeared, the sea urchin population exploded and started to destroy the vast beds of kelp. Without the kelp, other species that lived in the ecosystem, including many species of fish and snails and other invertebrates, began to decline in number. Efforts to restore sea otter populations brought the kelp communities back to near normal in the late 20th century. Measuring ecological diversity is difficult because each of the Earth's ecosystems merges into the ecosystems around it. A lake, for example, might have a distinct shoreline, but the plants fringing its edges are quite different from the aquatic plants in the middle of the lake or the trees and shrubs surrounding the lake. Beavers may live in the lake, but they construct dams from trees that grow in adjacent ecosystems. Nutrients flow into the lake via streams and rivers beyond the lake's ecosystem. B Evolutionary Diversity Every species on Earth is related to every other species in a pattern every bit as complex as the patterns of energy flow within an ecosystem. In evolutionary diversity, the connection is not energy flow, but rather genetic connections that unite species. The more closely related any two species are, the more genetic information they will share, and the more similar they will appear. An ever-widening circle of evolutionary relatedness embraces every species on Earth. An organism's closest relatives are members of its own species--that is, other organisms with which it has the potential to mate and produce offspring. Members of a species share genes, the bits of biochemical information that determine, in part, how the animals look, behave, and live. One eastern gray squirrel, for example, shares the vast majority of its genes with other eastern gray squirrels, whether they live in the same area or are separated by thousands of miles. Members of a species also share complex mating behaviors that enable them to recognize each other as potential mates. When a female eastern gray squirrel is ready to mate, she exudes a scent that attracts male eastern gray squirrels. Mating and sharing a common supply of genes unite a species. For virtually every species there is a similar and closely related species in an adjacent habitat. West of the Rocky Mountains, one finds western gray squirrels instead of eastern gray squirrels. Although western gray squirrels are more similar to than different from their eastern counterparts, these animals do not share a common mating behavior with eastern gray squirrels. Even when brought into close proximity, eastern and western gray squirrels do not mate, and so constitute two distinct species. Each species has other, more remotely related species, which share a more general set of characteristics. Gray squirrels, chipmunks, marmots, and prairie dogs all belong to the squirrel family because they share a number of features, such as tooth number and shape, and details of skull and muscle anatomy. All of these animals are rodents, a large group of more distantly related animals who share similar chisel-like incisor teeth that grow continuously. All rodents are related to a broader group, mammals. Mammals have hair, raise their young on milk, and have three bones in the middle ear. All mammals, in turn, are more distantly related to other animals with backbones, or vertebrates. All these organisms are animals but share a common cell structure with plants, fungi, and some microbes. Finally, all living organisms share a common molecule, ribonucleic acid (RNA), and most also have deoxyribonucleic acid (DNA). These molecules direct the production of proteins--molecules responsible for the structure and function of virtually all living cells. This is the evolutionary chain of life. All species are descended from a single common ancestor. From that ancient single-celled microbe, all inherited RNA. As time goes by, species diverge and develop their own peculiar attributes, thus making their own contribution to biodiversity (see Evolution). III GLOBAL BIODIVERSITY CRISIS Most biologists accept the estimate of American evolutionary biologist Edward O. Wilson that the Earth is losing approximately 27,000 species per year. This estimate is based primarily on the rate of disappearance of ecosystems, especially tropical forests and grasslands, and our knowledge of the species that live in such systems. We can measure the rate of loss of tropical rain forests, for example, by analyzing satellite photographs of continents from different periods that show rates and amounts of habitat destruction--and from these measurements calculate the approximate number of species being lost each year. This extraordinary rate of extinction has occurred only five times before in the history of complex life on Earth. Mass extinctions of the geological past were caused by catastrophic physical disasters, such as climate changes or meteorite impacts, which destroyed and disrupted ecosystems around the globe. In the fifth mass extinction, which occurred more than 65 million years ago, the Earth was shrouded in a cloud of atmospheric dust--the result of meteorite impact or widespread volcanic activity. The resulting environmental disruption caused the demise of 76 percent of all species alive at the time, including the dinosaurs. Today's sixth extinction is likewise primarily caused by ecosystem disturbance--but this time the destroying force is not the physical environment, but rather humankind. The human transformation of the Earth's surface threatens to be every bit as destructive as any of the past cataclysmic physical disasters. IV HUMAN IMPACT The underlying cause of biodiversity loss is the explosion in human population, now at 6 billion, but expected to double again by the year 2050. The human population already consumes nearly half of all the food, crops, medicines, and other useful items produced by the Earth's organisms, and more than 1 billion people on Earth lack adequate supplies of fresh water (see World Food Supply). But the problem is not sheer numbers of people alone: The unequal distribution and consumption of resources and other forms of wealth on the planet must also be considered. According to some estimates, the average middle-class American consumes an amazing 30 times what a person living in a developing nation consumes. Thus the impact of the 270 million American people must be multiplied by 30 to derive an accurate comparative estimate of the impact such industrialized nations have on the world's ecosystems. The single greatest threat to global biodiversity is the human destruction of natural habitats. Since the invention of agriculture about 10,000 years ago, the human population has increased from approximately 5 million to a full 6 billion people. During that time, but especially in the past several centuries, humans have radically transformed the face of planet Earth. The conversion of forests, grasslands, and wetlands for agricultural purposes, coupled with the multiplication and growth of urban centers and the building of dams and canals, highways, and railways, has physically altered ecosystems to the point that extinction of species has reached its current alarming pace. In addition, overexploitation of the world's natural resources, such as fisheries and forests, has greatly outstripped the rate at which these systems can recover. For example, 12 of the 13 largest oceanic fisheries are severely depleted. Modern fishing techniques, such as using huge fishing nets and bottom vacuuming techniques, remove everything in their paths--including tons of fish and invertebrates of no commercial use. These victims, as well as porpoises and seals that are also hauled in as accidental catches, are permanently removed from their populations, significantly altering the ecosystems in which they live. As human populations have grown, people have spread out to the four corners of the Earth. In the process, whether on purpose or by accident, they have introduced nonnative species that have created ecological nightmares, disrupting local ecosystems and, in many cases, directly driving native species extinct. For example, the brown tree snake was introduced to the island of Guam, probably as a stowaway on visiting military cargo ships after World War II (1939-1945). The snake devastated the native bird population, driving over half a dozen native species of birds to extinction--simply because the native birds had not been exposed to this type of predator and did not recognize the danger posed by these snakes. V PRESERVING BIODIVERSITY As the scope and significance of biodiversity loss become better understood, positive steps to stem the tide of the sixth extinction have been proposed and, to some extent, adopted. Several nations have enacted laws protecting endangered wildlife. An international treaty known as the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) went into effect in 1975 to outlaw the trade of endangered animals and animal parts. In the United States, the Endangered Species Act (ESA) was enacted in 1973 to protect endangered or threatened species and their habitats. The Convention on Biological Diversity, held in Rio de Janeiro, Brazil, in 1992 and ratified by more than 160 countries, obligates governments to take action to protect plant and animal species. In the last three decades, focus has shifted away from the preservation of individual species to the protection of large tracts of habitats linked by corridors that enable animals to move between the habitats. Thus the movement to save, for example, the spotted owl of the Pacific Northwest, has become an effort to protect vast tracts of old-growth timber (see National Parks and Preserves). Promising as these approaches may be, conservation efforts will never succeed in the long run if the local economic needs of people living in and near threatened ecosystems are not taken into account. This is particularly true in developing countries, where much of the world's remaining undisturbed land is located. At the end of the 20th century, international organizations such as the World Bank and the World Wildlife Fund launched a movement for all countries in the developing world to set aside 10 percent of their forests in protected areas. But many communities living near these protected areas have relied on the rain forest for food and firewood for thousands of years. Left with few economic alternatives, these communities may be left without enough food to eat. To address this problem, the burgeoning field of conservation biology emphasizes interaction with the people directly impacted by conservation measures. Conservation biologists encourage such people to develop sustainable economic alternatives to destructive harvesting and land use. One alternative is harvesting and selling renewable rain forest products, such as vegetable ivory seeds from palms, known as tagua nuts, and brazil nuts. Where protection measures permit, rain forest communities may undertake sustainable rain forest logging operations, in which carefully selected trees are extracted in a way that has minimal impact on the forest ecosystem. Still other communities are exploring medicinal plants for drug development as ways to strengthen and diversify their economies. Conservation biologists also work with established industries to develop practices that ensure the health and the sustainability of the resources on which they depend. For example, conservation biologists work with fishers to determine how many fish the fishers can harvest without damaging the population and the ecosystem as a whole. The same principles are applied to the harvesting of trees, plants, animals, and other natural resources. Preserving biodiversity also takes place at the molecular level in the conservation of genetic diversity. All around the world efforts are being made to collect and preserve endangered organisms' DNA, the molecule that contains their genes. These collections, or gene banks, may consist of frozen samples of blood or tissue, or in some cases, they may consist of live organisms. Biologists use gene banks to broaden the gene pool of a species, increasing the likelihood that it will adapt to meet the environmental challenges that confront it. Many zoos, aquariums, and botanical gardens work together to carefully maintain the genetic diversity in captive populations of endangered animals and plants, such as the giant panda, the orangutan, or the rosy periwinkle. Captive animals are bred with wild populations, or occasionally released in hopes that they will breed freely with members of the wild population, thus increasing its genetic diversity. These gene banks are also an essential resource to replenish the genetic diversity of crops, enabling plant breeders and bioengineers to strengthen their stocks against disease and changing climate conditions. Contributed By: Niles Eldredge Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.