Biological diversity and probability of local extinction of ecosystems

1. With a mathematical model, we demonstrate that two principle components of biological diversity have antagonistic effects on the probability of eco system functioning (i.e. on ecosystem reliability, sensu Naeem 1998). Incre asing the number of functionally analogous species in the system is shown t o enhance reliability, while increasing the number of functions or processe s which these species support (functional diversity) has a negative influen ce. 2. Probability of functioning was calculated from the likelihood that essen tial ecosystem functions are lost, which was based on the probability of lo cal extinction of the species that support these functions. 3. A third component of diversity, heterogeneity among extinction probabili ties of different species, enhanced probability of ecosystem functioning fo r species belonging to the same functional group, but only in species-poor systems. In contrast, heterogeneity among species of different functional g roups drastically reduced probability of ecosystem functioning and this red uction was more pronounced when more groups were present. 4. The model further predicts an asymptotic relationship between number of functionally analogous species and probability of functioning, rather than a linear or complex one. In case the species are very prone to extinction, saturation occurs at higher species numbers, which indicates there is less species redundancy in vulnerable systems.