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Species diversity is the number of different species that are represented in a given community (a dataset). The effective number of species refers to the number of equally abundant species needed to obtain the same mean proportional species abundance as that observed in the dataset of interest (where all species may not be equally abundant). Species diversity consists of two components: species richness and species evenness. Species richness is a simple count of species, whereas species evenness quantifies how equal the abundances of the species are.〔Hill, M. O. (1973) Diversity and evenness: a unifying notation and its consequences. Ecology, 54, 427–432〕〔Tuomisto, H. (2010) A diversity of beta diversities: straightening up a concept gone awry. Part 1. Defining beta diversity as a function of alpha and gamma diversity. Ecography, 33, 2-22. 〕〔Tuomisto, H. 2010. A consistent terminology for quantifying species diversity? Yes, it does exist. Oecologia 4: 853–860. 〕 ==Calculation of diversity== Species diversity in a dataset can be calculated by first taking the weighted average of species proportional abundances in the dataset, and then taking the inverse of this. The equation is:〔〔〔 :. The proportional abundances themselves are used as weights. The equation is often written in the equivalent form: : The value of ''q'' defines which kind of mean is used. ''q'' = 0 corresponds to the weighted harmonic mean, which is 1/S because the values cancel out. ''q'' = 1 is undefined, except that the limit as ''q'' approaches 1 is well defined: : value. In practice, ''q'' modifies species weighting, such that increasing ''q'' increases the weight given to the most abundant species, and fewer equally abundant species are hence needed to reach mean proportional abundance. Consequently, large values of ''q'' lead to smaller species diversity than small values of ''q'' for the same dataset. If all species are equally abundant in the dataset, changing the value of ''q'' has no effect, but species diversity at any value of ''q'' equals species richness. Negative values of ''q'' are not used, because then the effective number of species (diversity) would exceed the actual number of species (richness). As ''q'' approaches negative infinity, the generalized mean approaches the minimum value. In many real datasets, the least abundant species is represented by a single individual, and then the effective number of species would equal the number of individuals in the dataset.〔〔 The same equation can be used to calculate the diversity in relation to any classification, not only species. If the individuals are classified into genera or functional types, represents the proportional abundance of the ''i''th genus or functional type, and ''q''''D'' equals genus diversity or functional type diversity, respectively. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「species diversity」の詳細全文を読む スポンサード リンク
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