A conceptual problem that faces all species-based indicators is whether all species matter equally. If not, on what basis is the weighting assigned? Where a subset of species is used for practical reasons, can they be assumed to represent the unsampled species?
Species richness (i.e., the number of recognized and recorded species within a given set of taxonomic groups and within a given bounded area) is the simplest and most widely used biodiversity index. The data are derived from collection labels in museums or field observations and may be extrapolated over unsampled locations to create inferred species distribution maps, using either explicit techniques such as habitat modeling using GLIM or GARP (Crawley 1993; Stockwell 1994) or expert judgment. The principal problems with species richness as a biodiversity indicator are as follow:
It is critically dependent on the quality and completeness of collection and classification. Changes in richness often are simply a result of taxonomic revisions or observations rather than changes in the underlying biodiversity. Some stability has been achieved in richness numbers for well-known taxa such as birds, mammals, and some plant groups (Govaerts 2001; Bramwell 2002; Scotland and Wortley 2003) in well-studied areas, but for most of the biological realm, less than half of the postulated extant species have been formally described (WCMC 2000). In some parts of the world, even the plant and vertebrate observations are extremely incomplete (Prance et al. 2002).
Richness is strongly spatially dependent. Typically, the species richness in an ecological region rises to an asymptote in relation to the area sampled (the species—area curve). The shape of the curve varies between ecological regions and taxa and can itself be used as a biodiversity index (Cowling et al. 1989). Comparing species richness between locations without taking this relationship into account is misleading. Special procedures, such as rarefaction (Hurlbert 1971), correct for biases related to unequal sampling area. Species richness is a very insensitive indicator of biodiversity loss. It provides no indication of changes in the abundance of component species in a community or of changes in their phylogenetic and functional diversity traits. A decrease in richness occurs only through extinction. Extinction is the loss throughout the world of a species or variety, whereas extirpation is the loss of a species or variety in a portion of its range. Species richness per se does not distinguish between native and introduced species. In disturbed areas total species richness may increase because of introduced species, while populations of native species are reduced but not entirely extirpated. In this case species richness may provide perverse signals. It is therefore recommended (CGER 2000) that counts of introduced species be kept separate from those of native species. Maximizing richness by introducing alien species is a perverse objective.
Extinction, or the risk of it, has been widely used as a measure of biodiversity loss (e.g., IUCN 2002). It has the advantages of being extremely easily grasped and compelling, but as a biodiversity management indicator it has several drawbacks:
It is surprisingly difficult to prove that a species is extinct. How do you know that it is not just hiding? If a species is not known to science, its extinction is invisible.
By the time a species has gone extinct (and been shown to be so), it is far too late to do anything about it. For this reason, the IUCN has created a variety of lev els of threat, which are indicators of the last phase of the lengthy process of biodiversity loss.
Species extinction is a natural process, balancing, in the long term, the process of spe-ciation. What rate of extinction is clearly too high?
Endemism is a widely applied refinement on species richness. This is the number of species found only in a specified area and nowhere else on Earth (in the wild). Endemic richness, at the species or higher taxonomic level and sometimes in conjunction with an assessment of threat, is often used as an indicator of biodiversity hotspots (Reid et al. 1993; Williams et al. 1998; Myers et al. 2000). Obviously, the loss of an endemic species within its entire range is more critical than the local loss of an otherwise widespread species. However, endemism has the same problems as species richness.
A sophisticated set of indices have been developed by conservation biologists for the purpose of optimizing the design of networks of protected areas (Margules and Pressey 2000). They are based on the assumption that a system of protected areas should represent the biodiversity in each region and separate it from processes that threaten its existence. Complementarity measures thus are based on quantitative targets that specify how much of the landscape is needed to conserve a representative set of sample species or habitats. Additionally, they may include considerations of threat or vulnerability.
The fundamental unit of biodiversity is an evolutionary one (Faith 2002). Santini and Angulo (2001) propose an index for the estimation of evolutionary potential, which they define as the potential of a member of the genealogical hierarchy to persist for ecologically significant periods of time. It is the balance between diversification and extinction of the evolutionary unit considered. Phylogenetic diversity measures indicate the amount of branch length or evolutionary history spanned by a set of taxa (Faith and Williams 2006).
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