Theoretical Basis for Biodiversity Definitions

A scientific definition of biodiversity might be "the complexity of living systems at all organization levels." Many definitions are valid in the context of their specific use, and no simple definition can cover all aspects: natural versus human-altered diversity, evenness versus richness, the various spatial (a, b, and g biodiversity) and temporal dimensions (phylogenetic biodiversity), and the biological incompatibilities of increasing diversity at all organizational levels simultaneously. Because of the broad definition, it is very difficult to derive verifiable targets and measurements of biodiversity at this time. Which biodiversity should be conserved at the expense of which other biodiversity?

The conceptual framework attributed to Noss (1990) is a useful way to organize the various interrelated facets of biodiversity (Figure 16.1). It proposes that biodiversity is expressed at three main levels of organization (ecosystem, species, and gene) and in three aspects (compositional, structural, and functional). Thus, focusing for instance at the species level, a sample composed of several different species is more diverse than one with fewer species. Even with only one species, a sample that includes both big and small individuals, perhaps organized into clumps, is structurally more diverse than one in which all the individuals are the same size, organized in a perfect grid. If several species were present but all did exactly the same thing, functionally they would be less diverse than a sample that included species that had very different roles (e.g., a plant, herbivore, carnivore, and decomposer).

The same three aspects of biodiversity can be defined at the supraorganism scale (the ecosystem) and the suborganism scale (the gene). A landscape that is a mixture of forest, grassland, and cropland is compositionally, structurally, and functionally more diverse than one that is forest only. A cloned crop in which each individual is identical to every other is less diverse than a traditional landrace that contains genetic variation. On land, ecosystems are mapped almost entirely based on the distribution of easily identified plant structural formations, such as forests or grasslands. Structure often is closely linked to function; the biggest potential divergence is between composition and function. For instance, the microclimate in a forest is more closely related to the relative proportion of tall, medium, and short plants than to the variety of species present.

The fundamental evolutionary process generating biodiversity is mutation and its stabilization within populations by speciation. This is a slow and still poorly understood process. Ultimate biodiversity loss occurs by extinction (of specific genes, species, or ecosystems), which occurs at an unsteady and only roughly quantified rate, even in the absence of human threats. It is nevertheless apparent that the earth is in a period of net biodiversity loss (Leakey and Lewin 1995).

Bio Diversity Development Human

Figure 16.1. Compositional, structural, and functional attributes of biodiversity at four levels of organization (Noss 1990).


Figure 16.1. Compositional, structural, and functional attributes of biodiversity at four levels of organization (Noss 1990).

However, biodiversity loss entails much more than extinction and occurs at all levels of organization (CBD 2003a, 2003b, 2003d). At the ecosystem level it consists of a reduction in the extent, condition, or productivity of ecosystems; at the species level it consists of a decline in the abundance (ultimately extinction), distribution, or sustainable use of populations; and at the gene level it consists of loss of gene-level diversity within populations (genetic erosion). Anthropogenically driven biodiversity loss manifests as a decrease in abundance of many species and an increase in a few, leading to homogeneity at multiple scales.

Absolute biodiversity loss has to be measured relative to some baseline state, whose choice raises both political and practical concerns. A baseline state is not needed in order to determine relative biodiversity loss, but at least two measurements in time are needed. Four baselines have been proposed for use by the CBD (2003e):

Before any human interference (which is both illogical and unfeasible for many long-

inhabited parts of the world) Before major interference by industrial society (recommended as most appropriate) From the time the CBD entered into force (i.e., 1993; results in a bias toward developed countries)

Extinction threat according to the World Conservation Union (IUCN) Red List categories

Other possible baselines include viable population levels, species richness, a specific reference year, maximum sustainable yield levels, or a defined desired state.

Richness and evenness are another set of ideas widely used in biodiversity observation. They have their origin in information theory (Pielou 1969). Information diversity indices combine the number of classes with the proportions in each particular class. Richness is a count of how many different varieties (classes) exist in a given sample. Evenness measures the degree of dominance among the different varieties. Many ecological diversity indices have been proposed (Magurran 2004). Interpretation of these indices is complex, and a range of approaches have been suggested (French 1994; Iszak and Papp 2000). Such indicators do not distinguish between native, introduced, and cultivated species, which is often desired by policymakers. A native species, as opposed to an introduced species, is one that occurs naturally in a particular area. Cultivated species usually are specifically bred for purposes of cultivation. A minimum description of diversity includes both richness and evenness concepts, in the same way that an analysis of poverty usually includes measures of both absolute income and income inequality.

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