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Biodiversity is the term applied to the variety of organisms that occupy a given region. This includes all organisms that live in the region, from microscopic protists to large mammals. The region can be a political unit such as a country, a geographic feature such as a mountain range, or the entire world.

The term "biodiversity" is a combination of two words, "biology" and "diversity." The union of these two words is fairly recent, being inspired by the growing realization that the number of species in the world is seriously declining. Used in this context, it has taken on a greater meaning than just the variety of species, having grown to include three closely related levels: genetic diversity, taxonomic diversity, and ecosystem diversity. Biodiversity is created by complex physical and biological environments (ecosystem diversity) that allow organisms to evolve specializations, and genetic barriers (genetic diversity) that allow them to speciate (taxonomic diversity).

The importance of biodiversity has been recognized by people of many cultures and backgrounds who understand the multitude of functions it serves for humans, from providing food to filtering waste. Threats to biodiversity include direct killing of species by hunting, contaminating the environment with toxins, and habitat loss . The loss of biodiversity through extinction must ultimately be overcome by drastic changes of human behavior. Otherwise, humans will destroy the very environment that supports them.

Levels of Biodiversity

Genetic Diversity.

The first level of biodiversity, genetic diversity, is the level at which we can most clearly observe the evolution of diversity. Genetic diversity includes the many kinds of genes that are available for given members of a species, such as a family, a population, or the entire species. This variety of genes allows the species to have many kinds of heritable traits that allow it to survive through changing environments.

For example, in a particularly cold winter, many individuals of a species may die from lack of insulation, but if the population as a whole has genetic diversity for a trait such as fat storage, then at least some members of the population will survive and the species will not become extinct. The next year, more offspring will have the valuable trait and the species will evolve to tolerate the cooler environment.

Species with little genetic diversity, such as farm hybrids (special breeds of crops or livestock that are all closely related), have limited ability to adapt to changing weather conditions or insect pests. Species lacking genetic diversity cannot adapt to a changing environment and may become extinct without help like the careful maintenance that goes into farm crops and animals.

At the opposite extreme, characteristics of genetically healthy populations are a high population size that includes many individuals that are unrelated to each other. Often, the existence of disjunct populations, those separated by some geographic barrier that only occasionally lets migrants through, ensures that there are always unrelated individuals.

Taxonomic Diversity.

The next level of biodiversity, taxonomic diversity, refers to the variety of individuals at a given hierarchical level in the scientific naming system. This could be the number of different species, genera, families, or kingdoms. For example, a cornfield may have hundreds of birds living in it, but they may represent only three species that are all in one family, meaning that there is low taxonomic diversity. A similar-sized area in a nearby forest may also have hundreds of birds living in it, but these birds may be from twenty different species that belong to eight families and three orders, representing a higher level of diversity. Around the world, crops and livestock typically consist of only one species. These monocultures support a low level of biodiversity.

Ecological Diversity.

Ecological diversity is the variety of habitat types that are available in a given area. These habitats can have different physical characteristics such as temperature and soil type, as well as different organisms inhabiting them. When the habitat with all its organisms and their complex interactions are considered together, it is termed an ecosystem. Ecosystem diversity is typically the level that is discussed in relation to biodiversity.

Beyond ecosystems is a division termed "landscape" that consists of all of the ecosystems in a defined region, such as a drainage basin. Biomes are groups of similar landscapes, such as all the mountain ranges in the world. Finally, the largest division is the biosphere, which refers to all life on Earth.

A mountain range is as an example of ecological diversity that illustrates the evolution and patterns of biodiversity. Because of the latitude of the range selected as an example here, the temperatures are warm and plants grow throughout the year. The topography and weather patterns typically make one side of the mountain moist while the other is dry, meaning that each side grows its own kinds of plants. There will also be variation in soil type, as the soils are made from dead plants, and variation in insects because many insects can eat only certain species of plants.

Soil type will determine what kind of ground-dwelling invertebrates and other decomposers can live there, as well as providing habitat for reptiles, amphibians, and small mammals that build their burrows in the soil and feed on the invertebrates. This kind of variation at the base of the food chain determines that there will be variation at all the higher levels as well, including predators.

Another major habitat characteristic is elevation. Higher elevation means colder temperatures and less oxygen, which also dictates that tougher, scrubbier plants will live near the top and that there will be less soil and fewer species at high elevations. Barriers that are inhospitable to some taxa, such as a mountaintop, a river, or a deep canyon, add complexity to the habitat that consequentially creates separate populations that do not communicate very often. These separated populations allow for greater genetic, and ultimately taxonomic, diversity.

From these examples we can make the generalization that biomes in warmer climates , with greater energy from primary producers and with more varied and complex habitat types, have a higher biodiversity than those without those traits. For example, a mountain range of the same overall characteristics at a higher latitude will have less soil and less energy because there is less primary production from the plants, which go dormant for much of the year. This will cause the overall number of species and biodiversity to be lower.

The Importance of Biodiversity

The value of biodiversity has been argued by many different people for a variety of reasons, but they all point to a unified ideal of conservation. Aldo Leopold is known as the father of environmental ethics in the United States. In books such as Sand County Almanac and Sketches Here and There (1949), he stressed that humans must change their role from consumer of the natural world to cohabitator of it. This change is needed to preserve biodiversity, which would ensure that natural resources are available for future generations.

The value of biodiversity is also recognized by various groups and organizations in modern society. They include waste managers who use wetlands to clean runoff; pharmacists who search for new drugs in rare species; a food industry that interbreeds wild species to improve domestic ones; a pet industry that imports and breeds rare animals; hobbyists who bird-watch, camp, and photograph in search of new species; hunters and fishermen who selectively harvest to eat and teach their families about the wilderness; scientists who use species to study evolution; and, finally, conservationists interested in preserving biodiversity not only for what it can do for them, but for its inherent value in that it lives and breathes as we do.

The Decline of Biodiversity

The decline of biodiversity is documented for prehistoric times and can be the result of natural events that may or may not be related to the fitness of the species that become extinct. A constant level of background extinction has always existed, but it is mass extinction events that cause concern about the future of biodiversity.


The largest extinction event recorded occurred 250 million years ago, when 95 percent of marine species died in response to an uplift of the species-rich continental shelf that was caused by plate tectonics . Another famous example of mass extinction is the impact of the Chicxulub meteorite, which is thought to have left a dense cloud over the sky worldwide for a decade, causing a decrease in primary production (plant activity) and a subsequent extinction of many taxa. This impact coincides with the end of the dinosaur age and probably contributed to the extinction of many of those lines.

Extinction rates at the beginning of the twenty-first century are undeniably higher than background rates, but the exact rate calculated depends on what method of calculation is employed. Using estimates from recent past extinction rates based on fossils, mammal extinctions were once one per two hundred years, and most recently were twenty species in the twentieth century. At the turn of the twenty-first century, the rate of bird extinction is 1,000 times the average over the past 2,000 years. Combining the two calculations provides an estimate of a 1 percent loss of species diversity over the twentieth century, a number much greater than any prehuman impact. Using habitat loss as a predictor of species loss, it is estimated that between 2 and 25 percent of biodiversity will be lost over the twenty-first century.

Causes of extinction.

Biodiversity decline can result from excessive hunting, environmental contamination, or habitat loss and there are a variety of ways to combat these sources of decline. Excessive hunting typically impacts large species that come into frequent contact with humans, usually because the humans are moving into the animal's habitat. These species are killed out of ignorance, because they are seen as a safety threat, or because they are desired for their fur or meat or as trophies. Many times these species are top predators, so their loss is felt throughout the food chain as populations of prey items go unchecked, which causes subsequent problems for the ecosystem and humans. The regulation of hunting, however, involves innumerable complications when impoverished people rely on hunting for their livelihood and when regulation is not well-funded.

Contamination of the air, land, and water results largely from the generation of energy and the use of machines such as the automobile. Power plants and cars produce huge amounts of pollution that have far-reaching impacts because the pollution is spread by wind and river to formerly pristine areas. Contamination of soils and waters also results from the use of pesticides and fertilizers associated with farming as well as from human waste generated in large urban centers. However, the major threat to bio-diversity is habitat loss. Human activities alter the environment to the degree that it can no longer sustain species where they once lived.

The solution to these problems must start with global recognition of the importance of conservation. Biodiversity will need to be maintained in those places where it still exists by creating and managing large protected areas. Some species will need to be helped along artificially by maintaining them in captivity and creating seed banks. Previously destroyed habitat will need to be restored by revegetating and repairing the damage that has been done. Management strategies will have to be created that allow for the conservation of land in concert with human goals. To support all of these strategies, a financial, legal, and political infrastructure will need to be created.

It is important to recognize that Earth's declining biodiversity is a serious global problem. It will be up to educators and future generations to stress the importance of conservation and find means to preserve biodiversity, the immense variety of organisms and interactions that support life on Earth.

see also Ecology; Habitat; Habitat Loss.

Jean K. Krejca


Becher, Anne. Biodiversity. Santa Barbara, CA: ABC-CLIO, 1998.

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Dunell, Lee. State of the Ark: An Atlas of Conservation in Action. London: Gaia Books, 1986.

Elredge, Niles. Life in the Balance: Humanity and the Biodiversity Crisis. Princeton, NJ: Princeton University Press, 1998.

Jeffries, Michael J. Biodiversity and Conservation. London: Routledge, 1997.

Leopold, Aldo. A Sand County Almanac and Sketches Here and There. New York: Oxford University Press, 1949.

Miller, G. Tyler, Jr. Environmental Science, 3rd ed. Belmont, CA: Wadsworth, 1991.

Rosenzweig, Michael L. Species Diversity in Space and Time. New York: Cambridge University Press, 1995.

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The continued health of human societies depends upon a natural environment that is productive and contains a wide diversity of plant, animal, and microbe species. Life on the earth comprises at least 10 million species of plants, animals, and microbes, while in the United States there are an estimated 750,000 species, of which small organisms such as anthropods and microbes comprise 95 percent.

The sustainability of the forest ecosystems and other natural ecosystems are in danger from the expanding world population, which now totals more than 6 billion. With an estimated growth rate of 1.4 percent per year, it is projected to reach 12 billion by the year 2050. Further, due in large part to the growing human population and diverse human activities (supported in large part by fossil fuels), the current extinction rate of species ranges from approximately 1,000 to 10,000 times higher than natural extinction rates. This is alarming for several reasons. Foremost, biodiversity is essential for the sustainable functioning of agricultural, forest, and natural ecosystems upon which human survival and health depends. The loss of a key species (e.g., loss of a predator) creates an imbalance among the remaining species, and can sometimes result in the collapse of the entire ecosystem. Altering a habitat may also improve the environment for an infectious disease, like dengue.

Species diversity affects the quantity and quality of human food supply. For example, conserving pollinators and natural enemies of pests is essential for successful grain, fruit, and vegetable production. Improving food production decreases malnutrition. Yet, at present, the rapidly expanding human population is intensifying the need for increased food supplies. In the year 2000, more than 3 billion people were suffering from malnutritionthe largest number and proportion of people to date. Each year, between 6 million and 14 million people die from the effects of malnutrition.

In many parts of the world, especially in developing countries (e.g., in the Sahelian region of Africa), severe shortages of vitamin A are causing blindness and even death. Worldwide, approximately 250 million children are vitamin A deficient, and each year vitamin A deficiency causes approximately 2 million deaths and 3 million serious eye problems, including blindness.

Similarly, iron intake per person has been declining, especially in sub-Saharan Africa, South Asia, the People's Republic of China, and South America, because overall shortages of food result in inadequate nutrition. In 1998 more that 2 billion persons were sufficiently iron deficient to cause anemia in 1.2 billion people. An estimated 20 percent of the malnutrition deaths are attributed to severe anemia.

Malnutrition is also associated with parasitic infections that are found in areas were conditions of poverty and inadequate sanitation also exist. The health of malnourished individuals, especially children, is seriously affected by parasitic infections, because their presence reduces the availability of nutrients. Parasitic infections diminish appetites while increasing the loss of nutrients by causing diarrhea and dysentery. Hookworms, for instance, can suck as much as 30 milliliters of blood from an infected individual each day, lowering his or her resistance to other diseases. Because an estimated 5 to 20 percent of an individual's daily food intake is used by the body to offset the effects of parasitic illnesses, the overall nutritional status of a parasite-infected person is greatly diminished over time.

As a human population continues to expand and biodiversity declines, waste grows and its disposal becomes a major environmental problem. Each year the total quantity of waste produced by humans, livestock, and crops weighs about 38 billion tons worldwide. Numerous invertebrate animals and microbes function to degrade and recycle wastes. Their preservation in ecosystems is essential to maintain a healthy and productive environment.

Worldwide chemical waste and pollution are also major environmental problems. In the twenty-first century in the United States, 80,000 different chemicals are used and released into the soil, water, and air; worldwide, an estimated 100,000 chemicals are used. In the United States, more than 1,100 kilograms of chemicals per person are used each year; nearly 10 percent of these are known carcinogens. Each year nearly 3 billion kilograms of pesticides are applied worldwide. These toxic chemicals cause 26 million human poisonings annually, with about 220,000 deaths, and affect approximately 750,000 people with chronic diseases like cancer.

Approximately 75 percent by weight of the chemicals released into the environment can be degraded by biological organisms. Thus, species biodiversity helps provide continuous cleanup of contaminated sites (such as residue of pesticides in agriculture), and has a significant advantage over other techniques. Conserving beneficial natural enemies not only controls crop pests but also helps reduce the amount of pesticides applied.

In addition to degrading chemicals, some invertebrate and microbe species also degrade and recycle biological pollutants in water resources. Again, the biological pollution problem is particularly serious in developing nations. About 1.2 billion people in the world lack clean, safe water because most household and industrial wastes are dumped directly into rivers and lakes without treatment. This pollution contributes to the rapidly increasing incidence of diseases in humans. Waterborne infections account for 80 percent of all infectious diseases worldwide and 90 percent of all infectious diseases found in developing countries. A lack of sanitary conditions contributes to about 2 billion human infections of diarrhea, resulting in about 4 million deaths, per year, mostly among infants and young children.

Sometimes altering a natural habitat inadvertently leads to the spread of an infectious disease. Diseases like schistosomiasis that are associated with contaminated fresh water are expanding worldwide. In 1999 it was estimated that schistosomiasis caused 1 million deaths per year. The escalation of the incidence of this disease followed an increase in suitable habitats for the snail that serves as the intermediate host of the causative agent, Schistosoma mansoni. Thus, construction in 1968 of the Aswan High Dam in Egypt and its related irrigation systems was followed by an explosion in the prevalence of Schistosoma mansoni, which increased in the human population from 5 percent in 1968 to 77 percent in 1993.

Considered together, the natural biodiversity of plants, animals, and microbes functions in many ways to enhance the health and quality of life enjoyed by human society. In view of the likely continued growth in human population, and the resultant alteration of the earth's fragile natural ecosystem, greater efforts must be made to conserve biodiversity as a natural and essential treasure.

David Pimentel

(see also: Climate Change and Human Health; Demographic Trap, Drinking Water; Ecosystems; Endangered Species Act; Environmental Determinants of Health; Famine; Groundwater Contamination; Land Use; Municipal Solid Waste; Nutrition; Ocean Dumping; Pesticides; Pollution; Population Density; Population Growth; Sanitation; Species Extinction; Sustainable Development; Wastewater Treatment; Water Quality )


Heywood, V. H. (1995). Global Biodiversity Assessment. Cambridge, UK: Cambridge University Press.

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Biodiversity describes the variety of biological organisms in a given habitat , area, or ecosystem . It includes several components involving variation in species, ecosystems, and genetics.

Species diversity is perhaps what most people think of when they think of biodiversity. Species biodiversity describes the number of different species of plants, animals, fungi, protists, or bacteria within an area of interest. Although there are approximately 1.5 million named species globally, the large majority of species have yet to be studied or named, and there are likely to be as many as 10 million species on Earth.

Ecosystem diversity describes the variety of habitat types found within a given area. For example, there are numerous types of terrestrial (land-based) ecosystems in the United States, including forests, grasslands, wetlands , and deserts. There also are many types of aquatic (water-based) ecosystems, including lakes, rivers, estuaries , coral reefs, and intertidal habitats.

Genetic diversity describes genetic variation within species and represents a third component of biodiversity. Genetic variation allows species to adapt to changes in their environment. Species that are reduced to very small populations lose much of the gene pool and hence can lose much of this adaptability.

Aquatic Biodiversity

Aquatic biodiversity describes the diversity of species and ecosystems found in and around aquatic habitats such as rivers, lakes, and oceans. As with terrestrial ecosystems, aquatic biodiversity varies from region to region. Aquatic biodiversity is greatest in tropical latitudes. For example, an estimated 3,000 species of fish are found in the Amazon River alone. Coral reef habitats also have extremely high biodiversity; nearly a quarter of all known marine species are found in coral reefs. The Great Barrier Reef, off the coast of Australia, is the largest coral reef system in the world. It supports over 700 species of coral, in addition to 1,600 fish species and 4,000 species of mollusks .

In the Antarctic Ocean, on the other hand, only 120 fish species are found. These species possess special molecular, biochemical "antifreeze" properties to deal with the cold water temperatures. However, Antarctic habitats nonetheless support many unique aquatic groups, such as the albatross, penguin, and large numbers of marine mammals such as the whale and seal.*

Many fresh-water habitats also harbor a high proportion of unique species. This is due to the fact that, unlike oceans, fresh-water habitats often are isolated from one another, with natural barriers between them that are difficult to cross. This results in the evolution of distinct species in different fresh-water habitats. The preservation of fresh-water habitats therefore is particularly critical to conserving aquatic biodiversity.

The Value of Biodiversity

The value of biodiversity is an issue that has caused considerable debate, given that the preservation of habitats often conflicts with the desires of developers. Yet there are several reasons for valuing biodiversity.

First, biodiversity is essential to the functioning of ecosystems. Each species plays a unique role within an ecosystem, and every species is dependent on others for food, shelter, or other resources. The loss of a single species therefore can have profound effects for the ecosystem as a whole. Second, all species are potential sources of genetic variation for the development of new types of agricultural crops, as well as of medical drugs for treatment of human diseases. Third, biota (living organisms) have scientific and educational value. Finally, species have aesthetic and recreational valueconsider, for example, the popularity of activities such as snorkeling, scuba diving, and hiking.

Scientists have shown that habitats with greater biodiversity are more resilientthat is, they are better able to adjust to and recover from various disturbances. Because different species may perform overlapping functions in a biologically diverse ecosystem, a disturbance that affects one species may have lesser impact on the ecosystem as a whole. Habitats with little diversity are more vulnerable, because a disturbance affecting one species may cause the entire network of interactions to collapse.

Ecosystems approaches to natural resource management address interactions among species and among food webs , as well as the cycling of resources such as carbon, water, and nitrogen. These ecosystems approaches focus not on single species, but on the preservation of complex sets of interactions among species. Preservation of large, intact areas of habitat is necessary for the continued functioning of ecosystems.

Threats to Biodiversity

Aquatic biodiversity is threatened on many fronts. Fresh-water habitats support many of the most highly threatened animal groups. These include fish, mussels, amphibians , and crustaceans .

Fresh-water habitats are threatened by many factors, including pollution from industry, increased acidification, and agricultural runoff containing residues of fertilizers or pesticides . In addition, the building of dams destroys many river ecosystems. Development can harm aquatic habitats or remove them altogether, as when marshy areas are filled.

Aquatic ecosystems also are particularly fragile because the disturbance of a watershed can affect multiple components downstream, including rivers, lakes, estuaries, and oceans. Perhaps the largest threat to ocean biodiversity is overfishing. In addition to depleting commercial species of fish, bivalves, and crustaceans, many fishing methods cause the needless deaths of noncommercial fish species as well as numerous reptiles, birds, and marine mammals.

see also Amphibian Population Declines; Balancing Diverse Interests; Birds, Aquatic; Bivalves; Cephalopods; Corals and Coral Reefs; Crustaceans; Ecology, Fresh-Water; Ecology, Marine; Endangered Species Act; Estuaries; Fish; Fish and Wildlife Issues; Fisheries, Fresh-Water; Fisheries, Marine; Fishes, Cartilaginous; Instream Water Issues; Life in Extreme Water Environments; Life in Water; Marine Mammals; Oceans, Polar; Oceans, Tropical; Reptiles.

Jennifer Yeh


Byatt, Andrew, Alastair Fothergill, and Martha Holmes. The Blue Planet: A Natural History of the Oceans. New York: DK Publishers, 2001.

Gould, James L., and William T. Keeton, with Carol Grant Gould. Biological Science, 6th ed. New York: W. W. Norton & Co., 1996.


This encyclopedia contains photographs of several types of ecosystems. Entries with notable images include:

Entry Photograph

"Corals and coral reef

Coral Reefs"

"Desert Hydrology" desert

"Estuaries" estuary

"Forest Hydrology" forest

"Glaciers and snow and ice

Ice Sheets"

"Oceans, Polar" ocean

"Tides" intertidal

"Wetlands" wetland

* See "Marine Mammals" for photographs of a killer whale, manatee, elephant seal, and dolphin.

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Biodiversity is the sum total of life on Earth; the entire global complement of terrestrial, marine, and freshwater biomes and ecosystems , and the speciesplants, animals, fungi, and microorganismsthat live in them, including their behaviors, interactions, and ecological processes. Biodiversity is linked directly to the nonliving components of the planetatmosphere, oceans, freshwater systems, geological formations, and soilsforming one great, interdependent system, the biosphere.

Humankind's Relationship to Biodiversity

Humans depend entirely on this biodiversity and are an integral part of it. Directly or indirectly, be it from wild or domesticated components of biodiversity, humankind derives many goods critical to its sustenance, wellbeing, health, and enjoyment, such as food, medicine, building materials, and industrial products. Also, people enjoy many ecosystem services, including water regulation and supply, erosion control, soil formation, nutrient storage and cycling, pollination, pollution breakdown and absorption, climate stability, protection and recovery from natural disasters, and buffering against the spread of disease. These services, provided by nature free of charge, have an estimated value of $33 trillion per year.

Even though continued human welfare depends on it, our knowledge of biodiversity is seriously inadequate. As of 1998, scientists have described between 1.4 and 1.8 million species. However, later estimates indicate that the total number of species ranges between 5 and 30 million, and some scientists believe it may be higher than 100 million.

Clearly, much more work is needed to quantify and describe all biodiversity at three main levels: genetic diversity, or the variation of genes within species; species diversity, or the variety of species within a biome or ecosystem, measured in species richness, species abundance, and taxonomic diversity; and ecosystem diversity, or the broad differences between ecosystem structures and biome types, and the diversity of habitats and ecological processes occurring within each of them. Taxonomists and other scientists in fields such as zoology, botany, ecology, and genetics study biodiversity.

Threats to Biodiversity

Species are becoming extinct faster than scientists can discover them. The loss of biodiversity is an irreversible process: once a species becomes extinct its loss is permanent and irrevocable. Late-twentieth-century estimates cite the extinction rate between one thousand and ten thousand times greater than it would be naturally. This means that Earth is losing species at the fastest rate in the planet's 4.5 billion-year history and, unlike prior extinction episodes (such as the mass extinction of dinosaurs 65 million years ago), this extinction spasm is mainly the result of human activity and not of a cosmic event. If extinctions continue at the current rate, in the next one hundred years humankind runs the risk of losing half of the planet's biodiversity.

Most threats to biodiversity have to do with pressures on natural resources due to human activities. These include habitat destruction and conversion of natural ecosystems to agriculture; flooding for hydroelectric projects; large-scale extraction of natural resources such as mining and logging; excessive hunting and overfishing; pollution from agricultural pesticides, human waste, and industrial processes; and poorly planned urban and suburban sprawl.

WILSON, E. O. (1829)

U.S. evolutionary biologist and Pulitzer Prizewinning author. Wilson is the world's authority on ants and biodiversity and was an early advocate of studying the behavior of humans and other animals in the context of evolution and adaptation, socalled "sociobiology."

Conserving Biodiversity

Conserving biodiversity is an urgent matter of common concern and should be an integral part of the development process, as was outlined in the Convention on Biological Diversity. This global, comprehensive agreement was drafted at the 1992 Rio de Janeiro Earth Summit and signed by 160 nations to address all aspects of biological diversity. Its objectives include "the conservation of biodiversity, its sustainable use and the fair sharing of the benefits derived from the utilization of genetic resources."

One conservation strategy aimed at reaching this goal recognizes that biodiversity is not evenly distributed over the planet: certain regions have higher species richness (the number of species in an area) and endemism (the number of species in that area that occur nowhere else) than others. Ironically, many of these sensitive areas are also preferred by humans to inhabit, placing tremendous pressure on local biodiversity. These areas are called the "biodiversity hotspots"; twenty-five of them have been described thus far, including Madagascar, the tropical Andes, the Philippines, and the Atlantic forest of Brazil. Conservationists believe that urgent conservation efforts should be targeted at these regions. Equally important are the socalled wilderness areas: Amazonia, the Congo Basin, and Papua New Guinea. These areas are also high in biodiversity but are not so immediately threatened.

see also Biome; Conservation; Ecosystem; Endangered Species; Extinction; Invasive Species

Cristina G. Mittermeier and Russell A. Mittermeier


Costanza, Robert, et al. "The Value of the World's Ecosystem Services and Natural Capital." Nature 387, no. 15 (May 1997): 253260.

May, R. M. "How Many Species Are There on Earth?" Science 241 (1998): 14411449.

Mittermeier, Russell A., P. Robles Gil, and Cristina G. Mittermeier. Megadiversity: Earth's Biologically Wealthiest Countries. Mexico: Cemex, 1997.

. Hotspots: Earth's Biologically Richest and Most Endangered Terrestrial Ecorregions. Mexico: Cemex, 1999.

Stuart, S. Species: Unprecedented Extinction Rate, and It's Increasing. Gland, Switzerland: IUCN, 1999.

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The term biodiversity refers to the wide range of organismsplants and animalsthat exist within any given geographical region. That region may consist of a plot of land no more than a few square meters or yards, a whole continent, or the entire planet. Most commonly, discussions of biodiversity consider all the organisms that interact with each other in an extended geographical region, such as a tropical rain forest or a subtropical desert.

Concerns about biodiversity are relatively new. Only during the last quarter of the twentieth century did scientists begin to appreciate the vast number of organisms found on Earth and the complex ways in which they interact with each other and with their environments. Biologists have now discovered and named about 1.7 million distinct species of plants and animals. As many as 50 million species, however, are thought to exist.

Biodiversity in the tropics is of special interest since the richness of species found there is so great. According to some estimates, 90 percent of all plant, animal, and insect species exist in tropical regions. At the same time, surveys of organisms in the tropics have been very limited. Those studies that have been conducted provide only a hint of the range of life that may exist there. As an example, one study of a 108-square kilometer (42-square mile) reserve of dry forest in Costa Rica found about 700 plant species, 400 vertebrate species, and 13,000 species of insects. Included among the latter group were 3,140 species of moths and butterflies alone.

Human threats to biodiversity

One reason for the growing interest in biodiversity is the threat that human activities may pose for plant and animal species. As humans take over more land for agriculture, cities, highways, and other uses, natural habitats are seriously disrupted. Whole populations may be destroyed, upsetting the balance of nature that exists in an area. The loss of a single plant, for example, may result in the loss of animals that depend on that plant for food. The loss of those animals may, in turn, result in the loss of predators who prey on those animals.

As human populations grow, the threat to biodiversity will continue to grow with it. And as more people place greater stress on the natural environment, greater will be the loss of resources plant and animal communities need to survive.

Why is biodiversity important?

Maintaining biodiversity in a region and across the planet is important for a number of reasons. First, some people argue that all species

because they existhave a right to continue to exist in their own natural habitats, untouched by human development. Second, humans depend on many of the plants and animals that make up an ecological community. For example, one-quarter of all the prescription drugs in the United States contain ingredients obtained from plants. And third, humans themselves benefit from the interaction among organisms in a biologically diverse community: plants help clean the water and air, provide oxygen in the atmosphere, and control erosion. "Biodiversity," according to the biologist Peter Raven, "keeps the planet habitable and ecosystems functional."

Protection of threatened biodiversity

One of the great issues in environmental science today is how biodiversity can be preserved both in specific geographical regions and across the planet. One proposal that has been made involves the use of ecological reserves. Ecological reserves are protected areas established for the preservation of habitats of endangered species, threatened ecological communities, or representative examples of widespread communities. By the end of the 1990s, there were about 7,000 protected areas globally with an area of 651 million hectares (1.6 billion acres). Of this total, about 2,400 sites comprising 379 million hectares (936 million acres) were fully protected and could be considered to be true ecological reserves.

Ideally, the design of a national system of ecological reserves would provide for the longer-term protection of all native species and their natural communities including terrestrial (land-dwelling), freshwater, and marine (saltwater) systems. So far, however, no country has put in place a comprehensive system of ecological reserves to fully protect its natural biodiversity. Moreover, in many cases existing reserves are relatively small and are threatened by environmental change, illegal poaching of animals and plants, and tourism.

The World Conservation Union, World Resources Institute, and United Nations Environment Program are three important agencies whose purpose is to conserve and protect the world's biodiversity. These agencies have developed the Global Biodiversity Strategy, an international program to help protect plant and animal habitats for this and future generations. Because this program began only in the late 1970s, it is too early to evaluate its success. However, the existence of this comprehensive international effort is encouraging, as is the participation of most of Earth's countries, representing all stages of economic development.

[See also Ecosystem; Endangered species ]

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biodiversity (biological diversity) The existence of a wide variety of species (species diversity) or other taxa of plants, animals, and microorganisms in a natural community or habitat, or of communities within a particular environment (ecological diversity), or of genetic variation within a species (genetic diversity). The maintenance of a high level of biodiversity is important for the stability of ecosystems. Certain habitats, especially rainforests, have a rich species diversity, which is threatened by the continued destruction of habitats (see deforestation; desertification). Such ecosystems typically support large numbers of rare species, and population sizes of individual species tend to be small; they are therefore especially vulnerable to habitat destruction. Biodiversity in natural habitats also represents an important pool of species and genetic material of potential use to human societies. For example, wild plants continue to be used as a source of new drugs and other products, and the development of new strains and varieties of crop plants with increased disease resistance usually depends on incorporating genetic material from wild plants.

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Biodiversity exists at three interrelated levels: species diversity, genetic diversity, and community-level diversity. When we talk about plant biodiversity, we refer to the full range of plant species, the genetic variation found within those species, and the biological communities formed by those species. For vascular plants, biodiversity includes all species of ferns, gymnosperms , flowering plants, and related smaller groups such as clubmosses and horsetails. The genetic variation found within populations and among populations arises through the mutation of individual genes or chromosomes and is rearranged by genetic recombination during the sexual process. Genetic variation is important not only for the survival and evolution of species; it is also important to people for breeding improved crop plants with higher yields.

Biological diversity also refers to all biological communities, including temperate forests, tropical forests, grasslands, shrub lands, deserts, freshwater wetlands, and marine habitats. Each of these biological communities represents an adaptation of plants to particular regimes of climate, soil, and other aspects of the environment. This adaptation involves ecosystem interactions of each biological community with its physical and chemical environment. For example, the ability of a forest community to absorb rain water and slowly release the water into streams and the ability of a swamp to process and detoxify polluted water are both aspects of ecosystem-level biological diversity that are of central importance to human societies.

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


Falk, D. A, C. I. Millar, and M. Olwell, eds. Restoring Diversity; Strategies for the Reintroduction of Endangered Plants. Washington, DC: Island Press, 1996.

Mittermeier, R. A., N. Myers, P. R. Gil, and C. G. Mittermeier. Hotspots: Earth's Richest and Most Endangered Terrestrial Ecoregions. Mexico City: Agrupacion Sierra Madre, SC, 1999.

Primack, R. A Primer of Conservation Biology. Sunderland, MA: Sinauer Associates, 2000.

Quammen, D. The Song of the Dodo: Island Biogeography in an Age of Extinctions. New York: Scribner, 1996.

Schneider, S. Laboratory Earth: The Planetary Gamble We Can't Afford to Lose. New York: Basic Books, 1998.

Stein, B. A., and S. R. Flack. Species Report Card: The State of U.S. Plants and Animals. Arlington, VA: The Nature Conservancy, 1997.

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"Biodiversity." Plant Sciences. . 30 Apr. 2017 <>.

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biodiversity A portmanteau term, which gained popularity in the late 1980s, used to describe all aspects of biological diversity, especially including species richness, ecosystem complexity, and genetic variation.

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"biodiversity." A Dictionary of Ecology. . 30 Apr. 2017 <>.

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biodiversity A portmanteau term, which gained popularity in the late 1980s, that describes all aspects of biological diversity, especially including species richness, ecosystem complexity, and genetic variation.

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bi·o·di·ver·si·ty / ˌbīōdiˈvərsitē/ • n. the variety of life in the world or in a particular habitat or ecosystem.

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"biodiversity." The Oxford Pocket Dictionary of Current English. . 30 Apr. 2017 <>.

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biodiversity: see biological diversity.

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biodiversitybanditti, bitty, chitty, city, committee, ditty, gritty, intercity, kitty, nitty-gritty, Pitti, pity, pretty, shitty, slitty, smriti, spitty, titty, vittae, witty •fifty, fifty-fifty, nifty, shifty, swiftie, thrifty •guilty, kiltie, silty •flinty, linty, minty, shinty •ballistae, Christie, Corpus Christi, misty, twisty, wristy •sixty •deity, gaiety (US gayety), laity, simultaneity, spontaneity •contemporaneity, corporeity, femineity, heterogeneity, homogeneity •anxiety, contrariety, dubiety, impiety, impropriety, inebriety, notoriety, piety, satiety, sobriety, ubiety, variety •moiety •acuity, ambiguity, annuity, assiduity, congruity, contiguity, continuity, exiguity, fatuity, fortuity, gratuity, ingenuity, perpetuity, perspicuity, promiscuity, suety, superfluity, tenuity, vacuity •rabbity •improbity, probity •acerbity • witchetty • crotchety •heredity •acidity, acridity, aridity, avidity, cupidity, flaccidity, fluidity, frigidity, humidity, hybridity, insipidity, intrepidity, limpidity, liquidity, lividity, lucidity, morbidity, placidity, putridity, quiddity, rabidity, rancidity, rapidity, rigidity, solidity, stolidity, stupidity, tepidity, timidity, torpidity, torridity, turgidity, validity, vapidity •commodity, oddity •immodesty, modesty •crudity, nudity •fecundity, jocundity, moribundity, profundity, rotundity, rubicundity •absurdity • difficulty • gadgety •majesty • fidgety • rackety •pernickety, rickety •biscuity •banality, duality, fatality, finality, ideality, legality, locality, modality, morality, natality, orality, reality, regality, rurality, tonality, totality, venality, vitality, vocality •fidelity •ability, agility, civility, debility, docility, edibility, facility, fertility, flexility, fragility, futility, gentility, hostility, humility, imbecility, infantility, juvenility, liability, mobility, nihility, nobility, nubility, puerility, senility, servility, stability, sterility, tactility, tranquillity (US tranquility), usability, utility, versatility, viability, virility, volatility •ringlety •equality, frivolity, jollity, polity, quality •credulity, garrulity, sedulity •nullity •amity, calamity •extremity • enmity •anonymity, dimity, equanimity, magnanimity, proximity, pseudonymity, pusillanimity, unanimity •comity •conformity, deformity, enormity, multiformity, uniformity •subcommittee • pepperminty •infirmity •Christianity, humanity, inanity, profanity, sanity, urbanity, vanity •amnesty •lenity, obscenity, serenity •indemnity, solemnity •mundanity • amenity •affinity, asininity, clandestinity, divinity, femininity, infinity, masculinity, salinity, trinity, vicinity, virginity •benignity, dignity, malignity •honesty •community, immunity, importunity, impunity, opportunity, unity •confraternity, eternity, fraternity, maternity, modernity, paternity, taciturnity •serendipity, snippety •uppity •angularity, barbarity, bipolarity, charity, circularity, clarity, complementarity, familiarity, granularity, hilarity, insularity, irregularity, jocularity, linearity, parity, particularity, peculiarity, polarity, popularity, regularity, secularity, similarity, singularity, solidarity, subsidiarity, unitarity, vernacularity, vulgarity •alacrity • sacristy •ambidexterity, asperity, austerity, celerity, dexterity, ferrety, posterity, prosperity, severity, sincerity, temerity, verity •celebrity • integrity • rarity •authority, inferiority, juniority, majority, minority, priority, seniority, sonority, sorority, superiority •mediocrity • sovereignty • salubrity •entirety •futurity, immaturity, impurity, maturity, obscurity, purity, security, surety •touristy •audacity, capacity, fugacity, loquacity, mendacity, opacity, perspicacity, pertinacity, pugnacity, rapacity, sagacity, sequacity, tenacity, veracity, vivacity, voracity •laxity •sparsity, varsity •necessity •complexity, perplexity •density, immensity, propensity, tensity •scarcity • obesity •felicity, toxicity •fixity, prolixity •benedicite, nicety •anfractuosity, animosity, atrocity, bellicosity, curiosity, fabulosity, ferocity, generosity, grandiosity, impecuniosity, impetuosity, jocosity, luminosity, monstrosity, nebulosity, pomposity, ponderosity, porosity, preciosity, precocity, reciprocity, religiosity, scrupulosity, sinuosity, sumptuosity, velocity, verbosity, virtuosity, viscosity •paucity • falsity • caducity • russety •adversity, biodiversity, diversity, perversity, university •sacrosanctity, sanctity •chastity •entity, identity •quantity • certainty •cavity, concavity, depravity, gravity •travesty • suavity •brevity, levity, longevity •velvety • naivety •activity, nativity •equity •antiquity, iniquity, obliquity, ubiquity •propinquity

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