Two hypothetical communities with identical species richness ( S = 6), organized by hierarchical Linnaean classi fi cations into taxonomic trees. Community ‘a’ is less diverse than ‘b’ based on taxonomic relation- ships with 6 species represented by only two genera as opposed to three in ‘b.’ 

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... is most frequently expressed as species richness and often in combination with some measure of species abundance distributions. Species richness ( S ) is simply the number of species present in an assemblage and can be modi fi ed to include also the total number of individuals in that assemblage. Species abundance distributions describe the number of individuals of each species in an assemblage and how individuals are distributed among species. Uneven species distributions are more commonly recognized as having dominant species. Evaluating species richness and species abundance distributions is dependent on sampling effort, illus- trated by species accumulation curves that show ever-increasing effort is needed to discover new species. This requirement also applies when comparing biodiversity indices that must be generated using the same sampling protocols, i.e., sample size and effort, for the comparisons to be valid. Other major drawbacks that apply to most of richness and evenness-based diversity indexes are that there is no statistical framework to determine the departure of measured S from expectation and richness can vary markedly with different habitat type. A relatively new index for measuring biodiversity is average taxonomic distinctness (Clarke and Warwick 1998). A nontechnical interpretation of average taxonomic distinctness (abbreviated as either AvTD or ∆ + ) is that it is the average distance, based on Linnaean classi fi cation, between any two randomly chosen species in a classi fi cation tree representing a species assemblage (Figure 1) and is calculated ...