Biodiversity

Measuring Biodiversity

Biological diversity can be measured in various ways, each of which captures some of the overall meaning of biological diversity. The most common method of measuring biological diversity is simply to count the number of species occurring in one particular place, such as a forest or a grassland. Since it is not possible to count every species of plant, insect, fungus, and microorganism, the usual procedure is to count certain types of organisms, such as birds, butterflies, all flowering plants, or just tree species. This type of local diversity of species is usually referred to as species richness or alpha diversity. A tropical rain forest might contain three hundred or more tree species in a square of forest measuring 400 meters on a side, whereas a temperate forest of equal area might contain only forty tree species. Biological diversity can also be measured in larger areas. For example the country of Colombia has more than fifty thousand species of higher plants, in contrast to sixteen hundred species for the United Kingdom and nearly sixteen thousand for Australia. This type of regional or large-scale diversity is referred to as gamma diversity.

Another way to measure biodiversity is to consider the number or percentage of a region's species that are endemic to that region. For example, of the United Kingdom's fifteen-hundred native plant species, only sixteen species or 1 percent are endemic. The overwhelming majority of the United Kingdom's plants can be found in other neighboring countries, such as Ireland, France, and Germany. In contrast, 14,074 of the 15,000 plant species of Australia are endemic and found in no other country.

Individual species can also be compared for their evolutionary uniqueness. Species that are not closely related to other species are generally considered to have greater value to overall biological diversity than species that have many close relatives. For example, the maidenhair tree, Ginkgo biloba, is the only species in its genus, and Ginkgo is the only genus in the gymnosperm family Ginkgoaceae. In contrast, the common dandelion, Taraxacum officinale, has many related species in the same genus and is a member of a large family, the Asteraceae, with twenty-five thousand species and eleven hundred genera. Using this approach, a species that was the only member of its genus and family would have greater biodiversity value than a species that had many relatives in the same genus and belonged to a family with many genera. In contrast, a few biologists would argue that a species in a large genus has greater value because this species has the greater potential to undergo further evolution than a species with no close relatives that may be an evolutionary dead end.

Extent of Diversity

There are around 250,000 living species of higher plants in the world today. Of these species, the overwhelming majority are flowering plants. Flowering plants are grouped into more than three hundred families, including such large and economically important families as the Poaceae, which contains the grasses and the cereal crops we depend on for food and animal fodder; the Fabaceae, which includes the beans and peas we need for protein in our diet; and the Rosaceae, which is important for fruit trees and ornamentals such as apples, pears, and roses.

Higher plants exhibit a great diversity of growth forms, leaf shapes and sizes, flower and fruit types, seed types, and particular adaptations for growing in different environments. Growth forms include trees, shrubs, annual herbs, perennial herbs, climbers, and aquatic plants.

Plant diversity is not equally distributed across the world's surface. Only a relatively few land plants are adapted to salt water, and these are found rooted in shallow waters. So the large oceanic expanses of the world are devoid of higher plants. On the land surface, the greatest diversity of plants is found in the tropical lowland and montane rain forests of the Americas, central Africa, and Southeast Asia. In such forests there is a great diversity of plant species in the form of trees, shrub, herbs, and climbers. There is also an abundance of epiphytes, in particular orchids and bromeliads, that perch on the branches of the trees. Illustrating this tropical diversity, there are only around thirty tree species in all of northern Canada, in contrast to more than one thousand tree species in just the southern countries of Central America.

There is also great species diversity in the temperate regions of the world that have mild, wet winters and dry, hot summers, such as the Mediterranean basin, the California region, central Chile, the cape region of South Africa, and southwest Australia. In such areas, many plants have adaptations to drought, such as succulent cacti, which store water in their stems, and annual plants that grow, reproduce, and die in one growing season.

Certain regions of the world are known as hot spots of biodiversity because of their high concentrations of species overall, their high percentages of species that are endemic, and the high degree of threat that those species face. In addition to rain forest areas and localities with Mediterranean climates, many of these are islands, such as the Caribbean Islands, Madagascar and nearby islands, New Caledonia, New Zealand, Sri Lanka, and the islands between New Guinea and peninsular Malaysia. Biodiversity hot spots encompass the entire range of 44 percent of the world's plant species, 25 percent of the bird species, 30 percent of the mammal species, 38 percent of the reptile species, and 54 percent of the amphibian species on only 1.4 percent of Earth's total land surface. The premier hot spot is the tropical Andes, in which 45,000 plant species, 1,666 bird species, 414 mammal species, 479 reptile species, and 830 amphibian species occur in the tropical forests and high-altitude grasslands that occupy less than 0.25 percent of Earth's land surface. This approach can also be applied to individual countries. In the United States hot spots for endangered species occur in the Hawaiian Islands, the southern Appalachian Mountains, the arid Southwest, and the coastal areas of the lower forty-eight states, particularly California, Texas, and Florida.

Threats to Biological Diversity

Biological diversity is being lost today at all levels, including genetic variation, species, and biological communities. The most serious threat is the extinction of species, because once a species is lost, it can never be regained. The loss of genetic variation is occurring in two different ways: when populations of a species are eliminated and when populations become smaller in size. This loss of populations is seen most immediately in the local extinction of species. In a study of a conservation area in Massachusetts, one-third of the native plant species present one hundred years ago could no longer be found today. They were not replaced by other native species, but there was an increase in the number of nonnative species. This park is now poorer in total species, and many species still present have fewer populations. Many species that were formerly listed as common now have only a few individuals left.

Biological diversity is most severely threatened when entire biological communities are lost. In many tropical countries of the world, the tropical rain forests that are so rich in species have been largely destroyed. Examples of countries with devastated forests are Madagascar (87 percent lost), Rwanda (84 percent), Vietnam (83 percent), and the Philippines (94 percent). With the loss of these communities comes the extinction of plant and animal species, the loss of genetic variation within remaining species, and the loss of the ecosystem services provided by these communities, such as flood control, soil erosion protection, and the production of wood and food. Other habitats almost completely destroyed include tropical deciduous forests, of which more than 98 percent have been destroyed in Central America, and temperate grasslands, which are readily converted to agriculture and ranching. In the United States, only around 560 acres of the tallgrass prairie of Illinois and Indiana remains undisturbed, only about one ten-thousandth of the original area. Wetlands including swamps, bogs, floodplains, and vernal pools are similarly suffering devastation. As these habitats are damaged by human activity and converted to other uses, the species they contain decline in abundance and eventually become extinct. Habitats that are restricted in area and contain high concentrations of endemic species are particularly vulnerable, such as the rain forests of Hawaii and isolated mountain peaks in the southwestern United States.

In general, the rate of extinction for plants has been lower than that for animals. To date, there are recorded extinctions of around four hundred plant species, about 0.2 percent of the total in contrast with around 2.1 percent of mammals and 1.3 percent of birds already extinct. The lower percentages of plants that are extinct are related in part to our ability to protect small populations of plants in nature reserves. In contrast, many animals have a greater need to migrate and have often been extensively exploited. At the turn of the twenty-first century, around 9 percent of plant species are in danger of extinction, a figure only slightly lower than that for birds (11 percent) and mammals (11 percent). The extinction rates for certain groups of plants are much higher than this average value. For example, 32 percent of gymnosperms and 33 percent of palms are threatened with global extinction in the wild due to the limited distribution of many species' specialized habitat requirements and the intensive collection of plants for horticulture.

Factors Threatening Species

Species are threatened with extinction primarily because of habitat destruction. Species are also driven to extinction when their habitat is degraded to the point where they can no longer exist. This might happen when a grassland is heavily grazed by domestic animals, a forest is repeatedly logged, or uncontrolled fires burn shrub land. Fully 81 percent of the endangered species of the United States are threatened by habitat degradation and loss. Species are also lost from habitats fragmented by human activity, when habitats are broken up into smaller pieces by roads, fences, power lines, residential areas, and ranches. The remaining fragments may be so altered in micro-climate, and so much more vulnerable to other human activities, that many plant species are no longer able to survive.

The second most significant threat to species diversity is competition and predation from exotic invasive species, which is a threat for 57 percent of the endangered plant species of the United States. In many cases, exotic species of animals such as cattle, sheep, goats, rabbits, and pigs selectively remove certain native plant species. For example, pigs introduced in Hawaii have removed all wild individuals of numerous plant species. Invasive exotic plants have often overwhelmed natural communities and outcompeted the native species. For example, in bottomland communities of the southern United States, Japanese honeysuckle plants have replaced the rich wildflower communities, and in the rangeland of the western United States European grasses outcompete native grasses and wildflowers. As a result, native species decline at the expense of the introduced species.

Overharvesting of plants, often for food, medicinal purposes, or by horticulturists, threatens 10 percent of the endangered plant species of the United States. A notable example is ginseng, an herb used in Asian medicine, which has been so overharvested throughout its range that only a small number of plants remain. Many rare wildflowers, such as orchids, have been so severely overcollected by gardeners that they are in danger of extinction in the wild. Information on the location of the last remaining plants is often kept secret to prevent the theft of these individuals.

Pollution threatens 7 percent of the plant species of the United States. Water pollution can alter the water chemistry so severely that aquatic plants cannot grow. Increased inputs of nitrogen and phosphorus compounds into the water from sewage and agricultural fertilizers can result in algal blooms that shade out and kill native plants. In the land environment air pollution in the form of smog, acid rain, and nitrogen deposition can cause plants to slow down in growth or die. In some cases, this death may be related to the decline and death of the sensitive soil fungi (mycorrhizae) that have mutualistic relations with plants, providing water and mineral nutrients and receiving carbohydrates in return. And lastly, about 1 percent of plant species is threatened by disease and parasites. While this number may not seem very great, some of the most important woody plants in the forests of North America, such as chestnuts, elms, and dogwoods, are in severe decline due to introduced diseases.

What Can Be Done?

The most important way to protect plant biological diversity is to establish protected areas that include high concentrations of species, particularly those species in danger of extinction or in decline. These protected areas may be established by governments, conservation organizations, or private individuals. Management plans must be developed and implemented, and these protected areas must be monitored to ensure they are meeting their goals. Many management plans for protected areas include some forms of public education, because public support is often crucial for the success of a park.

Where it is not possible to maintain plant species in the wild due to ongoing threats, plants can often be grown in botanical gardens or kept as stored seed samples in seed banks. Networks of botanical gardens and seed banks are making a concerted effort to increase their holdings of endangered species and species of potential agricultural and economic importance. The goal of many botanical gardens is to increase knowledge of plants, to educate the public concerning plants, and to return plants eventually to their natural habitats.

The diversity of plant species provides us with the agriculture crops that are our food, many of the medicines that keep us healthy, wood that is needed in construction, fodder that feeds our domestic animals, ornamental plants that enrich our gardens and homes, and even the oxygen that we breathe. People could not live without the diversity of plants, and many plant species will live in the wild only if we take care of them.

SEE ALSO ASTERACEAE; BOTANICAL GARDENS AND ARBORETA; ENDANGERED SPECIES; GINKGO; HUMAN IMPACTS; INVASIVE SPECIES; RAIN FORESTS; SEED PRESERVATION; WETLANDS.

Richard B. Primack

Bibliography

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