  In this study we are concerned with the general problem of choosing from a
set of endangered species T a subset S of k species to protect as a priority.
Here, the interest to protect the species of S is assessed by the resulting
expected phylogenetic diversity (ePD) of the set T, a widely used criterion for
measuring the expected amount of evolutionary history associated with T. We
consider that the survival of the protected species is assured and, on the
contrary, that there is a risk of extinction for the unprotected species. The
problem is easy to solve by a greedy type method if the extinction
probabilities of the unprotected species are known but these probabilities are
generally not easy to quantify. We show in this note that the choice of the
precise values attributed to the extinction probabilities-provided it respects
the rank of imperilment of each species-is not as decisive as might be feared
for the considered problem. The values of these probabilities have a clear
impact on the selection of the species to be protected but a little impact on
the resulting ePD. More precisely, if T1 and T2 are the two optimal subsets of
species corresponding to two scenarios (two different sets of probabilities)
the ePDs of T1 and T2 , calculated with the probabilities of the first
scenario-or with the probabilities of the second scenario-are not very
different.
