Going beyond richness: Modelling the BEF relationship using species identity, evenness, richness and species interactions via the DImodels R package, and a comparison with traditional approaches

BEF studies aim at understanding how ecosystems respond to a gradient of species diversity. Diversity-Interactions models are suitable for analysing the BEF relationship. These models relate an ecosystem function response of a community to the identity of the species in the community, their evenness (proportions) and interactions. The no. of species in the community (richness) is also implicitly modelled through this approach. It is common in BEF studies to model an ecosystem function as a function of richness; while this can uncover trends in the BEF relationship, by definition, species diversity is much broader than richness alone, and important patterns in the BEF relationship may remain hidden. We compare DI models to traditional modelling approaches to highlight the advantages of using a multi-dimensional definition of species diversity. DI models can capture variation due to species identities, species proportions and species interactions, in addition to richness effects. We also introduce the DImodels R package for implementing DI models. Through worked examples, we show that using DI models can lead to considerably improved model fit over other methods. Collapsing the multiple dimensions of species diversity to a single dimension (such as richness) can result in valuable ecological information being lost. Predicting from a DI model is not limited to the study design points, the model can extrapolate to predict for any species composition and proportions. Overall, DI models lead to enhanced inference compared to other approaches. Expressing the BEF relationship as a function of richness alone can be useful to capture overall trends, however, there are multiple ways to quantify the species diversity of a community. DI modelling provides a framework to test the multiple aspects of species diversity and facilitates uncovering a deeper ecological understanding of the BEF relationship.