8.3 When Space Matters: Alpha, Beta and Gamma Diversities
Species diversity varies through space. Trivial as it may seem, this observation had
to wait two seminal papers by Whittaker (1960, 1972) to receive appropriate
attention. In Whittaker’s conception, alpha (α) is local diversity, beta (β) is the
spatial variation of diversity, and gamma (γ) is regional diversity. These concepts
have since gained huge interest among ecologists.
Alpha diversity has been presented in Sect. 8.2.2. In various ways it measures the
variation in species identity of individuals observed at a given site (Legendre and
Legendre 2012 Sect. 6.5).
Gamma diversity is the species diversity of the whole region concerned by a
study. Since the region is generally not known in its entirety, one estimates gamma
diversity by pooling the sampling units available and computing the same indices as
for alpha diversity, under the assumption that the sampling units are representative of
the species composition of the region.
Beta diversity is another matter. It can be construed as the variation of species
composition among sites in the area of interest (Legendre et al. 2005, Anderson et al.
2006, Anderson et al. 2011) and can be studied in many different ways. It deserves a
separate section.
8.4 Beta Diversity
Beta diversity can be estimated on the basis of either presence-absence or quantitative data. Different approaches have been proposed to study beta diversity. Some of
them attempt to summarize it by a single number. Others take advantage of the
variation component of beta diversity and study this variation for itself using
methods rooted in the concepts presented in Chaps. 3, 5, 6 and 7 of this book. We
will present some of these methods below (Sect. 8.4.2 and 8.4.3), but let us first
briefly summarize the simplest ways of measuring beta diversity.
8.4.1 Beta Diversity Measured by a Single Number
Whittaker proposed a first measure of beta diversity for species presence-absence
data: β ¼ S=α, where S is the number of species in the pooled community composition vector (i.e., gamma diversity) and α is the mean number of species at the
n sampling sites. β tells us how many more species are present in the whole area than
in an average individual site. This index is said to be multiplicative since it is a ratio.
8.4 Beta Diversity
379
Species diversity varies through space. Trivial as it may seem, this observation had
to wait two seminal papers by Whittaker (1960, 1972) to receive appropriate
attention. In Whittaker’s conception, alpha (α) is local diversity, beta (β) is the
spatial variation of diversity, and gamma (γ) is regional diversity. These concepts
have since gained huge interest among ecologists.
Alpha diversity has been presented in Sect. 8.2.2. In various ways it measures the
variation in species identity of individuals observed at a given site (Legendre and
Legendre 2012 Sect. 6.5).
Gamma diversity is the species diversity of the whole region concerned by a
study. Since the region is generally not known in its entirety, one estimates gamma
diversity by pooling the sampling units available and computing the same indices as
for alpha diversity, under the assumption that the sampling units are representative of
the species composition of the region.
Beta diversity is another matter. It can be construed as the variation of species
composition among sites in the area of interest (Legendre et al. 2005, Anderson et al.
2006, Anderson et al. 2011) and can be studied in many different ways. It deserves a
separate section.
8.4 Beta Diversity
Beta diversity can be estimated on the basis of either presence-absence or quantitative data. Different approaches have been proposed to study beta diversity. Some of
them attempt to summarize it by a single number. Others take advantage of the
variation component of beta diversity and study this variation for itself using
methods rooted in the concepts presented in Chaps. 3, 5, 6 and 7 of this book. We
will present some of these methods below (Sect. 8.4.2 and 8.4.3), but let us first
briefly summarize the simplest ways of measuring beta diversity.
8.4.1 Beta Diversity Measured by a Single Number
Whittaker proposed a first measure of beta diversity for species presence-absence
data: β ¼ S=α, where S is the number of species in the pooled community composition vector (i.e., gamma diversity) and α is the mean number of species at the
n sampling sites. β tells us how many more species are present in the whole area than
in an average individual site. This index is said to be multiplicative since it is a ratio.
8.4 Beta Diversity
379