BIODIVERSITY, DEFINITION OF Ian R. Swingland The Durrell Institute of Conservation and Ecology I. What Is Biodiversity? that is associated with a given habitat or ecosystem II. Definition of Biodiversity and that may increase awareness of the need for III. Genetic Diversity conservation action. IV. Species Diversity genetic diversity Genetic variety found within or V. Ecosystem Diversity among species; this diversity allows the population VI. Biodiversity: Meaning and Measurement or species to adapt and evolve in response to chang- VII. Biodiversity: Changes in Time and Space ing environments and natural selection pressures. VIII. Loss of Biodiversity and Causes keystone species 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 abundance of all species living in a given area. species richness Absolute number of species living in biodiversity/biological diversity Species, genetic, and a given area (also called alpha diversity), giving equal ecosystem diversity in an area, sometimes including weight to all resident species. associated abiotic components such as landscape fea- use values Values that are obtained by using a natural tures, drainage systems, and climate. resource, such as timber, fuelwood, water, and land- 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 gamma diversity, ende- micity, and higher taxon richness. ecosystem diversity Diversity of habitats, ecosystems, 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 organisms. Encyclopedia of Biodiversity, Volume 1 Copyright 2001 by Academic Press. All rights of reproduction 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 humans. 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, biomass, cover, rate) diversity’’ has become synonymous with life on earth, 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 environmental technology 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 genes, 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 animals and plants’’ (Dod- ‘‘practical’’ value systems of direct use/genetics 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 landscapes 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 Land Management advisory one component of biodiversity. Biodiversity is also more group included ecological processes in their definition than species diversity (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 function. 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 systematics and evolutionary studies. Molecu- word invented for convenience. Its invention has had lar biology 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 scientists 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., evolution). sary for the proper functioning of ecosystems. Conser- Fundamental divisions remain, such as ‘‘straight’’ parsi- vation (i.e., management)