One justification for species conservation is the possibility that spe
cies will be found to be beneficial. Existing models of the value of a
collection of potentially beneficial species typically assume that th
e probability that each species is beneficial does not depend on the o
utcome for other species. It is also typically assumed that beneficial
species are perfect substitutes. This paper describes a model under w
hich both these assumptions are relaxed. (C) 1995 Academic Press, Inc.