Evidence is increasing for positive effects of a-diversity on ecosystem function- ing. We highlight here the crucial role of b-diversity--a hitherto underexplored facet of biodiversity--for a better process-level understanding of biodiversity change and its consequences for ecosystems. A focus on b-diversity has the potential to improve predictions of natural and anthropogenic influences on diversity and ecosystem functioning. However, linking the causes and conse- quences of biodiversity change is complex because species assemblages in nature are shaped by many factors simultaneously, including disturbance, envi- ronmental heterogeneity, deterministic niche factors, and stochasticity. Because variability and change are ubiquitous in ecosystems, acknowledging these inherent properties of nature is an essential step for further advancing scientific knowledge of biodiversity--ecosystem functioning in theory and practice.