Assessing the importance of species and their assemblages for the biodiversity-ecosystem multifunctionality relationship.

Biodiversity changes, such as decline in species richness and biotic homogenization, can have grave consequences for ecosystem functionality. Careful investigation of biodiversity-ecosystem multifunctionality linkages with due consideration of conceptual and technical challenges is required to make the knowledge practically useful in managing social-ecological systems. In this paper, we introduced different methods to assess perspectives regarding the issue of diversity-multifunctionality, including a possible multifunctional redundancy/uniqueness, and the influences of the number and identity of functions on multifunctionality. In particular, we aimed to align methods with detecting the mechanisms underpinning diversity-multifunctional relationships that are free from statistical biases. Based on a set of novel methods that excluded analytical biases resulting from differences in the number and identities of multiple functions considered, we found that a substantial portion of species disproportionately supported ecosystem functions and that the diversity effects on multifunctionality were more markedly observed when more functions were considered. These results jointly emphasize that individual species are, to some extent, both functionally unique as well as redundant, highlighting the complexity and necessity for managed assemblages to retain high levels of diversity. We also observed that the relative magnitude of uniqueness or redundancy can differ between species and functions and therefore should be defined in a multifunctional context. We further found that only a small subset of species was identified as significantly less important, especially at low levels of multifunctionality. Taken together, given the low level of multifunctional redundancy we identified, we stress that unraveling the hierarchical roles of biodiversity at different levels, such as individual species and their assemblages, should be a high research priority, in both theory and practice.