  If predictions for species extinctions hold, then the `tree of life' today
may be quite different to that in (say) 100 years. We describe a technique to
quantify how much each species is likely to contribute to future biodiversity,
as measured by its expected contribution to phylogenetic diversity. Our
approach considers all possible scenarios for the set of species that will be
extant at some future time, and weights them according to their likelihood
under an independent (but not identical) distribution on species extinctions.
Although the number of extinction scenarios can typically be very large, we
show that there is a simple algorithm that will quickly compute this index. The
method is implemented and applied to the prosimian primates as a test case, and
the associated species ranking is compared to a related measure (the `Shapley
index'). We describe indices for rooted and unrooted trees, and a modification
that also includes the focal taxon's probability of extinction, making it
directly comparable to some new conservation metrics.
