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Bioscience Topics

Biodiversity

  • Photo: Public Library of Science (PLoS)

Biodiversity is the 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 more than 10 million different species inhabit Earth.

Biodiversity underlies everything from food production to medical research. Humans 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 our domesticated plants and animals came from wildliving ancestral species. In addition, almost 40 percent of the pharmaceuticals used in the United States either are based on or are synthesized from natural compounds found in plants, animals or microorganisms.

The array of living organisms found in a particular environment combined with the physical and environmental factors that affect them is an ecosystem. Healthy ecosystems are vital to life; they regulate many of the chemical and climatic systems that make available clean air, clean water and plentiful oxygen. Forests, for example, regulate the amount of carbon dioxide in the air, produce oxygen as a byproduct of photosynthesis 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 — fewer than 20 percent of those estimated to exist. Of those identified, only a fraction has been examined for potential medicinal, agricultural or industrial value. Much of Earth’s great biodiversity rapidly is disappearing, even before we know what is missing. Most biologists agree that life on Earth now is 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. Because of this, scientists around the world are focusing their research on cataloging and studying global biodiversity in an effort to better understand it and slow the rate of loss. As a result, the majority of current biodiversity research concentrates on preserving biodiversity and assessing environmental quality and change.

Benefits of Biodiversity

Biodiversity plays an important role in the way ecosystems function and in the services they provide. The following is a list of some of the benefits, or services, of biodiversity:

Types of Biodiversity

Biodiversity includes three main types: diversity within species (genetic diversity), between species (species diversity) and between ecosystems (ecosystem diversity).

Genetic Diversity

Every species on Earth is related to every other species through genetic connections. The more closely related any two species are, the more genetic information they will share, and the more similar they will appear. An organism’s closest relatives are members of its own species, or 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.

For virtually every species there is a similar and closely related species in an adjacent habitat. Western, instead of eastern, gray squirrels are found west of the Rocky Mountains. 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 — so they constitute two distinct species.

Each species also has other, more remotely related species with which it shares 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).

While all species have descended from a single, common ancestor, species diverge and develop their own peculiar attributes with time, thus making their own contribution to biodiversity.

Species Diversity

Species diversity is the variety of species within a habitat or a region. Species are the basic units of biological classification and thus the normal measure of biological diversity. Species richness is the term that describes the number of different species in a given area. The world total is estimated at five to 10 million species, though only 1.75 million have been named scientifically so far.

Some habitats, such as rainforests and coral reefs, have many species. For example, tropical North and South America has about 85,000 flowering plant species, tropical and subtropical Asia has more than 50,000 and tropical and subtropical Africa has about 35,000. By contrast, all of Europe has 11,300 vascular plants. Yet other areas, such as salt flats or a polluted stream, have fewer species. Species are grouped together into families according to shared characteristics.

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 and their connections through the flow of energy, nutrients and matter. Those connections occur as the organisms of different species interact with one another. The ultimate source of energy in almost every ecosystem is the sun. The sun’s radiant energy is converted to chemical energy by plants. That 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, which releases nutrients back into the soil. Thus, an ecosystem is a collection of living components (microbes, plants, animals and fungi) and nonliving components (climate and chemicals) that are connected by energy flow. Measuring ecological diversity is difficult because each of Earth’s ecosystems merges into the ecosystems around it.

Related Issues

Biodiversity Agreements

Concern about environmental destruction led to the signing of several national and international agreements.

In 1972, the United Nations Conference on the Human Environment resolved to establish the United Nations Environment Programme. Governments signed a number of regional and international agreements to tackle specific issues, such as protecting wetlands and regulating the international trade in endangered species. Those agreements, along with controls on toxic chemicals and pollution, have helped slow the tide of destruction — but they have not reversed it.

An international treaty known as the Convention on International Trade in Endangered Species of Wild Fauna and Flora went into effect in 1975 to outlaw the trade of endangered animals and animal parts. In the United States, the Endangered Species Act was enacted in 1973 to protect endangered or threatened species and their habitats. In 1987, the World Commission on Environment and Development (the Brundtland Commission) concluded that economic development must become less ecologically destructive. Then, in 1992 at the United Nations Conference on Environment and Development in Rio de Janeiro, Brazil, a set of binding agreements was signed at the Convention on Biological Diversity. It was the first global agreement on the conservation and sustainable use of biological diversity. More than 150 governments signed the document at that conference, and since then more than 187 countries have ratified the agreement. The convention has three main goals: the conservation of biodiversity, sustainable use of the components of biodiversity and sharing the benefits arising from commercial and other uses of genetic resources in a fair and equitable way.

Ethical and Social Considerations

Human Impact

Most biologists accept the estimate of American evolutionary biologist Edward O. Wilson that Earth is losing approximately 27,000 species per year. His estimate is based primarily on the rate of disappearance of ecosystems, especially tropical forests and grasslands, and on our knowledge of the species that live in such systems. That extraordinary rate of extinction has occurred only five times before in Earth’s history. Mass extinctions of the geological past were caused by catastrophic physical disasters, such as climate changes or meteorite impacts, which destroyed and disrupted global ecosystems. Today’s sixth extinction also primarily is being caused by ecosystem disturbance — but this time the destroying force is not the physical environment, but rather humankind. The human transformation of Earth’s surface threatens to be as destructive as any of the past cataclysmic physical disasters.

The underlying cause of biodiversity loss is the explosion in human population, which now stands at six billion and which is expected to double again by 2050. The human population already consumes almost half of all the food, crops, medicines and other useful items produced by Earth’s organisms, and more than one billion people on the planet lack adequate supplies of fresh water. 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 also must 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 almost 300 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 five million to the current six billion. During that time, but especially in the past several centuries, humans have radically transformed 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, physically has altered ecosystems to the current alarming pace of species extinction.

Conservation

As the scope and significance of biodiversity loss have become better understood, positive steps to stem the tide of the sixth mass extinction have been proposed and, to some extent, adopted. Several nations have enacted laws protecting endangered wildlife.

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.

But as promising as these approaches are, conservation efforts never will succeed in the long run if the local economic needs of people living in and near threatened ecosystems are not taken into account. That 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 those protected areas have relied on the rain forest for food and firewood for thousands of years. Left with few economic alternatives, those communities could be left without enough food to eat.

To address the problem, conservation biology emphasizes interaction with the people directly impacted by conservation measures. These 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.

Efforts are being made around the world to collect and preserve endangered organisms’ DNA. These collections, or gene banks, might consist of frozen samples of blood or tissue or, in some cases, might consist of actual live organisms. Biologists use gene banks to broaden the gene pool of a species, increasing the likelihood 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 and the rosy periwinkle. Captive animals are bred with wild populations or occasionally released in hopes they will breed freely with members of the wild population, thus increasing their genetic diversity. These gene banks also are 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.