Valuing bits of biodiversity: units, traits and triage.

Species are a key measure of biodiversity and are also generally considered valuable. But why? I will present my (evolutionary biologist's) thinking on three aspects of what might be called the Normative Postulate Problem:
(i) I will examine how species are considered in Endangered Species legislation, and suggest that the concept is somewhat confused,
(ii) I will review a few ways to quantify the relative value of different species as the products of past evolution, and, if time permits
(iii) I will muse on what we might actually be valuing when we value species.
Differential value matters when trade-offs and triage are necessary.