Introduction

It is generally agreed that a measure of biodiversity of a site ideally ought to say something about how different the inhabitants are from each other. Taxonomic related-ness of the individuals or species in a sample is the key concept on which taxonomic diversity and distinctness measures are based. It is well known that in impacted assemblages of organisms the taxonomic spread of species is reduced, and in extreme cases they may be sibling species belonging to the same genus, or at least very closely related (Figure 1). Unimpacted assemblages, on the other hand, have a wider taxonomic spread and the species belong to many different genera, families, orders, classes, and phyla. This concept of taxo-nomic relatedness is totally independent of the numbers of species present, and measures based on it overcome many of the problems inherent in species-richness measures, at least to some degree.

It is clear that a sample consisting of ten species from the same genus should be seen as much less biodiverse than another sample of ten species, all of which are from different families or higher taxa. Species richness (S), widely used as the preferred measure of biodiversity at the organismal level of biological organization, clearly suffers from this problem, but it also has some other major drawbacks, many of which apply equally to other diversity indices such as the Shannon-Wiener index (see Shannon-Wiener Index), Margalefs richness index (see Margalefs Index) and Pielou's evenness index. Observed richness is heavily dependent on sample size or sampling effort and in most cases the asymptote of species accumulation curves is rarely reached so that observed species richness S is highly sensitive to sample size and totally noncomparable across studies involving unknown, uncontrolled, or simply differing degrees of sampling effort. The same is true, to a lesser extent, of many other standard diversity indices (Figure 2). Also, while observed species-richness measures can be compared across sites (or times) which are subject to strictly controlled and equivalent sampling designs, values of S cannot be compared with some absolute standard, that is, we cannot generally say what the expected richness would in the absence, for example, of anthropogenic impact. Further, it would be

Figure 1 Drawings of the five dominant harpacticoid copepod species from a pristine environment (a, seaweeds from the Isles of Scilly, UK) and a polluted environment (b, a Belgian sluice dock). Those from the pristine environment differ greatly in form and function and belong to various families, while those from the polluted environment are all sibling species of Tisbe.

Figure 1 Drawings of the five dominant harpacticoid copepod species from a pristine environment (a, seaweeds from the Isles of Scilly, UK) and a polluted environment (b, a Belgian sluice dock). Those from the pristine environment differ greatly in form and function and belong to various families, while those from the polluted environment are all sibling species of Tisbe.

preferable to work with a biological index whose relation to the degree of perturbation was purely monotonic (increasing or decreasing, but not both) but, in accordance with the 'intermediate disturbance hypothesis' (see Intermediate Disturbance Hypothesis), under moderate levels of disturbance species richness may actually increase before decreasing again at higher impact levels. Finally, richness can vary markedly with differing habitat type, and again, the ideal would be a measure which is less sensitive to differences in natural environmental variables but is responsive to anthropogenic disturbance.

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