Kevin J. Gaston defines biodiversity and lays out obstacles to its better understanding in this chapter. Biodiversity is the variety of life in all of its many manifestations. This variety can usefully be thought of in terms of three hierarchical sets of elements, which capture different facets: genetic diversity, organismal diversity, and ecological diversity. There is by definition no single measure of biodiversity, although two different kinds of measures (number and heterogeneity) can be distinguished. Pragmatically, and rather restrictively, biodiversity tends in the main to be measured in terms of a number measures of organismal diversity, and especially species richness. Biodiversity has been present for much of the history of the Earth, but the levels have changed dramatically and have proven challenging to document reliably. Biodiversity is variably distributed across the Earth, although some marked spatial gradients seem common to numerous higher taxonomic groups. The obstacles to an improved understanding of biodiversity are: (i) its sheer magnitude and complexity; (ii) the biases of the fossil record and the apparent variability in rates of molecular evolution; (iii) the relative paucity of quantitative sampling over much of the planet; and (iv) that levels and patterns of biodiversity are being profoundly altered by human activities.
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