30
2.7.6 Beta Diversity Metrics
Whittaker’s 1960 definition of beta diversity (Eq. 2.3) quantified the degree of differentiation among communities in relation to environmental gradients. Under this definition, beta diversity is defined as the ratio between regional (gamma) and local (alpha)
diversities (Eq. 2.3) and measures the number of different communities in a region and
the degree of differentiation between them (Whittaker 1960; Jost 2007). Indices such
as Bray-Curtis dissimilarity and Jaccard and Sørensen indices evaluate similarity of
communities based on the presence or abundance of species within them. Metrics of
similarity used for species have been adapted for phylogenetic and functional trait distances (Bryant et al. 2008; Graham and Fine 2008; Kembel et al. 2010; Cardoso et al.
2014) and can equally be applied to spectral information (Gamon et al., Chap. 16).
While the ratio between regional and local communities provides a simple means
to estimate beta diversity, there are many different ways to calculate taxonomic,
functional, and phylogenetic beta diversity that can be grouped into pairwise and
multiple-site metrics (reviewed in Baselga 2010). Notably, beta diversity can be
partitioned into components that capture species replacement—the “turnover component”—caused by the exchange of species among communities and differences in
the number of species, the “nestedness component,” caused by differences in the
number of species among communities. The turnover component can be interpreted
as the difference between two community assemblages that contain contrasting subsets of species from a regional source pool, while the nestedness component represents the difference in species composition between two communities due to
attrition of species in one assemblage relative to the other (Baselga 2010; Cardoso
et al. 2014). Examining these different components of beta diversity for multiple
dimensions of plant diversity provides a means to discern the role of historical and
ongoing environmental sorting processes in the distribution of plant diversity at
continental extents (Pinto-Ledezma et al. 2018b). In contrast to traditional diversity
metrics, spectral diversity (alpha and beta) is only beginning to receive attention in
biodiversity studies (Rocchini et al. 2018). Although different approaches have been
proposed (Schmidtlein et al. 2007; Féret and Asner 2014; Rocchini et al. 2018;
Laliberté et al. 2019), the estimation and mapping of dissimilarities in spectral composition (i.e., the variation among pixels) is similar to traditional estimations of beta
diversity. For example, Laliberté et al. (2019) adapted the total community
composition variance approach (Legendre and De Cáceres 2013) to estimate
spectral diversity as spectral variance, partitioning the spectral diversity of a region
(gamma diversity) into additive alpha and beta diversity components.
2.8 Links Between Plant Diversity, Other Trophic Levels,
and Ecosystem Functions
Plant diversity has consequences for other trophic levels, sometimes reducing herbivory on focal species (Castagneyrol et al. 2014), but also increasing the diversity of
insects and their predators in an ecosystem (Dinnage et al. 2012; Lind et al. 2015).
J. Cavender-Bares et al.
2.7.6 Beta Diversity Metrics
Whittaker’s 1960 definition of beta diversity (Eq. 2.3) quantified the degree of differentiation among communities in relation to environmental gradients. Under this definition, beta diversity is defined as the ratio between regional (gamma) and local (alpha)
diversities (Eq. 2.3) and measures the number of different communities in a region and
the degree of differentiation between them (Whittaker 1960; Jost 2007). Indices such
as Bray-Curtis dissimilarity and Jaccard and Sørensen indices evaluate similarity of
communities based on the presence or abundance of species within them. Metrics of
similarity used for species have been adapted for phylogenetic and functional trait distances (Bryant et al. 2008; Graham and Fine 2008; Kembel et al. 2010; Cardoso et al.
2014) and can equally be applied to spectral information (Gamon et al., Chap. 16).
While the ratio between regional and local communities provides a simple means
to estimate beta diversity, there are many different ways to calculate taxonomic,
functional, and phylogenetic beta diversity that can be grouped into pairwise and
multiple-site metrics (reviewed in Baselga 2010). Notably, beta diversity can be
partitioned into components that capture species replacement—the “turnover component”—caused by the exchange of species among communities and differences in
the number of species, the “nestedness component,” caused by differences in the
number of species among communities. The turnover component can be interpreted
as the difference between two community assemblages that contain contrasting subsets of species from a regional source pool, while the nestedness component represents the difference in species composition between two communities due to
attrition of species in one assemblage relative to the other (Baselga 2010; Cardoso
et al. 2014). Examining these different components of beta diversity for multiple
dimensions of plant diversity provides a means to discern the role of historical and
ongoing environmental sorting processes in the distribution of plant diversity at
continental extents (Pinto-Ledezma et al. 2018b). In contrast to traditional diversity
metrics, spectral diversity (alpha and beta) is only beginning to receive attention in
biodiversity studies (Rocchini et al. 2018). Although different approaches have been
proposed (Schmidtlein et al. 2007; Féret and Asner 2014; Rocchini et al. 2018;
Laliberté et al. 2019), the estimation and mapping of dissimilarities in spectral composition (i.e., the variation among pixels) is similar to traditional estimations of beta
diversity. For example, Laliberté et al. (2019) adapted the total community
composition variance approach (Legendre and De Cáceres 2013) to estimate
spectral diversity as spectral variance, partitioning the spectral diversity of a region
(gamma diversity) into additive alpha and beta diversity components.
2.8 Links Between Plant Diversity, Other Trophic Levels,
and Ecosystem Functions
Plant diversity has consequences for other trophic levels, sometimes reducing herbivory on focal species (Castagneyrol et al. 2014), but also increasing the diversity of
insects and their predators in an ecosystem (Dinnage et al. 2012; Lind et al. 2015).
J. Cavender-Bares et al.