One reason with the IUCN recognizes different categories of species (critically endangered, endangered, vulnerable, near threatened, least concern) is that these categories seem like a useful tool in deciding how to allocate limited conservation resources. If our goal is to prevent extinction of as many species as we can, surely we should focus our attention first on those species that are in the most immediate danger of extinction.7 Well, Wilson et al.  argue that ``allocating resources solely to the most endangered species will typically not minimize the number of extinctions....'' To see why, we have to take a brief look at their model.
The authors consider a simple conceptual model in which there is some number of species that exist in one of three states: extinct, endangered, or recovered. Extinction is forever, so extinct species stay extinct. If a species is endangered, it may (a) recover with probability if there is a management intervention that costs , (b) recover even if there is no management with probability , or go extinct with probability . Each species has its own values for , , , and .
Given these simple assumptions and the assumption that there is a fixed budget to allocate, the authors determine how best to allocate those resources to maximize the number of species extant at some future time. Their results are illustrated in Figure 2. Under a longer planning horizon, resources are shifted towards species with a lower probability of extinction and a lower cost of recovery. Why? Because under a longer time horizon, some species that are less in danger of extinction immediately are likely to go extinct unless we pay attention to them.