As in all indicator systems, it is useful to distinguish between indicators of the state of biodiversity and indicators of the things causing changes in the state (drivers, e.g., harvest pressure, pollution, and fragmentation) and responses to the state (e.g., the area under formal protection). Because of data deficiencies, we are often forced to use proxies and surrogates as indicators of state and drivers. Exhaustive surveys in highly diverse regions cost a great deal of time and money. Therefore, simpler surrogates for mapping diversity, such as higher-taxon richness or the richness of indicator taxon groups, have been proposed and tested (Williams et al. 1998), with partial success. Bioindicator approaches measure the abundance of sensitive species (sentinels or detectors) or genetic markers in assessing ecosystem health but are not measures of biodiversity per se.
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