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Home , Alpha diversity, Biodiversity, Gamma diversity

, DEFINITION OF

Ian R. Swingland The Durrell Institute of Conservation and

I. What Is Biodiversity? that is associated with a given or II. Definition of Biodiversity and that may increase awareness of the need for III. conservation action. IV. Diversity genetic diversity Genetic variety found within or V. among species; this diversity allows the VI. Biodiversity: Meaning and Measurement or species to adapt and evolve in response to chang- VII. Biodiversity: Changes in and ing environments and pressures. VIII. Loss of Biodiversity and Causes Species that has a disproportionately IX. Maintaining Biodiversity greater effect on the ecological processes of an eco- X. Contextual Variations of the Definition system, and whose loss would result in significantly XI. Implications of Variations in the Definition greater consequences for other species and biotic in- teractions. organismal (species) diversity Number and relative GLOSSARY of all species living in a given area. Absolute number of species living in biodiversity/biological diversity Species, genetic, and a given area (also called ), giving equal ecosystem diversity in an area, sometimes including weight to all resident species. associated abiotic components such as fea- use values Values that are obtained by using a natural tures, drainage systems, and . , such as timber, fuelwood, , and - diversity indices Measures that describe the different scapes. These include direct, indirect, option, and components of biodiversity, such as species richness nonuse values. (alpha diversity), beta and , ende- micity, and higher richness. ecosystem diversity Diversity of , , and the accompanying ecological processes that THE WORD BIODIVERSITY IS A MODERN CON- maintain them. TRACTION OF THE TERM BIOLOGICAL DIVERSITY. endemicity State of a species or other taxon being re- Diversity refers to the range of variation or variety or stricted to a given area, such as a specific habitat, differences among some set of attributes; biological di- region, or continent. versity thus refers to variety within the living world or flagship species Charismatic or well-known species among and between living .

Encyclopedia of Biodiversity, Volume 1 Copyright  2001 by Academic Press. All rights of in any form reserved. 377 378 BIODIVERSITY, DEFINITION OF

I. WHAT IS BIODIVERSITY? authors), which has been defined as the number of species in an area and their relative abundance (Pie- The term ‘‘biodiversity’’ was first used in its long version lou, 1977). (biological diversity) by Lovejoy (1980) and is most DeLong (1996) offered a more comprehensive defi- commonly used to describe the number of species. Rec- nition: ognizing that conventional methods of determining, and separating, species were inadequate, others elabo- Biodiversity is an attribute of an area and spe- rated the definition by including the variety and vari- cifically refers to the variety within and among ability of living organisms. living organisms, assemblages of living organisms, These reduced and simple definitions, which em- biotic communities, and biotic processes, whether brace many different parameters, have been much elab- naturally occurring or modified by . Bio- orated and debated in the last three decades (see Section diversity can be measured in terms of genetic II); upon this definition hangs the outcome of important diversity and the identity and number of different scientific considerations, international agreements, types of species, assemblages of species, biotic conventions, conservation initiatives, political debates, communities, and biotic processes, and the and socio-economic issues. Indeed, while the word ‘‘bio- amount (e.g., abundance, , cover, rate) diversity’’ has become synonymous with on , and structure of each. It can be observed and the term is commonly used in the fields of politics measured at any spatial scale ranging from mi- and in addition to various crosites and habitat patches to the entire bio- scientific disciplines (Ghilarov, 1996). The U.S. Strategy sphere. Conference on Biological Diversity (1981) and the Na- tional Forum on Biodiversity (1986) in Washington, This definition allows for modification according to D.C., were the critical debates in crafting a definition, the context in which it is used. and it was the proceedings from the latter, edited by Various authors have proposed specific and detailed E. O. Wilson, that ‘‘launched the word ‘biodiversity’ elaborations of this definition. Gaston and Spicer (1998) into general use’’ (Harper and Hawksworth, 1994). proposed a three-fold definition of ‘‘biodiversity’’— In measuring biodiversity, it is necessary to decon- ecological diversity, genetic diversity, and organismal struct some of the separate elements of which biodiver- diversity—while others conjoined the genetic and or- sity is composed. It has become widespread practice ganismal components, leaving genetic diversity and to define biodiversity in terms of , species, and ecological diversity as the principal components. These ecosystems, for example, ‘‘the abundance, variety, and latter two elements can be linked to the two major genetic constitution of native and ’’ (Dod- ‘‘practical’’ value systems of direct use/ and indi- son et al., 1998). Biodiversity also encompasses all five rect use/ecological described by Gaston and Spicer living kingdoms, including fungi. However, biodiver- (1998). Other workers have emphasized a hierarchical sity does not have a universally agreed on definition approach or hierarchies of life systems. and it is often re-defined on each occasion according In contrast, some argue that biodiversity, according to the context and purpose of the author. to the definition of biological, does not include the diversity of abiotic components and processes, and that it is inaccurate to identify ecological processes, ecosys- tems, ecological complexes, and as compo- II. DEFINITION OF BIODIVERSITY nents of biodiversity. The term ecological, as used in the sense of ecological system (ecosystem), encom- ‘‘Biodiversity’’ is a relatively new compound word, but passes both biotic and abiotic components and pro- biological diversity (when referring to the number of cesses. Therefore, ecological diversity is a more appro- species) is not. Over the last decade its definition has priate term for definitions that include the diversity of taken a more reductionist turn. Possibly the simplest ecological processes and ecosystems. However, ecologi- definition for biodiversity, lacking in specificity or con- cal processes, it has been argued, should be included text, is merely the number of species. Yet many have in the definition of biodiversity, the reasoning being argued that biodiversity does not equate to the number that ‘‘although ecological processes are as much abiotic of species in an area. The term for this measure is as biotic, they are crucial to maintaining biodiversity.’’ species richness (Fiedler and Jain, 1992), which is only Similarly, a U.S. Bureau of advisory one component of biodiversity. Biodiversity is also more group included ecological processes in their definition than (simply called diversity by some of biodiversity in response to criticism that the Office BIODIVERSITY, DEFINITION OF 379 of Technology Assessment’s (1987) definition did not full scope of what the term means, not just what can consider ecosystem form and . Other writers be measured and managed. In contrast, monitoring or point out that even though ecological processes are management objectives must be attainable to be effec- often cited as being crucial to maintaining biodiversity tive. Recognizing the distinction between a definition (Reid and Miller, 1989; Noss and Cooperrider, 1994; and management objectives should reduce the confu- Samson and Knopf, 1994), this does not warrant the sion between the meaning of biodiversity and the objec- inclusion of ecological processes into the meaning of tives for achieving biodiversity goals. biodiversity. For example, Reid and Miller (1989) and Biodiversity is a broad totality and often embraces Agarwal (1992) distinguished between biodiversity and elements beyond species diversity or numbers. For ex- the processes and ecological diversity that maintain it. ample, a major debate in biological sciences over many Nevertheless, the jargon word ‘‘biodiversity’’ is, by its decades has been that of pattern versus process, espe- very origin, fundamentally indefinable, being a populist cially in and evolutionary studies. Molecu- word invented for convenience. Its invention has had lar and systematics have enabled ecologists to beneficial effects by fuelling research projects, mainly see that inferred history is important in framing appro- in ecology and systematics, and have been priate questions, and this understanding has precipi- drawn into contributing to the debate by the need to tated a real integration of these twin hierarchies— show that biodiversity is useful to humans and neces- pattern (e.g., diversity) and process (e.g., ). sary for the proper functioning of ecosystems. Conser- Fundamental divisions remain, such as ‘‘straight’’ parsi- vation (i.e., management) of biodiversity is axiomatic mony (i.e., pattern) versus maximum likelihood (i.e., to these two concerns and lies behind the scientific process) in the phylogenetic interpretation of se- need to define the term within whatever context is quence data. appropriate, since no general definition will be suitable It is apparent that the term biodiversity still lacks when applied across a range of situations. consistent meaning within the field of Biodiversity conservation requires the management management. Michael Soule´ found it shocking that of natural , and this in turn requires the mea- ‘‘we are still trying to define biological diversity after surement of these resources. Biodiversity measurement all of the efforts of the Office of Technology Assess- implies the need for some quantitative value that can be ment and E. O. Wilson’s book, Biodiversity’’ (Hudson, ascribed to the various measurements so these values can 1991). It is still defined in different ways by different becompared.Amongthe firstscientiststomeasurediver- people; some characterize biodiversity as being a sity were Fisher, Corbet, and Williams (1943), who ap- widely used term ‘‘having no unified definition’’ and proximated the frequency distribution of the species rep- others emphasize or limit the meaning of biodiversity resented by 1,2,3,4. . . (and so on) individuals by the to that of native biodiversity. Some writers have logarithmic series Ͱx, Ͱx2/2, Ͱx3/3, Ͱx4/4...,where included alterations of biological communities the constant Ͱ has been found to be a measure of species in the scope of biodiversity (Bryant and Barber, 1994). diversity. Species diversity is low when the number of Angermeier (1994) argued that ‘‘the absence of a species is growing slowly with respect to the increase in ‘native’ criterion within the definition [of biodiversity] number of individuals, and it is high when the number severely compromises biodiversity’s utility as a mean- of species is growing quickly. ingful biological concept,’’ reasoning that native bio- If the need to quantify biodiversity drives the funda- diversity is more valuable than artificial diversity and mental meaning of biodiversity, the definition may be should be the primary focus of conservation efforts. limited to that which can be readily measured given The conservation of native biodiversity appears to be current understanding and technologies. Such a defini- the theme of biodiversity conservation texts (Wilson tion of biodiversity could change over time as ideas, and Peter, 1988; Hunter, 1996). Conversely, others technology, and resources for measuring diversity argued that an important component of biodiversity change. DeLong (1996) suggested that an operational is maintained by traditional farming techniques. In ‘‘clause’’ should be added to the definition of biodiver- the context of conserving biodiversity, Reid and Miller sity, namely, that ‘‘biodiversity is. . .as measured in (1989) and Bryant and Barber (1994) discussed the terms of. . . .’’ This approach provides a link to man- importance of genetic diversity within species of culti- agement while distinguishing between what biodiver- vated plants. Biodiversity within agricultural plants is sity is (a state or attribute) and how it is measured. It important for pest management in agroecosystems also allows the operational clause to be adjusted over and sustainable . time without changing the fundamental meaning of the An accepted fundamental definition of biodiversity term. A definition of biodiversity should portray the is needed for conservation planning, as are effective 380 BIODIVERSITY, DEFINITION OF communication and co-operation within and among level. None of these factors includes any explicit assess- different countries, governments, agencies, disciplines, ment of genetic diversity. organizations, and private landowners. Co-operation Although the word biodiversity has already gained among these entities has been identified as being neces- wide currency in the absence of a clear and unique sary for the conservation of biodiversity (Babbitt, 1994). meaning, greater precision will be required of its users Knopf (1992) asserted that the definitions of biodiver- if policy and programs are to be more effectively defined sity are ‘‘as diverse as the biological resource.’’ Defini- in the future. tions of biodiversity range in scope from ‘‘the number of different species occurring in some location’’ to ‘‘all of the diversity and variability in ’’ and ‘‘the vari- III. GENETIC DIVERSITY ety of life and its processes.’’ A more comprehensive definition is ‘‘the variety of living organisms, the genetic Genetic diversity is reliant on the heritable variation differences among them, the communities and ecosys- within and between of organisms. New tems in which they occur, and the ecological and evolu- arises in individuals by and chro- tionary processes that keep them functioning, yet ever mosome , and in organisms with sexual re- changing and adapting’’ (Noss and Cooperrider, 1994). production it can be spread through the population by This plethora of terms and definitions is one of the recombination. It has been estimated that in humans major stumbling blocks to reaching agreement in prob- and fruit flies alike, the number of possible combina- lem solving and decision making. If entities in a plan- tions of different forms of each gene sequence exceeds ning process view biodiversity in fundamentally differ- the number of in the . Other kinds of ent ways, agreement on management objectives and genetic diversity can be identified at all levels of organi- strategies for biodiversity conservation will be impaired. zation, including the amount of DNA per and chro- (Swingland, 1999). mosome structure and number. Selection acts on this The differences between these conceptual perspec- pool of genetic variation present within an inter- tives on the meaning of biodiversity, and the associated breeding population. Differential survival results in semantic problems, are not trivial. Management in- changes of the frequency of genes within this pool, tended to maintain one facet of biodiversity will not and this is equivalent to population evolution. Genetic necessarily maintain another. For example, a timber variation enables both natural evolutionary change and extraction program that is designed to conserve biodi- artificial to occur (Thomas, 1992). versity in the sense of site species richness may well Only a small fraction (Ͻ1%) of the genetic material reduce biodiversity measured as genetic variation of higher organisms is outwardly expressed in the form within the species harvested. Clearly, the mainte- and function of the ; the purpose of the re- nance of different facets of biodiversity will require maining DNA and the significance of any variation different management strategies and resources, and will within it are unclear (Thomas, 1992). Each of the esti- meet different human needs. mated 109 different genes distributed across the world’s Even if complete knowledge of particular areas could biota does not make an identical contribution to overall be assumed, and standard definitions of diversity are genetic diversity. In particular, those genes that control derived, the ranking of such areas in terms of their fundamental biochemical processes are strongly con- importance with respect to biological diversity remains served across different taxa and generally show little problematic. Much depends on the scale that is being variation, although such variation that does exist may used. Thus, the question of what contribution a given exert a strong effect on the viability of the organism; area makes to global biological diversity is very different the converse is true of other genes. A large amount of from the question of what contribution it makes to molecular variation in the mammalian immune system, local, national, or regional biological diversity. This is for example, is possible on the basis of a small number because, even using a relatively simplified measure, any of inherited genes (Thomas, 1992). given area contributes to biological diversity in at least three different ways—through its richness in numbers of species, through the (or geographical IV. SPECIES DIVERSITY uniqueness) of these species (e.g., Mittermeier et al., 1992), and on the basis of degree of threat. The relative Historically, species are the fundamental descriptive importance of these three factors will inevitably change units of the living world and this is why biodiversity at different geographical scales, and sites of high re- is very commonly, and incorrectly, used as a synonym gional importance may have little significance at a global of species diversity, in particular of ‘‘species richness,’’ BIODIVERSITY, DEFINITION OF 381 which is the number of species in a site or habitat. does a European alpine that may have no other Discussion of is typically presented species wholly dependent on it. in terms of global numbers of species in different taxo- nomic groups. An estimated 1.7 million species have been described to date; estimates for the total number V. ECOSYSTEM DIVERSITY of species existing on earth at present vary from 5 million to nearly 100 million. A conservative working While it is possible to define what is in principle meant estimate suggests there might be around 12.5 million. by genetic and species diversity, it is difficult to make When considering species numbers alone, life on a quantitative assessment of diversity at the ecosystem, earth appears to consist mostly of and microor- habitat, or level. There is no unique defini- ganisms. The species level is generally regarded as the tion or classification of ecosystems at the global level, most natural one at which to consider whole-organism and it is difficult in practice to assess ecosystem diversity diversity. While species are also the primary focus of other than on a local or regional basis, and then only evolutionary mechanisms, and the origination and ex- largely in terms of vegetation. Ecosystems are further tinction of species are the principal agents in governing divorced from genes and species in that they explicitly biological diversity, species cannot be recognized and include abiotic components, being partly determined enumerated by systematists with total precision. The by /parent material and climate. concept of what a species is differs considerably among To get around this difficulty, ecosystem diversity is groups of organisms. It is for this reason, among others, often evaluated through measures of the diversity of that species diversity alone is not a satisfactory basis the component species. This may involve assessment on which to define biodiversity. of the relative abundance of different species as well as Another reason why a straightforward count of the consideration of the types of species. The more that number of species provides only a partial indication of species are equally abundant, then the more diverse biological diversity concerns the concept of degree or that area or habitat. Weight is given to the numbers of extent of variation that is implicit within the term bio- species in different size classes, at different trophic lev- diversity. By definition, organisms that differ widely els, or in different taxonomic groups. Thus a hypotheti- from each other in some respect contribute more to cal ecosystem consisting only of several plant species overall diversity than those that are very similar. The would be less diverse than one with the same number greater the interspecific differences (e.g., by an isolated of species but that included and pred- position within the taxonomic hierarchy), then the ators. Because different weightings can be given to these greater contribution to any overall measure of global different factors when estimating the diversity of partic- biological diversity. Thus, the two species of Tuatara ular areas, there is no one authoritative index for mea- (genus Sphenodon) in New Zealand, which are the only suring ecosystem diversity. This obviously has impor- extant members of the order Rhynchocephalia, tant implications for the conservation ranking of are more important in this sense than members of some different areas. In examining (i.e., the highly species-rich family of lizards. A site with many change in species composition between areas), the only different higher taxa present can be said to possess more reliable predictor of community similarity is to compare taxonomic diversity than another site with fewer higher the species composition of the site immediately ad- taxa but many more species. Marine habitats frequently jacent. have more different phyla but fewer species than terres- trial habitats, that is, higher taxonomic diversity but lower species diversity. By this measure, the Bunaken VI. BIODIVERSITY: MEANING reef off the north of Sulawesi has the highest biodiversity on earth. Current work is attempting to AND MEASUREMENT incorporate quantification of the evolutionary unique- ness of species into species-based measures of biodi- A. Species Diversity versity. A. S. Corbet, upon analyzing a large collection of butter- The ecological importance of a species can have a flies from Malaya, remarked on the decrease in number direct effect on community structure, and thus on over- of new species with an increasing number of individu- all biological diversity. For example, a species of tropical als. He thought that the resulting distribution could be tree that supports an endemic described by a hyperbola, but R. A. Fisher, to whom of a hundred species makes a greater contribution Corbet sent his results, suggested that a negative bino- to the maintenance of global biological diversity than mial distribution would be much more appropriate 382 BIODIVERSITY, DEFINITION OF

(Williams, 1964). As mentioned earlier, Fisher, Corbet, distribution of relative abundance. A great variety of and Williams (1943) approximated the frequency dis- indices were proposed that assess the number of species tribution of the species represented by 1,2,3,4. . . (and and the proportions in abundance of different species. so on) individuals by the logarithmic series Ͱx, Ͱx2/2, Among others, there was the very popular index that Ͱx3/3, Ͱx4/4...,where the constant Ͱ is a measure is based on Shannon’s formula derived from informa- of species diversity. Species diversity is low when the tion theory: number of species rises slowly with an increase in the ϭ⌺ number of individuals, and diversity is high when the H pi log pi number of species rises quickly.

Species diversity measurement was thus clearly where pi is the proportion of the total number of individ- formulated more than 50 years ago and a particular uals that belong to the ith species. index was proposed. Fisher et al. attempted to find In a seminal work on the measurement of diversity, some general ‘‘rule’’ or ‘‘law’’ according to which the Whittaker (1972) introduced the concepts of alpha, numerical abundances of different species were related beta, and gamma diversity. The measurements just de- to each other. In many communities, the number of scribed, giving diversity values for single sites, are ex- species with given abundance could be approximated amples of alpha diversity. The beta and gamma diversity by the log-normal distribution. If species are classified concepts relate to changes in diversity between sites in accordance with their abundance in logarithmically at local (beta) and geographical (gamma) scales. An increasing classes—so-called ‘‘octaves’’ (i.e., the first essential part of these relational concepts is the idea of octave contains 1–2 individuals, the second contains species turnover—the degree to which species replace 2–4 individuals, the third has 4–8, the fourth has other species at different sites. For use in assessing the 8–16, and so on)—then the number of species per relative value of multiple sites for the conservation of ‘‘octave’’ shows a truncated normal distribution. If a biodiversity, the idea of species turnover is translated sample contains a high number of species and individ- into the principle of complementarity (see Section uals, we can usually obtain a log-normal distribution, VIII,A), which can be implemented in combination with and it is obviously more tractable than the logarith- a taxonomic . mic series. MacArthur (1957) went further by proposing an in- teresting model that assumed that boundaries between B. Taxonomic Diversity niches in resource–niche hypervolume are set at ran- Biodiversity measurements that measure genetic differ- dom, whereas the relative abundances of species are ence directly, or indirectly through use of the taxonomic proportional to these sections of hypervolume. This (cladistic) hierarchy (Williams et al., 1991), are cur- model became widely known as the ‘‘broken-stick’’ or rently being used. The indirect taxonomic approach is MacArthur’s model. The distribution of abundance pre- more practical because we already have a ‘‘rule of scribed by MacArthur’s model is much ‘‘flatter’’ (i.e., thumb’’ taxonomic hierarchy (which is being steadily the contrast between given species and the next in the improved through the application of cladistic analysis, sequence is less) than in the case of a logarithmic series notably to molecular data), whereas reliable estimates (Ghilarov, 1996). of overall genetic differences between taxa are virtually It has become clear that there is no universal type non-existent (abridged from Vane-Wright, 1992). of distribution of relative abundance that corresponds Based on the shared and unshared nodes between to all real communities, though such distributions taxa (equivalent to position in the taxonomic hierar- change in the course of succession according to a partic- chy), a number of taxonomic diversity indices have ular pattern. The of a few of the most abun- now been developed. Of these, the most distinct are dant species is more pronounced at the early stages root weight, higher taxon richness, and taxonomic dis- of succession, while later the species of intermediate persion. The first places highest individual value on abundance become more significant (Whittaker, 1972). taxa that separate closest to the root of the cladogram A comprehensive understanding of the underlying and comprise only one or relatively few species; in effect mechanisms that result in a given pattern of species this gives high weighting to relict groups (Vane-Wright, abundance still eludes scientists. 1996). Higher taxon richness favors taxa according to Another line of species diversity studies was con- their rank and number of included species. Dispersion, nected with the use of special indices proposed to mea- the most complex of the measures proposed so far (Wil- sure diversity without reference to some hypothetical liams et al., 1991), endeavors to select an even spread BIODIVERSITY, DEFINITION OF 383 of taxa across the hierarchy, sampling a mixture of high, by using one or more indices combining species rich- low, and intermediate ranking groups. ness and relative abundance within an area. Some at- For a given group these measures, together with tempts have also been made to measure change in spe- simple species richness if desired, can be used to com- cies richness (species turnover) between areas. These pare the biotic diversity of any number of sites. The solutions to the problem of measuring biodiversity are measures can also be expressed as percentages. Thus a limited because species richness takes no account of site with viable populations of all species in a group the differences between species in relation to their place would have a diversity score of 100%, whereas a site in the natural hierarchy. Moreover, relative abundance without any species of the group in question would is not a fixed property of a species, for it varies widely score zero. In reality, of course, most sites have only a from time to time and place to place. In many environ- selection of species, and so receive various intermediate ments most taxa are virtually or even completely un- scores. Such assessments allow us to compare all sites known. with each other, and rank them individually from high- Conservation , or applied ecologists, have est to lowest diversity (Vane-Wright, 1996). However, called for a measurement of diversity that is more clearly if we then take some conservation action (such as con- related to overall genetic difference. An example con- serving a particular site), the same measures are un- cerns the problem of differential . In World likely to be directly comparable for making a second Conservation Strategy (IUCN/UNEP/WWF, 1980), it is decision (such as choosing a second conservation site). noted that ‘‘the size of the potential genetic loss is related This is because, in most real situations at least, there to the taxonomic hierarchy because. . .different posi- will be considerable overlap in the presence of species tions in this hierarchy reflect greater or lesser degrees at particular sites. of genetic difference....Thecurrent taxonomic hier- archy provides the only convenient rule of thumb for determining the relative size of a potential loss of ge- C. Community Diversity netic material.’’ Early ecologists did not confine themselves to measur- ing species diversity. They also tried to understand the relationship of diversity with other features of D. Synthesis the community (e.g., Williams, 1964; Whittaker, A model incorporating island biogeographic theory, 1972). The dependence of species diversity on the species abundance, and , and that produces structural complexity of the environment was demon- a fundamental biodiversity number (␪) that is closely strated (MacArthur and MacArthur, 1961), as was associated with species richness and abundance in an the role of (Addicott, 1974) and periodical equilibrium meta-population, has been proposed in (Sousa, 1979) in determining a given Hubbell’s unified theory (1997). This model assumes level of diversity. The relationship between the species zerosum community dynamics or a saturated, totally diversity and standing crop of a community was also local community, which limits its applica- shown (Ghilarov and Timonin, 1972). tion, but it advances the study of species richness and Margalef (1957) was the first to use the Shannon relative abundance if others can extend its usefulness index (though expressed in a different form). He pro- to the nonequilibrium systems that characterize the posed to evaluate the level of community organization real world. in terms of information theory. Margalef stimulated many ecologists to quantitatively measure the species diversity of different communities and/or of the same VII. BIODIVERSITY: CHANGES IN TIME community in different stages of its development. At that time, there was a widespread belief that with a AND SPACE single numerical value, an assessment could be made of some very significant feature of community structure. A. Changes Over Time Many ecologists believed that in measuring species di- The record is very incomplete, which emphasizes versity at the community level they were using an ap- the marked variation between higher taxa and between proach that was fundamental to an understanding of species in different ecosystems in the extent to which diversity (Ghilarov, 1996). individuals are susceptible to preservation and subse- Ecologists have measured diversity either by estimat- quent discovery. Chance discovery has played a large ing species richness (number of species) in an area, or part in compiling the known fossil record, and interpre- 384 BIODIVERSITY, DEFINITION OF tation by paleontologists of the available material is areas that have been under human influence for ex- beset by differences of opinion. Thus, the record is tended periods, notably in temperate regions, mainte- relatively good for shallow-water, hard-bodied marine nance of existing levels of diversity may involve the , but poor for most other groups, such as maintenance of at least partially man-made landscapes plants in moist tropical uplands. and ecosystems, mixed with adequately sized areas of Two relevant points appear to be well substantiated. natural climax ecosystems. First, taxonomic diversity, as measured by the number of recognized phyla of organisms, was greater in Cambrian than in any later period. Second, it appears that species diversity and the number of families have under- VIII. LOSS OF BIODIVERSITY gone a net increase between the Cambrian and Pleisto- AND CAUSES cene epochs, although interrupted by isolated phases of mass extinction (few of which are reflected in the Species extinction is a natural process that occurs with- fossil record of plants). out the intervention of humans since, over geological time, all species have a finite span of existence. Extinc- tions caused directly or indirectly by humans are oc- B. Changes in Space curring at a rate that far exceeds any reasonable esti- Species diversity in natural habitats is high in warm mates of background extinction rates, and to the extent areas and decreases with increasing and alti- that these are correlated with habitat pertur- tude; additionally, terrestrial diversity is usually higher bation, they must be increasing. in areas of high rainfall and lower in drier areas. The Quantifying rates of species extinction is difficult richest areas are tropical moist forest and, if current and predicting future rates with precision is impossible. estimates of the number of microfaunal species (mainly The documentation of definite species extinctions is insects) of tropical moist are credible, then these only realistic under a relatively limited set of circum- areas, which cover perhaps 7% of the world’s surface stances, for example, where a described species is area, may well contain over 90% of all species. If the readily visible and has a well-defined range that can be diversity of larger organisms only is considered, then surveyed repeatedly. Unsurprisingly, most documented coral reefs such as Bunaken (see earlier) and, for plants extinctions are of species that are easy to record and at least, areas with a Mediterranean climate in South that inhabit sites that can be relatively easily invento- and Western may be as diverse. Gross ried. The large number of extinct species on oceanic genetic diversity and ecosystem diversity will tend to islands is not solely an artifact of recording, because be positively correlated with species diversity. island species are generally more prone to extinction What are not fully understood are the reasons for as a result of human actions. the large-scale geographic variation in species diversity, Most global extinction rates are derived from extrap- and in particular for the very high species diversity of olations of measured and predicted rates of habitat loss, tropical moist forests. The origin of diversity through and estimates of species richness in different habitats. the evolution of species and the maintenance of this These two estimates are interpreted in the light of a diversity both need more study before they are better principle derived from island , which understood. This will require consideration of the pres- states that the size of an area and of its species comple- ent and historic (in a geological or evolutionary sense) ment tend to have a predictable relationship. Fewer conditions prevailing in particular areas, principally cli- species are able to persist in a number of small habitat matic but also edaphic and topographic. Climatically fragments than in the original unfragmented habitat, benign conditions (warmth, moisture, and relative asea- and this can result in the extinction of species (MacAr- sonality) over long periods of time appear to be particu- thur and Wilson, 1967). These estimates involve large larly important. degrees of uncertainty, and predictions of current and Climax ecosystems will be more diverse than areas future extinction rates should be interpreted with con- at earlier successional stages, but an area with a mosaic siderable caution. The pursuit of increased accuracy in of systems at different successional stages will probably the estimation of global extinction rates is not crucial. be more diverse than the same area at climax provided It is more important to recognize in general terms the that each system occupies a sufficiently large area of extent to which populations and species that are not its own. In many instances, human activities artificially monitored are likely to be subject to fragmentation and maintain ecosystems at lower successional stages. In extinction (Temple, 1986). BIODIVERSITY, DEFINITION OF 385

Loss of biodiversity in the form of domesticated ani- within the next century as a result of changes in global mal breeds and plant varieties is of little significance climate and patterns. These will cause greatly in terms of overall global diversity, but elevated extinction rates. in these populations is of particular human concern in so far as it has implications for supply and the of locally adapted agricultural practices. For domesticated populations, the loss of wild relatives IX. MAINTAINING BIODIVERSITY of crop or timber plants is of special concern for the same reason. These genetic resources may not only A. In Situ Conservation underlie the of local agricultural systems The maintenance of biological diversity is the sustain- but may also, when incorporated into breeding pro- able management of viable populations of species or grams, provide the foundation of traits (disease resis- populations in situ or ex situ. The maintenance of a tance, nutritional value, hardiness, etc.) that are of significant proportion of the world’s biological diversity global importance in intensive systems and that will only appears feasible by maintaining organisms in their assume even greater importance in the context of future wild state and within their existing range. This allows . Erosion of diversity in crop gene pools for continuing of wild populations by natu- is difficult to demonstrate quantitatively, but can be ral evolutionary processes and, in principle, for current indirectly assessed in terms of the increasing proportion utilization practices to continue. For such maintenance of world cropland planted to high-yielding, but geneti- to succeed, it almost invariably requires enhanced man- cally uniform, varieties. Genetic modification of - agement through the integrated, community-based con- isms, varieties, or for food production, phar- servation of protected areas. maceuticals, and other products, which has caused Over the last thirty years, conservation biologists concern in some countries but not others, may also have struggled with the concept of the maintenance contribute to the loss of biodiversity. of biodiversity in highly diverse environments like Humans exterminate species either directly by hunt- rain forests. Analytical techniques (neural-net models) ing, collection, and persecution or indirectly through that allow us to reconstruct past distributions of and modification. Overhunting is forest types present an opportunity to predict past perhaps the most obvious direct cause of extinction in contractions and expansions of forest forms, and the animals, but it is undoubtedly far less important than likelihood of refugia surviving climate change. Such the indirect causes of habitat modification in terms of extrapolations must be treated with caution, as pollen overall loss of biodiversity. selectively affects samples from (for example) disproved modeling the targeted species, as well as plant and animal species predictions that savanna should have been whose populations are subsequently affected either neg- extant, when in fact tropical and temperate forests atively or positively, and so it has important implica- were present. Various authors also opposed the Pleisto- tions for the management of natural resources. Genetic cene refugia hypothesis (Haffer, 1969) for the Amazon diversity in a hunted population is liable to decrease region because some evidence demonstrated the lack as a result of the same factors. The genetic diversity of rain forest fragmentation during that era. In the represented by populations of crop plants or livestock biogeographical zones of the Australian wet , is also likely to decline as a result of mass production, there is a strong correlation between diversity patterns for the desired economics of scale demand high levels and reputed rain forest refugia in both species and of uniformity. genetic diversity. However, this appears to have been Sustained human activity will affect the relative caused by differential extinction rates in differently abundance of species and in extreme cases may lead to sized refugia rather than by in extinction. This may result from the habitat being made the Pleistocene. Others have emphasized that a greater unsuitable for the species (e.g., clear-felling of forests concentration on the Pliocene or before would be or severe of ) or through the habitat useful, since most tropical species occurred becoming fragmented (discussed earlier). Fragmenta- before the Pleistocene. tion divides previously contiguous populations of spe- The local-determination hypothesis of species diver- cies into small sub-populations. If these are sufficiently sity (Rosenzweig, 1995), which predicts similar species small, then chance processes lead to higher probabilities diversity in similar habitats, has also been challenged. of extinction within a relatively short time. Major In sister taxa of plants, the net diversification was changes in natural environments are likely to occur significantly higher in Asia than in North America 386 BIODIVERSITY, DEFINITION OF for genera shared between the two continents. Greater at a finer level (Balmford et al., 1996a, 1996b). insights into the effects of current ecology on the local However, species richness is not a good measure with diversity of an area may be assisted by considering the which to identify hotspots for conservation because relative ages of , which could establish species it overlooks , although as the sample proliferation rates between regions, thus advancing area for hotspots is increased, more rare species are the local versus regional diversity debate (Ricklefs included as a simple function of arithmetic progres- and Schluter, 1993). They also tested the taxon cycle sion. Rarity and endemicity are efficient indices for theory (Wilson, 1961) using phylogenies of spe- selecting the most parsimonious number of sites, but cies and showed that older species’ lineages had more compared to complementarity measures they are less restricted ranges, smaller habitat breadth, and more useful in defining conservation priorities. fragmented distributions, and were closer to extinction A good conservation measure is complementarity, than younger species. where the species complement of a reserve or area In efforts to conserve biodiversity, preserving genetic is identified and then further sites are found that add dissimilarity is often a higher priority than maintaining the greatest number of new species; this is akin to genes of considerable similarity. Recent work shows the portfolio approach (Swingland, 1997). Another that in increases from the method using integer linear programming to choose headwaters to the mouth as a gets broader and the optimal set of sites (maximal-covering-location; thus becomes a greater barrier to populations on oppo- Church et al., 1996) is limited to small datasets and site banks; this effect promotes species diversity does not achieve the greatest conservation gain for through allopatric speciation. Headwater species are the fewest additional sites. Clearly, combining an basal in the phylogeny, and shared haplotypes occur ecosystem portfolio approach with a richness or ende- only at the headwaters; this research is a contribution mism assessment would be effective, but differing to Wallace’s riverine diversification hypothesis in the approaches are needed according to the conservation . goal and data availability. A central question in the design of effective conserva- tion programs is what geographical regions to protect in order to maintain the most biological diversity. The B. term was coined by Myers (Myers, Viable populations of many organisms can be main- 1990) and most commonly refers to regions of high tained in cultivation or in . Plants may also be species richness. GAP analysis is used to identify gaps maintained in seed banks and germplasm collections; in existing networks (Scott et al., 1993); similar techniques are under development for animals it uses algorithms to select the minimum set of grid (storage of embryos, eggs, and sperm, i.e., ‘‘frozen cells that encompass the unprotected species. Rarity ’’) but are more problematic. Ex situ conservation and endemicity have been used to define hotspots in is extremely costly in the case of most animals, and (Balmford and Long, 1994), and spe- while it would in principle be possible to conserve a cies richness and endemism have been used to rank very large proportion of higher plants ex situ, this would countries (McNeely et al., 1990). Hotspots are also de- be feasible for only a small percentage of the world’s fined as those areas with the greatest number of threat- organisms. Furthermore, it often involves a loss of ge- ened species. netic diversity through founder effects and the high In setting conservation priorities, assumptions are probability of inbreeding (Milner-Gulland and Mace, made that indicator groups (e.g., macro-organisms 1998). such as , mammals, and plants) are good pre- dictors of biological diversity in general. Another question that arises is how best to analyze biodiversity information to generate accurate and useful analyses X. CONTEXTUAL VARIATIONS OF that will inform conservation decisions. On a large scale, some concordance is found between bird diver- THE DEFINITION sity across continents with diversity (Pearson and Cassola, 1992), and in endemism patterns across A. Derivation of ‘‘Biodiversity’’ taxa (Lawton, 1994); but at a finer spatial scale this The definition of biodiversity put forth by the Office correlation begins to break down. Richness in genera of Technology Assessment (1987) appears to be the and families are good predictors of species richness most widely cited basis for other published definitions BIODIVERSITY, DEFINITION OF 387

(Scott et al., 1995). However, the OTA did not explain provide impetus to define biodiversity in quantitative why they defined the term as they did, nor did they terms as Hunter (1996) recommended. cite any supportive documentation. One problem with relying solely on authoritative sources for definitions of biodiversity is that different authorities have defined C. Attributes of Biodiversity the term in fundamentally different ways. Another way of delineating the meaning of a term is to ‘‘Bio’’ is derived from the Greek word bios, meaning list its characteristics, properties, qualities, and parts. life. Biological and biotic are terms that refer to life, Noss (1990) recognized three main attributes of biodi- living organisms, assemblages of living organisms, and versity: composition, structure, and function. the activities and interactions of living organisms. The Composition addresses the identity and richness of scope of the term biological can be further understood biotic components, and the relative amount (e.g., abun- in the context of components and processes that are dance, cover, biomass) of each (Noss, 1990). Biotic considered biological. Defining biodiversity (i.e., diver- components of ecosystems include genes, organisms, sity) is more difficult because it continues to be defined family units, populations, age classes, species and other in several fundamentally different ways. In definitions taxonomic categories, trophic levels of animals (e.g., of biodiversity, diversity has been characterized as (1) herbivores, predators), animal guilds and assemblages, the number of different types of items, (2) the number plant communities, and interacting assemblages of of different types of items and their relative abundance, plants, animals, and (i.e., biotic com- and (3) variety. Characterization of diversity in discus- munities). sions of biodiversity has also included the structural Structural attributes of biodiversity refer to the vari- complexity of landscapes (Huston, 1994). ous vertical and horizontal components of a community or landscape (Noss, 1990) and the organizational levels of plant and animal populations and assemblages (Gas- ton and Spicer, 1998; Hunter, 1996). Considering only B. Classifying Biodiversity biotic, vegetative components of a landscape, horizontal The classification of biodiversity can be divided into structure consists of the size, shape, and spatial arrange- those authors who consider biodiversity to be a state ment and juxtaposition of different plant communities; and those who believe that it is a measure of the state. vertical structure consists of the foliage density and Most authors have defined biodiversity as a state height of different vegetation layers (Noss, 1990). Struc- or attribute, for example, ‘‘biodiversity is the variety ture can also refer to population, age and trophic struc- of. . .’’ or ‘‘variety and variability of. . .’’ (Noss and ture, and other levels of community organization Cooperrider, 1994). Standard dictionaries have (Hunter, 1996). classified diversity as a state, condition, or quality The inclusion of structure in the meaning of biodi- (Soukhanov et al., 1988). versity provides linkages with other concepts, such as Other definitions of biodiversity limited the scope habitat diversity and the plant community concept, for of the attribute to explicit, quantifiable dimensions both of which vegetation structure is an important dif- or measures, for example, ‘‘biodiversity is the number ferentiating attribute. Structure may have been left out of. . .’’ or ‘‘the number and relative abundance of. . .’’ of most definitions of biodiversity because the concept (Office of Technology Assessment, 1987). This empha- of biodiversity evolved from the concept of ecological sis on quantitative, operational definitions of biodiver- diversity, which primarily focused on species diversity sity and criticisms of non-quantitative definitions (Fisher et al., 1943). Interestingly, 20 years ago it was (Angermeier, 1994; Hunter, 1996) may signal a poten- asserted that measurements of diversity should not pre- tial shift in the classification of the term from an attri- clude structural diversity even though the term is most bute to a measure of an attribute. In the ecological and often used in reference to species diversity. Diversity natural resource management literature, Pielou (1977) can also be used in reference to niche width and the and others have treated diversity as a one- or two- structural complexity of habitats. dimensional attribute of a community (e.g., diversity is Biotic functions represent the third component of ‘‘the number of’’ or ‘‘the number and relative abundance biodiversity, and these include processes such as herbi- of’’). More recently, it has been defined as a measure vory, predation, , mortality, production, veg- or index of those attributes; for example, diversity is a etative succession, cycling and flow ‘‘measure of. . .’’ (Noss and Cooperrider, 1994). Opera- through biotic communities, colonization and extinc- tional definitions of biodiversity (Angermeier, 1994) tion, , and (Noss, 1990). Biotic 388 BIODIVERSITY, DEFINITION OF processes can be addressed in terms of the identity and verse large areas to collect representative samples number of different types of processes, as well as the rate of biological material. (e.g., predation rate) at which each process operates. • Direct use values that are non-extractive, but often Diversity of biotic components and processes can be require considerable on-site interaction of the user observed at many biogeographic scales, from microsites with the resource. This includes , recre- and larger-scale landscape elements (e.g., vegetation ation, on-site research, and other major ‘‘non-con- types, habitat types, range sites) to regional landscapes, sumptive’’ activities occurring principally in pro- , continents, hemispheres, and the entire bio- tected areas. These activities are characterized by sphere (Noss, 1990; Huston, 1994; Hunter, 1996). Al- the need to provide food, lodging, and transport to though these are scales at which biodiversity can be the participants. observed, they are not necessarily scales of biodiversity • Indirect use values that accrue on site. The primary because most include abiotic (e.g., geological) features. feature of these values is that they support or pro- Biodiversity can also be observed at several organism- tect the basic functioning of the protected area. Ex- based scales, including individual organisms, popula- amples include nutrient cycling, stabilization of tions, species, and assemblages (e.g., guilds and plant in erosion-prone areas, coastal zone stabiliza- communities), which themselves can be observed at tion, and biological support to local ecosystems. As various biogeographical scales. a result of their nature, the value of these on-site functions is likely to be a component of all the other direct and non-use values generated by the D. Biological Resource Asset and area. • Indirect use values that accrue off site. The value of Management Objectives these functions—such as watershed protection, nat- The contextual variations in the definition of biodiver- ural ecosystems protected as national parks in gen- sity depend on what use is being made of the biological erating income from tourism, protection of resource asset (or bioasset), and thus the asset manage- fisheries’ nurseries and subsistence fisheries, and cli- ment objective. Biological resource values consist of mate regulation—may be very large or very small direct use, indirect use, and option and non-use values. depending on their relative importance to the sup- For the purposes of assessing potential use, they can port or protection of off-site economic activity. be further classified as follows: • Option values. Because option values may be associ- ated with each and every use value, they are consid- • Direct use values of major extractive products. Princi- ered only where they may be of potential signifi- pally, this would include for timber and cance in conjunction with the particular type of commercial fisheries in the case of terrestrial and product or service. marine systems. Extraction of these products often • Nonuse values. By their very nature these values oc- involves substantial investment in capital equip- cur at a distance from the resource and require no ment by large non-local firms, and the products are extraction or physical interaction with the resource, transported and sold in well-developed markets far for example, , ethics, cultural belief, from their original source. and aesthetics. • Direct use values of ‘‘minor’’ extractive products. These are naturally or semi-naturally occurring products that require labor-intensive gathering or These foregoing values are only indirectly related to harvesting activities, often carried out by local peo- biological diversity. That is, a certain level of species ple. Examples include rattan, fuelwood, seaweed, richness is required for these functions but there is not wild , artisanal fisheries, aquarium fish, and necessarily a direct correlation between the value of the medicinal herbs. These may be collected for sale, ecosystem and its diversity. Thus, ecosystems barter, or home . are generally of far lower diversity than adjacent low- • Direct use values that require the extraction of only land terrestrial forests, but in resource terms they are a small amount of biological material for ex situ re- likely to be of comparable value. The savannas of eastern search or storage. This includes extraction of mate- and southern Africa, which are of great importance in rial for biological inventories, germplasm banks, generating revenues from tourism, are less diverse than and industrial research. Extraction is often accom- the moist forests in these countries, which have far less plished during short or long expeditions that tra- potential for tourism. BIODIVERSITY, DEFINITION OF 389

E. Cave Canem or the for this reason it is constantly redefined on nearly ev- ery occasion. One of the many reasons for this state of affairs is At present, humans actively exploit a relatively small that the definition of biodiversity affects objectives in proportion of the world’s biological diversity. Many national and regional research and conservation man- other potential, yet undiscovered, optional and non- agement, and in international funding priorities. One use values of biodiversity exist. These factors support could easily promote a timber extraction or non-timber a precautionary approach to maintaining biological di- forest product program that conserves species richness versity. In this case, the precautionary principle argues (i.e., numbers of species) at the expense of genetic that actions should be taken to prevent further loss of diversity. Indeed, a current research program to stimu- biodiversity and potentially irreversible consequences late or increase the range of tropical tree species not before all biological uncertainties are resolved. Yet in currently in trade, as a way to take the pressure off conserving biodiversity, there must come a point at over-exploited species, may be misguided. It may lead which the projected costs required to protect and main- to increased genetic as well as species impoverishment tain it will outweigh any probable benefits. when expand the number of species they take If species are to be viewed as a resource, and their and select only the best and most mature specimens, maintenance is to be cost-effective, conservation should thus removing the most productive and healthiest ge- concentrate on systems and areas rich in species, and netic stock. on those species known to be useful, or regarded as Apart from the principal definitions of biodiversity having a high probability of being useful. Thus biodiver- discussed earlier, such as the highest number of species sity and its conservation would be defined purely along (i.e., species richness) and the highest level of species operational or cost–benefit lines. This bioasset perspec- endemicity (Myers, 1990) or taxal endemicity (called tive on biodiversity would therefore rest upon economic critical analysis), interpretations of pure or ap- arguments more than biological ones. plied definitions are becoming more common within Biodiversity has been identified as important for eco- the vocabulary of conservation and biodiversity utiliza- system , medicinal values, agricultural purposes, tion when determining biodiversity management priori- and aesthetic and recreational values (Noss and Coo- ties. Some examples are national biodiversity programs perrider, 1994). Noss (1990) characterized an opera- that maintain ‘‘biodiversity portfolios’’; biodiversity de- tional definition as one that is responsive to real-life fined as flagship or keystone species diversity; viability management and regulatory questions, adding that such modeling (population viability analysis) defining the a definition is unlikely to be found for biodiversity. species’ populations to be prioritized; population analy- Angermeier (1994) referred to an operational definition sis defining sustainability and thus defining a species’ in a similar way, and Hunter (1996) suggested that a status; projects that focus on the feasibility of integrat- quantitative definition is needed for monitoring biodiv- ing the targeted species, assemblages, or ecosystems ersity and developing management plans. On the other with the needs of local human populations and sustain- hand, some writers assert that the confounding of defi- able use; and (lastly) political exigency (Swingland, nition and application is partly to blame for the confu- 1997). Although the conservation policy of a country sion over how biodiversity concepts can be practi- may be driven by more pressing needs—family plan- cally implemented. ning, education, politics, internal conflict, financial planning and investment, individual vested interests— current policy and decisions are also being made on the foregoing biodiversity bases rather than along strict XI. IMPLICATIONS OF VARIATIONS IN academic lines. THE DEFINITION Endemicity and species richness are useful starting points in defining priorities on the global level, but The need for an unequivocal and precise meaning of without information on the possibility of extinction biodiversity that is scientifically sensible and universally using viability modeling or population analysis, the ur- applicable is imperative to help guide the design of gency of a given conservation action cannot be assessed. policy and programs for the future, as well as to make Moreover, with the increasing emphasis on the integra- critical decisions in the present. Currently, such a defi- tion of local people into conservation programs to - nition does not exist. As a concept, biodiversity is both mize long-term costs and to provide a more stable basis ubiquitous and useful, particular and confusing; and for the people and their , the poten- 390 BIODIVERSITY, DEFINITION OF tial for community-based conservation, coupled with Balmford, A., Green, M. J. B., and Murray, M. G. (1996a). Using sustainable use, cannot be ignored. Since national or higher-taxon richness as a surrogate for species richness: I. Re- gional tests. Proc. Roy. Soc. London, Ser. B 263, 1267–1274. external funding will generally provide essential sup- Balmford, A., Jayasuriya, A. H. M., and Green, M. J. B. (1996b). Using port for most projects, the ecological importance of an higher-taxon richness as a surrogate for species richness: II. Local area relative to others using an ecosystem diversity (or applications. Proc. Roy. Soc. London, Ser. B 263, 1571–1575. portfolio) approach will be a major selection criterion. Bryant, D., and Barber, C. (1994). In World Resources: 1994–95, The presence of a flagship or keystone species will also pp. 147–164. World Resources Institute/Oxford University Press, be significant in raising such funds. Clearly political New York. Church, R. L., Stoms, D. M., and Davis, F. W. (1996). Reserve selec- exigencies or pure chance can enter the situation, and tion as a maximal covering location problem. Biol. Conservation scientists have yet to articulate whether genetic diver- 76, 105–112. sity should be used as the key measure. In the absence DeLong, D. C. (1996). Defining biodiversity. Wildl. Soc. Bull. 24, of realistic methods of quantifying these biodiversity 738–749. characteristics, they must remain imponderable objec- Dodson, S. I., Allen, T. F. H., Carpenter, S. R., Ives, A. R., Jeanne, tives for the moment. R. L., Kitchell, J. F., Langston, N. E., and Turner, M. G. (1998). Ecology. Oxford University Press, New York. The differing approaches being advocated for biodi- Fiedler, P. L., and Jain, S. K. (eds.). (1992). : versity conservation are not just guided by the available The Theory and Practice of , Preservation and methodologies but are also symptomatic of the underly- Management. Chapman and Hall, New York. ing philosophies. The evolution-based approach is pre- Fisher, R. A., Corbet, A. S., and Williams, C. B. (1943). The relation dominantly the preserve of biologists, and it is con- between the number of species and the number of individuals in a random sample of an animal population. J. Anim. Ecol. 12, 42–58. cerned with the maintenance of diversity as an Gaston and Spicer (1998). Biodiversity: An Introduction. Blackwell unqualified objective unaffected by economics. The , Oxford, United Kingdom. need for conservation and the uses of biodiversity—the Ghilarov, A. (1996). What does ‘‘biodiversity’’ mean—Scientific prob- resource-based argument—are what are used to ‘‘sell’’ lem or convenient myth? Ecol. Evol. 11, 304–306. the proposition to decision makers and policy-makers. Ghilarov, A., and Timonin, A. G. (1972). Relations between biomass Where these factors come together, the ideal of ecologi- and species diversity in marine and freshwater zooplankton com- munities. Oikos 23, 190–196. cal sustainability and the conservation methods of Haffer, J. (1969). Speciation in Amazon forest birds. Science 165, achieving it will be possible. Because so much is now 131–137. formally invested in using the word biodiversity, its Harper, J. L., and Hawksworth, D. L. (1994). Biodiversity: Measure- definition will continue to play a crucial role in both ment and estimation. Philos. Trans. Roy. Soc. London, Ser. B conservation planning and public policy. 345, 5–12. Hubbell, S. P. (1997). A unified theory of biogeography and relative species abundance and its application to tropical rain forests and See Also the Following Articles coral reefs. Coral Reefs 16, S9–S21. Hudson, W. E. (ed.). (1991). Landscape Linkages and Biodiversity. BIODIVERSITY, ORIGIN OF • ECOLOGY, CONCEPT AND , Washington, D.C. THEORIES OF • ECOSYSTEM, CONCEPT OF • GENETIC Hunter, M. L., Jr. (1996). Fundamentals of Conservation Biology. Black- DIVERSITY • HABITAT AND NICHE, CONCEPT OF • LOSS OF well Science, Cambridge, Massachusetts. BIODIVERSITY, OVERVIEW • MEASUREMENT AND ANALYSIS Huston, M. A. (1994). Biological Diversity: The Coexistence of Species OF BIODIVERSITY • , METHODS OF on Changing Landscapes. Cambridge University Press, New York. IUCN/UNEP/WWF. (1980). World Conservation Strategy: Living Re- source Conservation for . International Bibliography Union for Conservation of Nature and Natural Resources/U.N. Addicott, J. F. (1974). Predation and prey community structure: An Environment Programme/World Wildlife Fund, Gland, Swit- experimental study of larvae on the protozoan commu- zerland. nities of pitcher plants. Ecology 55, 475–492. Knopf, F. L. (1992). Focusing conservation of a diverse wildlife Agarwal, A. (1992). Sociological and political constraints to biodiver- resource. Trans. North Am. Wildl. and Nat. Resources Conf. 57, sity conservation: A case study from . In Conservation of 241–242. Biodiversity for Sustainable Development (O. Sandlund, K. Hindar, Lawton, J. H. (1994). Population dynamic principles. Philos. Trans. and A. H. D. Brown, eds.), pp. 293–302. Scandinavian University Roy. Soc. London, Ser. B 334, 61–68. Press, Oslo, . Lovejoy, T. E. (1980). The Global 2000 Report to the President (G. O. Angermeier, P. L. (1994). Does biodiversity include artificial diver- Barney, ed.), Vol. 2, The Technical Report, pp. 327–332. , sity? Conservation Biol. 8, 600–602. New York. Babbitt, B. (1994). Protecting biodiversity. Nature Conservancy 44, MacArthur, R. H. (1957). On the relative abundance of bird species. 1621. Proc. Natl. Acad. Sci. USA 45, 293–295. Balmford, A., and Long, A. (1994). Avian endemism and forest loss. MacArthur, R. H., and MacArthur, J. (1961). On bird species diversity. Nature 372, 623–624. Ecology 4, 594–598. BIODIVERSITY, DEFINITION OF 391

MacArthur, R. H., and Wilson, E. O. (1967). The Theory of Island Scott, J. M., Davis, F., Csuti, B., Noss, R., Butterfield, B., Grove, C., Biogeography. Princeton University Press, Princeton, New Jersey. Anderson, S, H., Caicco, S., Derchia, F., Edwards, T. C., Jr., Margalef, R. (1957). La teoria de la informacı´on en ecologia. Memorias Ullman, J., and Wright, R. G. (1993). GAP analysis: A geographic de la Real Academia de Ciencias y Artes (Barcelona), 3rd series approach to protection of biological diversity. Wildl. Monogr. 32, 373–449. 123, 1–41. McNeely, J. A., Miller, K. R., Reid, W. V., Mittermeier, R. A., and Soukhanov, A. H., Ellis, K. E., Harris, D. R., DeVenne, P. B., and Werner, T. B. (1990). Conserving the World’s Biological Diversity. Severynse, M. (eds.). (1988). Webster’s II New Riverside University International Union for Conservation of Nature and Natural Dictionary. 1563 pp. The Riverside Publ. Co., New York. Resources/World Resources Institute/Conservation Interna- Sousa, W. P. (1979). Disturbance in marine intertidal boulder fields: tional/World Wildlife Fund–U.S./World Bank, Gland, Switzer- The nonequilibrium maintenance of species diversity. Ecology land/Washington, D.C. 60, 1225–1239. Milner-Gulland, E. J., and Mace, R. (1998). Conservation of Biological Swingland, I. R. (1997). Global conservation and the sciences: People, Resources. Blackwell Science, Oxford, United Kingdom. policy and pennies. In Conservation, Restoration and Management Mittermeier, R. A., Carr, J. L., Swingland, I. R., Werner, T. B., and of Tortoises and (J. Van Abbema, ed.), pp. 453–464. New Mast, R. B. (1992). Conservation of and . In York and Tortoise Society, New York. Herpetology: Current Research on Amphibians and Reptiles. (K. Swingland, I. R. (1999). Commercialisation, structure and sustainabil- Adler, ed.), pp. 59–80. Society for the Study of Amphibians and ity of biodiversity conservation. In New Practices in Integrated Reptiles, Contributions to Herpetology. Protected Area Management (M. Walkey, I. R. Swingland, and S. Myers, N. (1990). Threatened biotas: ‘‘Hotspots’’ in tropical forests. Russell, eds.). Chapman and Hall, London. Environmentalist 8, 1–20. Temple, S. A. (1986). Predicting impacts of Noss, R. F. (1990). Indicators for monitoring biodiversity: A hierar- on forest birds: A comparison of two models. In Wildlife 2000: chical approach. Conservation Biol. 4, 355–364. Modelling Habitat Relationships of Terrestrial (J. Verner, Noss, R. F., and Cooperrider, A. Y. (1994). Saving Nature’s Legacy: M. L. Morrison, and C. J. Ralph, eds.), pp. 301–304. University Protecting and Restoring Biodiversity. Island Press, Washington, of Wisconsin Press, Madison. D.C. Thomas, R. (1992). Genetic diversity. In Global Biodiversity: Status Office of Technology Assessment. (1987). Technologies to Maintain of the Earth’s Living Resources: (World Conservation Monitoring Biological Diversity. Summary. U. S. Govt. Printing Office, Wash- Centre, eds.), pp. 1–6. Chapman and Hall, London. ington, D.C. Vane-Wright, R. I. (1992). Systematics and diversity. In Global Biodi- Pearson, D. L., and Cassola, F. (1992). World-wide species richness versity: Status of the Earth’s Living Resources: (World Conservation patterns of tiger beetles (Coleoptera: Cicindelidae): Indicator Monitoring Centre, eds.) pp. 7–12. Chapman and Hall, London. taxon for conservation and biodiversity studies. Conservation Biol. 6, 376–391. Vane-Wright, R. I. (1996). Systematics and the conservation of biolog- Pielou, E. C. (1977). Mathematical Ecology. John Wiley & Sons, ical diversity. Ann. Missouri Botanic Gardens. New York. Whittaker, R. H. (1972). Evolution and measurement of species diver- Reid, W. V., and Miller, K. R. (1989). Keeping Options Alive: The sity. Taxon 1, 213–251. Scientific Basis for Conserving Biodiversity. World Resources Insti- Williams, C. R. (1964). Patterns in the . Academic tute, Washington, D.C. Press, New York/London. Ricklefs, R. E., and Schluter, D. (1993). Species Diversity in Ecological Williams, P. H., Humphries, C. J., and Vane-Wright, R. I. (1991). Communities. University of Chicago Press, Chicago. Measuring biodiversity: Taxonomic relatedness for conservation Rosenzweig, M. L. (1995). Species Diversity in Space and Time. Cam- priorities. Australian Systematic 4, 665–679. bridge University Press, Cambridge, United Kingdom. Wilson, E. O. (1961). The nature of the taxon cycle in the Melanesian Samson, F. B., and Knopf, F. L. (1994). A framework to conserve fauna. Am. Nat. 95, 169–193. biological diversity through sustainable land management. Trans. Wilson, E. O. (1992). The Diversity of Life. W. W. Norton & Co., North Am. Wildl. and Nat. Resources Conf. 59, 367–377. New York. Scott, J. M., Abies, E. D., Edwards, T. C., Jr., Eng, R. L., Gavin, Wilson, E. O., and Peter, F. M. (eds.). (1988). Biodiversity. National T. A., Harris, L. D., Haufler, J. B., Healy, W. M., Knopf, F. L., Academy Press, Washington, D.C. Torgerson, O., and Weeks, H. P., Jr. (1995). Conservation of Zonneveld, I. S. (1988). Composition and structure of vegetation. In biological diversity: Perspectives and the future for the wildlife Vegetation Mapping (A. W. Ku¨ chler and I. S. Zonneveld, eds.), profession. TWS committee report. Wildl. Soc. Bull. 23, 646–657. pp. 25–35. Kluwer Academic Publisher, Dordrecht, .