Consequences of individual species loss in biodiversity experiments: An essentiality index

The importance of species richness for ecosystem productivity has received much attention in recent years, but the consequences of non-random species extinction remain poorly understood. We propose an index, 'essentiality', to assess the consequence of extinction of particular species for community biomass production. The essentiality of a species in a given community is the extent to which the functional role of that species is irreplaceable by the remaining species, calculated as the relative difference in yields between communities with and without the focal species. Species with zero essentiality play a completely replaceable functional role in their communities. Positive essentialities suggest irreplaceable contributions by species to their communities; species with essentiality values even higher than their dominance in the community may exhibit facilitative effects on the other species. Negative essentialities indicate interference of the focal species with other members of the same community. We applied this index to data from a microcosm experiment, in which five algal species were grown in monocultures and in all possible species combinations. Essentiality varied greatly across species. Two species had overall zero essentiality values throughout the experiment, two species had positive essentiality values which were lower than the species' dominance, and the remaining species had negative essentiality. Furthermore, two out of the five species had essentiality values dependent on species richness of communities from which they were lost; essentiality decreased with an increase in species richness. The essentiality index provides a straightforward measurement of functional consequences of individual species loss, which complements the existing analytical methods by focusing on the detection of facilitation and interference.