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 ecosystem functioning (i.e. on ecosystem reliability, sensu Naeem 1998). Increasing the number of functionally analogous species in the system is shown to enhance reliability, while increasing the number of functions or processes which these species support (functional diversity) has a negative influence. 2. Probability of functioning was calculated from the likelihood that essential ecosystem functions are lost, which was based on the probability of local extinction of the species that support these functions. 3. A third component of diversity, heterogeneity among extinction probabilities of different species, enhanced probability of ecosystem functioning for species belonging to the same functional group, but only in species-poor systems. In contrast, heterogeneity among species of different functional groups drastically reduced probability of ecosystem functioning and this reduction 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.