Biodiversity

I

Introduction

Biodiversity or Biological Diversity, sum of all the different species of animals, plants, fungi, and microbes 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 wild-living 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 wave of extinction 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.

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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).

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