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Phylogenetic Evenness

The extension of PD rarefaction to ∆PD allows for the measurement of phylogenetic evenness, which is essentially a measure of the distribution of individuals among branches in a phylogenetic tree (Webb and Pitman 2002). A phylogenetically even community is one where the most evolutionarily distinct species are also the most abundant. Because ∆PD will increase with both increasing phylogenetic evenness and phylogenetic diversity, it is more correctly a measure of entropy (Jost 2006), directly comparable to the PIE and Gini-Simpson indices. It has a particularly close relationship with the quadratic entropy measure of Rao (1982). Rao's quadratic entropy measures the average distance between individuals in an assemblage. When that distance is measured as patristic distance (path length on a phylogenetic tree), ∆PD will be approximately half of Rao's quadratic entropy. ∆PD is also similar in intent, but not in form, to the phylogenetic entropy index of Allen et al. (2009).

Low ecological evenness may be an indicator of disturbance where a small number of species are favoured. If those favoured species are also closely related, due to sharing a trait that allows exploitation of disturbance events, we can expect a reduction in phylogenetic evenness (Helmus et al. 2010). Medellin et al. (2000) surveyed the bat assemblages along a disturbance gradient in the Selva Lacandona, Chiapas, Mexico. The disturbance gradient consisted of four habitats, which, in order of disturbance, were cornfield, oldfield, cacao plantation and forest. Bats were sampled using mist nets and each habitat in the disturbance gradient was sampled using the same effort, thus making possible the comparison of habitats without the need for rarefaction. Medellin et al. (2000) found a trend of decreasing species richness and species evenness with increasing disturbance, and this trend is also reflected in the phylogenetic diversity and evenness of the assemblages (Table 2, Fig. 4).

The trend in phylogenetic evenness may simply be reflecting the abundance distribution among species. To determine the phylogenetic contribution to phylogenetic evenness, ∆PD was divided by the PIE index (Table 2). Since PIE is the probability that the second randomly selected individual is a different species to the

Table 2 Comparison of diversity measures for bat assemblages from four habitats along a disturbance gradient in the Selva Lacandona, Chiapas, Mexico

Habitat

Individuals

Species richness

PIE

Phylogenetic diversity

Phylogenetic evenness (∆PD)

Phylogenetic component (∆PD/PIE)

Cornfield

572

17

0.786

295

17.2

21.8

Oldfield

690

20

0.809

469

18.1

22.4

Cacao

699

21

0.851

493

18.2

21.3

Forest

444

27

0.884

609

20.4

23.0

Original data taken from Medellin et al. (2000). PIE refers to the Probability of Interspecific Encounter (Hurlbert 1971). Phylogenetic Diversity and phylogenetic evenness are measured in units of millions of years

Fig. 4 Individuals-based PD rarefaction curves for bat assemblages from four habitats along a disturbance gradient in the Selva Lacandona, Chiapas, Mexico.(See Medellin et al. (2000) for a description of the data. Phylogenetic evenness (∆PD) values are highest in the least disturbed habitat (Forest) and lowest in the most disturbed habitat (Cornfield)

first, we can divide ∆PD by PIE to get the expected branch length of that species (conditional on the second individual being a different species). This value is related to phylogenetic dispersion (∆PD from a species-based rarefaction curve) but differs due to the conditional probability structure, and effectively measures the pure phylogenetic contribution to ∆PD independent of the abundance distributions among species. We see, in this case, that the phylogenetic component generally decreases with increasing disturbance (Cacao being the exception), supporting the notion that disturbance favours more closely related species.

 
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