This paper presents a critique and extension of Weitzman's cost-effectiveness criterion for ranking biodiversity-protecting projects. Weitzman's result, which is entirely based on genetic diversity, is argued to be able to give rise to incorrect policy advice on biodiversity protection. The reason is that ecological relationships are not included. In order to do so, dependent survival probabilities are introduced. It is shown that these give rise to a different ranking of biodiversity-protecting projects. Subsequently, alternative evaluation criteria are discussed.