A Revised Assessment of Species Redundancy and Ecosystem Reliability

Naeem ( Conservation Biology 12: 39–45) formalized the relationships among species diversity, functional groups, species redundancy, and ecosystem reliability. Although we applaud his efforts and agree with many of his conclusions, we disagree with some fundamental assumptions used in the derivation of Naeem’s equation 2. These assumptions affect some of the conclusions concerning the role of redundancy. We discuss two possible interpretations of Naeem’s equation. Under the first interpretation, the equation relates to the probability that a species is present at a particular time t . Under the second interpretation, the equation relates to the probability that a species is present at least once during the interval of time between time 0 and time t . Under this second interpretation we also disagree with the application of Naeem’s equation 4. In his equation 2, Naeem relates “the probability that the i th species in the j th functional group is present by time t ” to the probability that the initial population survives until time t and the probability of colonization by time t , as follows