The relationship between climate and biodiversity has been long debated. In a changing environment, there is new emphasis to resolve this debate for practical reasons: to manage conservation efforts we need to understand how diversity will change from both our own actions and natural global cycles. In a new study published in AoB PLANTS, McBride et al. show that the roles played by different ecological and evolutionary factors in shaping plant diversity change across the world’s ecoregions, and—critically—that these differences scale with ecoregion size. Ecoregions that are both large and productive are globally important biodiversity sources that shape the biota of the smaller regions around them.