Despite significant advances in recent decades, understanding how biodiversity affects the stability of ecosystems remains topical in ecology. Here we reveal new selection and complementarity effects by which biodiversity affects ecosystem stability. Biodiversity can favor ecosystem resistance by making species less sensitive to perturbations and through a selection effect by which less sensitive species are favored by ecological assembly. Surprisingly, if ecosystem resilience can be favored by an increase in species resilience and a selection effect that benefits resilient species, it can also be favored by another selection effect by which changes in species resistance benefit the least resilient species. To exemplify the mechanisms at play and showcase the clarifying potential of our partitioning, we analyze a mechanistic model of grassland ecosystems subjected to drought. Our partitioning precisely explains how  unctional complementarity and differences in life-history strategies can allow more diverse grasslands to resist and swiftly recover from droughts

Despite growing interactions between ecology and evolution, there still remain opportunities to further integrate the two disciplines, especially when considering multispecies systems. Here, we discuss two such opportunities. First, we suggest to relax the focus on the distinction between evolutionary and ecological processes. This focus is particularly unhelpful in the study of microbial communities, where the very notion of species is hard to define. Second, we propose that key processes of evolutionary theory such as adaptation should be exported to hierarchical levels higher than populations to make sense of biodiversity dynamics. Together, we argue that broadening our perspective of eco-evolutionary dynamics to be more inclusive of all biodiversity, both phylogenetically and hierarchically, will open up fertile new research directions and help us to address one the major scientific challenges of our time, i.e. to understand and predict changes in biodiversity in the face of rapid environmental change.

Despite their close links, ecology and evolution have remained separate disciplines to this day. Breaking down the wall between the two disciplines is essential for at least two reasons. First, this wall is an obstacle to the study of most microorganisms, which constitute a large part of the Earth's biodiversity. Asexual reproduction, gene transfer and the lack of a clear definition of the species taxonomic level blur the distinction between ecological changes in species abundances and evolutionary changes in genotype frequencies in microbes. Second, a key question that biodiversity science will have to address in the coming decades is how ecological systems will cope with rapid environmental change. Generalising the concept of adaptation across multiple timescales and levels of organisation would provide an integrative framework for studying the combined ecological and evolutionary responses to environmental change, and thus help us to address one the major scientific challenges of our time.