These indicators are based on trends in the abundance (i.e., number, biomass, or cover) of individuals in a target population, which may be the total global population for that species or some well-defined part of it. Abundance-based indicators provide the advance warning of impending loss that richness-based indicators do not because the underlying information is continuous rather than binary (present or absent). Such indicators therefore are more sensitive and useful for policy purposes than richness-based indicators, particularly for setting policy targets.
Hughes et al. (1997) estimate that there are on average about 200 separate populations per species. Because of the amount of effort needed to get reliable, repeated abundance estimates of wild species from just one population, such censuses are limited to a tiny fraction of all known species. They are often either charismatic species, such as elephants, whales, and migrating waterfowl, or useful species, such as fish, timber, and domestic animal stocks. The Global Conservation Organization (WWF) marine ecosystem indicators are an attempt to apply population measures with a rapid, standardized sampling protocol (Wilkinson 2000). The Living Planet Index (Loh 2002) and species trend indices (e.g., Gregory et al. 2002) are a population approach based on a small set of species selected to represent the major groups. The Natural Capital Index and the Biodiversity Intactness Index are in principle both population-based measures.
Changes in species populations may be correlated with changes in diversity at the genetic level. The smaller the population, the smaller its genetic variation, particularly if losses are concentrated around the edges of the species' area of distribution.
Not all species have to be measured to indicate the overall process of biodiversity loss. A representative sample would be sufficient, and a deliberately selected set of indicators may be even more sensitive if the selection was well founded. Population-based indicators are the focus for much of the current activity in biodiversity indicator development.
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