The effect of diversity - first taxonomic, then phylogenetic and functional - on the regulation of multiple ecosystem functions has been the subject of unprecedented studies for over three decades. The impact of these studies has been enormous in the literature, and they have obscured the well-known role that particular species can have on the regulation of ecosystem functions beyond the effect of emergent properties of communities such as diversity. This systematic bias has several consequences: (i) species extinctions are not random, but it is difficult to infer the consequences of these extinctions for ecosystem functioning at both local and global scales; (ii) the putative key role of species in ecosystem functioning is an argument that has increasing weight for their conservation status (triage) but remains largely unquantified.
By combining original data from simulations using a forest-gap model, observations of natural grassland communities, and experiments under controlled conditions on herbaceous communities, we propose an analytical framework to systematically identify the key role that certain species may play in controlling ecosystem productivity and stability. We compare the explanatory power associated with the presence of these key species with that associated with different facets of diversity.
Results/Conclusions
The selection procedure identified several species with a significant peculiar effect on ecosystem productivity and stability that overcomes the effects of any other community-level metrics of diversity. Long-term simulations further highlighted an increasing predominant role of those species in extreme environments. Interestingly, the species selected in the different datasets most often expressed plant trait values that differed from the rest of the species, which positions them at the margins of the interspecific phenotypic space. These different case studies demonstrate the relevance of functional distinctiveness for studies in functional and ecosystem ecology and provide unique evidence that the marginal position of species in the phenotypic space can be added to the list of species features to consider in a conservation perspective. Finally, the experimental data allowed us to go deeper into the mechanisms underlying the role of functionally distinct species in the regulation of ecosystem productivity. Productivity was stimulated in at least two cases: (i) when communities contain species with distinct functional traits that allow them to escape interspecific competitive pressures, (ii) when communities contain species which are most affected by intraspecific competition in monoculture. Altogether, our findings call for a come back of a species-specific focus in biodiversity-ecosystem functioning studies.
