Biodiversity or biological diversity is the diversity of life, ranging from the level of genes to species. First coined at a 1986 conference and subsequently used in 1988 in the book BioDiversity, edited by E. O. Wilson, the term has grown in popularity and is used widely in science and conservation policy.
Biodiversity offers many benefits: ecological, economic, scientific, and moral. Despite this, there is a trend toward lower biodiversity as a result of human action, as monoculture is promoted in agriculture, habitats are replaced through commercial and residential use, and species become increasingly extinct. The extinction of species has become so pronounced that there is concern that we are witnessing the beginning of a new mass extinction. One of the challenges society faces is quantifying biodiversity and understanding how best to create regulations and a moral climate that both support maintaining biodiversity and human development.
Biodiversity is a neologism (recently created word, term, or phrase), literally meaning biological and diversity.
The term biological diversity was coined by Thomas Lovejoy, a tropical and conservation biologist. The word biodiversity itself was coined by W. G. Rosen in 1985 while planning the 1986 National Forum on Biological Diversity, organized by the National Research Council (NRC). The term first appeared in a publication in 1988 when entomologist E. O. Wilson used it as the title of the proceedings (Wilson 1988) of that forum (Wilson 1992). The word biodiversity was deemed more effective in terms of communication than biological diversity.
Since 1986, the terms and the concept have achieved widespread use among biologists, environmentalists, political leaders, and concerned citizens worldwide. This use has coincided with the expansion of concern over the rates of extinction observed in the last decades of the twentieth century.
There are varied definitions for the term biodiversity. One definition is "variation of life at all levels of biological organization" (Gaston and Spicer 2004). Biodiversity is also viewed as a measure of the relative diversity among organisms present in different ecosystems. In this definition, diversity includes variation within species and among species, and comparative diversity among ecosystems.
Biodiversity may also be defined as the “totality of genes, species, and ecosystems of a region.” An advantage of this definition is that it seems to describe most instances of its use, and one possibly unified view of the traditional three levels at which biodiversity has been identified:
- genetic diversity—diversity of genes within a species. There is a genetic variability among the populations and the individuals of the same species
- species diversity—diversity among species
- ecosystem diversity—diversity at a higher level of organization, the ecosystem
The 1992 United Nations Earth Summit in Rio de Janeiro defined biodiversity as "the variability among living organisms from all sources, including, inter alia, terrestrial, marine, and other aquatic ecosystems, and the ecological complexes of which they are part: this includes diversity within species, between species, and of ecosystems."
This is, in fact, the closest thing to a single, legally accepted definition of biodiversity, since it is the definition adopted by the United Nations Convention on Biological Diversity. The parties to this convention include almost all nations.
Levels of biodiversity
Some scientists, such as E. O. Wilson, hold that the gene is the fundamental unit of natural selection and thus of evolution, and therefore the real biodiversity is genetic diversity. Others, however, are opposed to the gene-centered view of evolution. Species diversity is an easier unit to study than genetic diversity.
For biologists, biodiversity is the diversity of populations of organisms and species, but also the way these organisms function. Organisms appear and disappear; sites are colonized by organisms of the same species or by another. Some species develop social organizations that improve their reproduction goals or involve other species that live in communities.
For ecologists, biodiversity is also the diversity of durable interactions among species. It not only applies to species, but also to their immediate environment (biotope) and the ecoregions the organisms live in. In each ecosystem, living organisms are part of a whole; they interact with one another, but also with the air, water, and soil that surround them.
Measurement of biodiversity
Biodiversity is a broad concept, so a variety of objective measures have been created in order to empirically measure and define biodiversity. Each measure of biodiversity relates to a particular use of the data.
For practical conservationists, the measure should quantify a value that is at the same time broadly shared among locally affected people.
For others, a broader and more economically valuable measure would allow the ensuring of continued possibilities for both adaptation and future use by people, assuring environmental sustainability. As a consequence, biologists argued that this measure is likely to be associated with the variety of genes. Since it cannot always be said which genes are more likely to prove beneficial, the best choice for conservation is to assure the persistence of as many genes as possible.
For ecologists, this gene-centered approach is sometimes considered inadequate and too restricted. In ecology, a diversity index is a statistic which is intended to measure the biodiversity of an ecosystem. More generally, diversity indices can be used to assess the diversity of any population in which each member belongs to a unique species. Estimators for diversity indices are likely to be biased, so caution is advisable when comparing similar values.
There are three common metrics used to measure species-level biodiversity, as outlined by Whittaker (1972).
- Species richness
- Simpson index
- Shannon index (called Shannon-Wiener information index in Whittaker 1970, and sometimes called Shannon-Weaver index)
These are either biased towards species richness or species evenness.
Species richness is the simplest measure of biodiversity and is simply a count of the number of different species in a given area. It is referred to in equations as S.
Species evenness is a measure of biodiversity that quantifies how equal the populations are numerically. So if there are 40 foxes, and 1,000 dogs, the population is not very even. But if there are 40 foxes and 42 dogs, the population is quite even. The evenness of a population can be represented by
where e = 2.7, a constant, D = Shannon-Weaver Information function, s = number of species in sample.
The value is constrained between 0 and 1. The less variation in populations between the species, the higher E is.
Biodiversity is usually plotted as taxonomic richness of a geographic area over a temporal scale. Whittaker (1972) described three terms for measuring biodiversity over geographic scales:
- Alpha diversity refers to diversity within a particular area, community, or ecosystem, and is measured by counting the number of taxa within the ecosystem (usually species)
- Beta diversity is species diversity between ecosystems; this involves comparing the number of taxa that are unique to each of the ecosystems.
- Gamma diversity is a measure of the overall diversity for different ecosystems within a region.
Five biodiversity measures are described below:
- Species richness
- Simpson's diversity index
- Shannon's diversity index
- Berger-Parker index
- Renyi entropy
Simpson's diversity index
If pi is the fraction of all organisms which belong to the i-th species, then Simpson's diversity index is most commonly defined as the statistic:
This quantity was introduced by Edward Hugh Simpson.
If ni is the number of individuals of species i which are counted, and N is the total number of all individuals counted, then
is an estimator for Simpson's index for sampling without replacement.
Note that , where values near zero correspond to highly diverse or heterogeneous ecosystems and values near one correspond to more homogeneous ecosystems. Biologists who find this confusing sometimes use 1 / D instead; confusingly, this reciprocal quantity is also called Simpson's index. A more sensible response is to redefine Simpson's index as
(called by statisticians the index of diversity), since
- this quantity has a simple intuitive interpretation: it represents the probability that if we randomly choose two individuals, that they will belong to distinct species,
- this quantity is comparable with the so-called Shannon-Weaver diversity index, which has an even better theoretical justification as a measure of statistical inhomogeneity.
Shannon's diversity index
Shannon's diversity index (sometimes called the Shannon–Weaver index) is simply the ecologist's term for the communication entropy introduced by Claude Shannon:
where pi is the fraction of individuals belonging to the i-th species.
This index takes into account the number of species and the evenness of the species. Its value is increased either by having more unique species, or by having a greater species evenness.
This is the most widely used diversity index. The intuitive significance of this index can be described as follows: Suppose we devise binary codewords for each species in our ecosystem, with short codewords used for the most abundant species, and longer codewords for rare species. As we walk around and observe individual organisms, we call out the corresponding codeword. This gives a binary sequence. If we have used an efficient code, we will be able to save some breath by calling out a shorter sequence than would otherwise be the case. If so, the average codeword length we call out as we wander around will be close to the Shannon diversity index.
It is possible to write down estimators which attempt to correct for bias in finite sample sizes, but this would be misleading since communication entropy does not really fit expectations based upon parametric statistics. Differences arising from using two different estimators are likely to be overwhelmed by errors arising from other sources. Current best practice tends to use bootstrapping procedures to estimate communication entropy.
Shannon himself showed that his communication entropy enjoys some powerful formal properties, and furthermore, it is the unique quantity which does so. These observations are the foundation of its interpretation as a measure of statistical diversity (or "surprise," in the arena of communications).
The Berger-Parker diversity index is simply
This is an example of an index which uses only partial information about the relative abundances of the various species in its definition.
The Species richness, the Shannon index, Simpson's index, and the Berger-Parker index can all be identified as particular examples of quantities bearing a simple relation to the Renyi entropy,
for α approaching respectively.
Unfortunately, the powerful formal properties of communication entropy do not generalize to Renyi's entropy, which largely explains the much greater power and popularity of Shannon's index with respect to its competitors.
Distribution of biodiversity
Biodiversity is not distributed evenly on earth. Biodiversity is generally highest in the tropics, lowest in the subtropical latitudes (desert regions) and the poles (tundra, high latitude deserts). Flora and fauna vary depending on climate, altitude, soils, and the presence of other species.
Biodiversity is also an important aspect of the study of ecoregions, a relatively large area of land or water that contains a geographically distinct assemblage of natural communities. The biodiversity of flora, fauna, and ecosystems that characterize an ecoregion tend to be distinct from that of other ecoregions. World Wildlife Fund ecologists currently divide the land surface of the earth into 8 major ecozones containing 867 smaller terrestrial ecoregions.
Ecozones are global divisions that have their own characteristic interplay of climatic factors, morphodynamics, soil-forming processes, living conditions for plants and animals, and production potentials for agriculture and forestry. Correspondingly, they are distinguished by different climates, landforms, soil units, plant formations and biomes, and land use systems. According to Schultz (1988, 2000, 2002, and 2005) nine ecozones can be defined:
- Polar subpolar zone
- Boreal zone
- Temperate (or Humid) midlatitudes
- Dry (or Arid) midlatitudes
- Subtropics with winter rain (or Mediterranean-type subtropics)
- Subtropics with year-round rain (or Humid subtropics)
- Dry tropics and subtropics (or Tropical/subtropical arid lands)
- Tropics with summer rain (or Seasonal tropics)
- Tropics with year-round rain (or Humid tropics)
These ecozones occur in bands, often fragmented because of the distribution of the continents and oceans, from the poles to the equator. Nearly all are present in both the Northern and Southern hemispheres. Many consider this classification to be quite decisive, and some propose these as stable borders for bioregional democracy initiatives.
The ecozones are very well-defined, following major continental boundaries, while the ecoregions are subject to more change and controversy.
Hotspots of biodiversity
One definition of a biodiversity hotspot is a region with many endemic species, or species exclusively native to a place or biota. As a result of the pressures of the growing human population, human activity in many of these areas is increasing dramatically, leading to threats to endemic species. These biodiversity hotspots were first identified by Dr. Norman Myers in two articles in the scientific journal The Environmentalist (1988 and 1990). Most of these hotspots are located in the tropics and most of them are forests.
One example of a biodiversity hotspot is Brazil's Atlantic Forest, which contains roughly 20,000 plant species, 1,350 vertebrates, and millions of insects, just under half of which are thought to occur nowhere else in the world.
Biodiversity and evolution
Biodiversity found on earth today is the culmination of 4 billion years of life on earth.
The original origin of life is not well known to science, though limited evidence suggests that life may already have been well-established only a few hundred million years after the formation of the earth. Until approximately 600 million years ago, all life consisted of bacteria and similar single-celled organisms.
The history of biodiversity during the Phanerozoic (the last 540 million years), starts with rapid growth during the Cambrian explosion—a period during which nearly every phylum of multicellular organisms first appeared. Over the next 400 million years or so, global diversity showed little overall trend, but was marked by periodic, massive losses of diversity classified as mass extinction events.
The apparent biodiversity shown in the fossil record suggests that the last few million years include the period of greatest biodiversity in earth's history. However, not all scientists support this view, since there is considerable uncertainty as to how strongly the fossil record is biased by the greater availability and preservation of recent geologic sections. Some (e.g., Alroy et al. 2001) argue that, corrected for sampling artifacts, modern biodiversity is not much different than biodiversity 300 million years ago (Smith 2001). Estimates of the present global macroscopic species diversity vary from 2 million to 100 million species, with a best estimate of somewhere near 10 million.
Most biologists agree, however, that the period since the emergence of humans is part of a new mass extinction, the Holocene extinction event, caused primarily by the impact humans are having on the environment. At present, the number of species estimated to have gone extinct as a result of human action is still far smaller than are observed during the major mass extinctions of the geological past. However, it has been argued that the present rate of extinction is sufficient to create a major mass extinction in less than 100 years. Others dispute this and suggest that the present rate of extinctions could be sustained for many thousands of years before the loss of biodiversity matches the more than 20 percent losses seen in past global extinction events.
New species are regularly discovered (on average about three new species of birds each year) and many, though discovered, are not yet classified. (An estimate states that about 40 percent of freshwater fish from South America are not yet classified.) Most of the terrestrial diversity is found in tropical forests.
Benefits of biodiversity
Biodiversity has contributed in many ways to the development of human culture, and, in turn, human communities have played a major role in shaping the diversity of nature at the genetic, species, and ecological levels.
There are four reasons commonly cited in the literature for the benefits of biodiversity.
Ecological role of biodiversity
All species provide some kind of function to an ecosystem. They can capture and store energy, produce organic material, decompose organic material, help to cycle water and nutrients throughout the ecosystem, control erosion or pests, fix atmospheric gases, or help regulate climate.
Ecosystems also provide various supports of production, such as soil fertility, pollinators of plants, predators, decomposition of wastes, and so forth, and services, such as purification of the air and water, stabilization and moderation of the climate, decrease of flooding, drought, and other environmental disasters.
These functions are important for ecosystem function and human survival.
Research suggests that a more diverse ecosystem is better able to withstand environmental stress and consequently is more productive. The loss of a species is thus likely to decrease the ability of the system to maintain itself or to recover from damage or disturbance. Just like a species with high genetic diversity, an ecosystem with high biodiversity may have a greater chance of adapting to environmental change. In other words, the more species comprising an ecosystem, the more stable the ecosystem is likely to be. The mechanisms underlying these effects are complex and hotly contested. However, it has become clear that there are real ecological effects of biodiversity.
Economic role of biodiversity
For all humans, biodiversity is first a resource for daily life. One important part of biodiversity is “crop diversity,” which is also called agrobiodiversity.
Most people see biodiversity as a reservoir of resources to be drawn upon for the manufacture of food, pharmaceutical, and cosmetic products. This concept of biological resources management probably explains most fears of resource disappearance related to the erosion of the biodiversity. However, it is also the origin of new conflicts dealing with rules of division and appropriation of natural resources.
Some of the important economic commodities that biodiversity supplies to humankind are:
- Food: crops, livestock, forestry, and fish;
- Medication: Wild plant species have been used for medicinal purposes since before the beginning of recorded history. For example, quinine comes from the cinchona tree (used to treat malaria), digitalis from the foxglove plant (chronic heart trouble), and morphine from the poppy plant (pain relief). According to the National Cancer Institute, over 70 percent of the promising anti-cancer drugs come from plants in the tropical rainforests. Animals may also play a role, in particular in research. It is estimated that of the 250,000 known plant species, only 5,000 have been researched for possible medical applications.
- Industry: for example, fibers for clothing, wood for shelter and warmth. Biodiversity may be a source of energy (such as biomass). Other industrial products are oils, lubricants, perfumes, fragrances, dyes, paper, waxes, rubber, latexes, resins, poisons, and cork, which can all be derived from various plant species. Supplies from animal origin include wool, silk, fur, leather, lubricants, and waxes. Animals may also be used as a mode of transport.
- Tourism and recreation: biodiversity is a source of economical wealth for many areas, such as many parks and forests, where wild nature and animals are a source of beauty and joy for many people. Ecotourism, in particular, is a growing outdoor recreational activity.
Ecologists and environmentalists were the first to insist on the economic aspect of biological diversity protection. Thus, E. O. Wilson wrote in 1992 that biodiversity is one of the bigger wealths of the planet, though few recognize it as such.
Estimation of the value of biodiversity is a necessary precondition to any discussion on the distribution of biodiversity richness. This value can be divided into use value (direct such as tourism or indirect such as pollination) and non-use or intrinsic value.
If biological resources represent an ecological interest for the community, their economic value is also increasing. New products are developed because of biotechnologies, and new markets created. For society, biodiversity also is a field of activity and profit. It requires a proper management setup to determine how these resources are to be used.
The majority of species have yet to be evaluated for their current or future economic importance.
Scientific role of biodiversity
Scientifically, biodiversity is important because each species can give scientists some clue as to how life evolved and will continue to evolve on earth. In addition, biodiversity helps scientists understand how life functions and the role of each species in sustaining ecosystems.
Ethical role of biodiversity
There is an ethical component to biodiversity if humans consider that other species have an intrinsic right to exist. Ecophilosophies such as deep ecology assert that a recognition of this intrinsic right makes it morally wrong to voluntarily cause extinction. The level of biodiversity is a good indicator of the state of our relationships with other living species. Biodiversity is also part of many cultures' spiritual heritage.
Threats to biodiversity
During the last century, erosion of biodiversity has been increasingly observed. Estimates of extinction rates are controversial. However, some recent calculations suggest that rates of species extinction are now on the order of 100 to 1,000 times those before humanity's dominance of earth, and these figures prove worse for well-known groups such as birds (Vitousek et al. 1997).
Conservationists maintain that human growth and development is potentially leading to an extinction rate similar to the end of the Permian and Cretaceous eras, where about 50 percent of the species in the fossil record died out (Raup and Sakowksi 1984). Some estimates put the loss at thousands of species per year, though these are based on Species-area theory and are controversial. This figure indicates unsustainable ecological practices, because only a small number of species come into being each year.
An increasing number of studies indicate that elevated rates of extinction are being driven by human use of organic resources. While most of the species that are becoming extinct are not food species, their biomass is converted into human food when their habitat is transformed into pasture, cropland, and orchards, and biomass is further reduced by conversion of habitat into residential or commercial uses. It is estimated that between 39 and 50 percent of the earth's land has been altered by human activity (Vitousek et al. 1997). Because an ecosystem decreases in stability as species are made extinct and biodiversity decreases, these studies warn that the global ecosystem is destined for significant threats if it is further reduced in complexity.
Not only species overuse and ecosystem degradation, but also conversion to very standardized ecosystems (e.g., monoculture following deforestation) are factors affecting biodiversity. Other issues affecting biodiversity include pollution by human activity and climate change driven by human activity. These have not yet been proven to have caused reduction in biodiversity, but the potential for them to do so is clear.
Dissenters such as economist Bjørn Lomborg (2001) argue that there is not enough data to support the view of mass extinction, and say abusive extrapolations are being made on the global destruction of rainforests, coral reefs, mangrove swamps, and other rich habitats.
There is also a growing awareness that the movement and the introduction of exotic species around the world by humans is a potent threat to biodiversity. When exotic species are introduced to ecosystems by humans and establish self-sustaining populations, the endemic species in that ecosystem, which have not evolved to cope with the exotic species in question, cannot automatically be expected to survive. Indeed, in many situations some will not. The exotic organisms in question may be predators and/or have features that make them very competitive, and similarly makes endemic species very defenseless and/or uncompetitive against these exotic species.
The rich diversity of unique species across many parts of the world exist only because they are separated by barriers—particularly seas and oceans—from other species of other land masses. These barriers keep highly fecund, ultra-competitive, generalist "super-species" in check. These barriers could never be crossed by natural processes, except through continental drift, a process that takes many millions of years. However, human travel by air and water has facilitated species invasion and increased the rate at which species spread. As a consequence of increased global human travel, some fear that many of the world's ecosystems will be dominated by a very few, cosmopolitan "super-species."
Biodiversity management: conservation, preservation, and protection
The conservation of biological diversity has become a global concern. Although not everybody agrees on extent and significance of current extinction, most consider biodiversity essential.
There are basically two main types of conservation options, in-situ conservation and ex-situ conservation. An example of an in-situ conservation effort is the setting-up of protection areas. An example of an ex-situ conservation effort, by contrast, would be planting germplasts in seedbanks. Such efforts allow the preservation of large populations of plants with minimal genetic erosion.
In-situ is usually seen as the ultimate conservation strategy. However, its implementation is sometimes unfeasible. For example, destruction of rare or endangered species' habitats sometimes requires ex-situ conservation efforts. Furthermore, ex-situ conservation can provide a backup solution to in-situ conservation projects. Some believe both types of conservation are required to ensure proper preservation.
Juridical status of biological diversity
Biodiversity must be evaluated, through observations, inventories, and so forth, if political decisions are to take it into account. Biodiversity is beginning to receive a juridical setting, defined by the following laws.
- "Law and ecosystems" relationship is very ancient and has consequences for biodiversity. It is related to property rights, private and public. It can define protection for threatened ecosystems, but also some rights and duties (for example, fishing rights, hunting rights).
- "Laws and species" is a more recent issue. It defines species that must be protected because they are threatened by extinction. Some people question application of these laws. The U.S. Endangered Species Act is an example of an attempt to address the "law and species" issue.
- "Laws and genes" is an approach only about a century old. While the genetic approach is not new (domestication, plant traditional selection methods), progress made in the genetic field in the past 20 years led to the obligation to tighten laws. With the new technologies of genetic engineering, people are going through gene patenting, processes patenting, and a totally new concept of genetic resource. A very hot debate today seeks to define whether the resource is the gene, the organism, the DNA, or the processes.
The 1972 UNESCO convention established that biological resources, such as plants, were the common heritage of mankind. These rules probably inspired the creation of great public banks of genetic resources, located outside the source-countries. New global agreements (e.g., Convention on Biological Diversity), now give sovereign national rights over biological resources (not property). The idea of static conservation of biodiversity is disappearing and being replaced by the idea of dynamic conservation, through the notion of resource and innovation.
The new agreements commit countries to conserve biodiversity, develop resources for sustainability, and share the benefits resulting from their use. Under these new rules, it is expected that bioprospecting or collection of natural products has to be allowed by the biodiversity-rich country, in exchange for a share of the benefits.
Sovereignty principles can rely upon what is better known as Access and Benefit Sharing Agreements (ABAs). The Convention on Biological Diversity spirit implies a prior informed consent between the source country and the collector, to establish which resource will be used and for what, and to settle on a fair agreement on benefit sharing. Bioprospecting can become a type of biopiracy when those principles are not respected.
Uniform approval for use of biodiversity as a legal standard has not been achieved, however. At least one legal commentator has argued that biodiversity should not be used as a legal standard, arguing that the multiple layers of scientific uncertainty inherent in the concept of biodiversity will cause administrative waste and increase litigation without promoting preservation goals. (See Bosselman 2004.)
Criticisms of the biodiversity paradigm
The “founder effect”
The field of biodiversity research has often been criticized for being overly defined by the personal interests of the founders (such as terrestrial mammals), giving a narrow focus, rather than extending to other areas where it could be useful. This is termed the "founder effect" (Irish and Norse 1996). France and Rigg reviewed biodiversity research literature in 1998 and found that a there was a significant lack of papers studying marine ecosystems, leading them to dub marine biodiversity research the "sleeping hydra."
Biodiversity researcher Sean Nee (2004) points out that the vast majority of earth's biodiversity is microbial, and that contemporary biodiversity science is "firmly fixated on the visible world" (Nee uses "visible" as a synonym for macroscopic). For example, microbial life is very much more metabolically and environmentally diverse than multicellular life.
- Bosselman, F. 2004. A dozen biodiversity puzzles. N.Y.U. Environmental Law Journal 364.
- France, R., and C. Rigg. 1998. Examination of the 'founder effect' in biodiversity research: patterns and imbalances in the published literature. Diversity and Distributions 4:77–86.
- Gaston, K. J., and J. I. Spicer. 2004. Biodiversity: An Introduction. 2nd ed. Blackwell Publishing. ISBN 1-4051-1857-1
- Irish, K. E., and E. A. Norse. 1996. Scant emphasis on marine biodiversity. Conserv. Biol. 10:680.
- Lomborg, B. 2001. The Skeptical Environmentalist. United Kingdom: University of Cambridge Press.
- Nee, S. 2004. More than meets the eye. Nature 429:804–805. doi: 10.1038/429804a online version
- Raup, D. M., and J. J. Sepkoski. 1984. Periodicity of extinction in the geologic past. Proceedings of the National Academy of Science 81:801–805.
- Schultz, J. 1995. The Ecozones of the World: The Ecological Divisions of the Geosphere. Berlin: Springer-Verlag. ISBN 3-540-58293-2
- Smith, A. B. 2001. Large-scale heterogeneity of the fossil record: implications for Phanerozoic biodiversity studies. Philosophical Transactions of the Royal Society B: Biological Sciences 356(1407):351–367.
- United Nations Environment Program. 1995. Global Biodiversity Assessment. ISBN 0-521564816. Used as a source by "Biodiversity," Glossary of terms related to the CBD, Belgian Clearing-House Mechanism (retrieved April 26, 2006).
- Vitousek, P. M., H. A. Mooney, J. Lubechenco, and J. M. Melillo. 1997. Human domination of Earth's ecosystems. Science 277:494–499.
- Whittaker, R. H. 1970. Communities and Ecosystems. New York: MacMillan Company.
- Whittaker, R. H. 1972. Evolution and measurement of species diversity. Taxon. 21,213–251.
- Wilson, E. O., ed. 1988. Biodiversity. National Academy Press. ISBN 0-309037832; ISBN 0-309037395 (pbk.) online edition
- Wilson, E. O. 1992. The Diversity of Life. Cambridge, MA: Belknap Press of Harvard University Press.
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