Many indicators can be used to measure a site's biodiversity. For example, species richness measures the number of different species present. This article will use a richer but slightly more complex indicator, phylogenetic diversity. It is based on the phylogenetic tree associated with a group of species and reflects their evolutionary history. According to some specialists, this diversity is disappearing even faster than species richness.
Taking species' evolutionary heritage into account
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A phylogenetic tree (PT) consists of a root vertex, internal vertices, leaf vertices representing the species, and branches connecting these vertices.
In tree in the box opposite, branch b1 connects the root to species e1, while branch b6 connects internal vertex i2 to internal vertex i3. The branches have an implicit orientation, from the root toward the leaves. We can therefore regard each branch as having an initial endpoint and a terminal endpoint. Each branch is assigned a length that, depending on the type of tree, reflects either the evolutionary changes that occurred from its initial endpoint to its terminal endpoint or, more simply, the time elapsed between the two. It thus provides an overall indicator of the amount of evolution associated with the branch.
