Changes in biodiversity reflect processes acting at multiple spatial scales, including globally, among habitats and within communities. This complexity makes it difficult to analyse the mechanisms that change biodiversity over time. To resolve this, we propose a novel approach to partition temporal changes in biodiversity into contributions from selection at multiple scales. We apply this approach to study changes in the biodiversity of invertebrate herbivores from a large-scale, plant community experiment. Though the experiment was designed to foster distinct insect communities due to differences in host plants, our approach shows that selection among these treatments was a negligible facet of diversity change. Instead, the dominant source of community dynamics was rapid changes in the relative abundances of individual species. These shifts produced surprisingly small changes in biodiversity. More broadly our work highlights how total change in biodiversity across a biogeographic region can be partitioned into logically distinct mechanisms.

Experiments suggest that increasing species’ diversity increases productivity over short observation periods. Theory predicts that diversity may increases productivity at equilibrium, such as in communities of coexisting competitors. In effect, experiment and theory are using different response variables. We illustrate how this gap can obscure some of the mechanisms through which diversity shapes productivity. To do this, we propose a formal link to dynamic models of population growth. Using this link, we show that selection (as it is formally defined in dynamic models) does not affect productivity. Instead, selection changes productivity over time. In contrast, niche complementarity can change productivity directly. We illustrate how to measure each of the terms in competition experiments. These results highlight how the effects of diversity on productivity can be obscured by our choice of response variable.

Changes in biodiversity reflect processes acting on the success of individual species at multiple spatial scales, including in communities, biogeographic regions, and globally. This complexity makes it difficult to analyse the mechanisms shaping diversity change using traditional approaches. To resolve this, we propose a novel approach to partition total biodiversity changes according to mechanisms reflecting species' success at multiple scales. We apply this approach to study changes in the diversity of invertebrate herbivores from a large-scale, plant community experiment. This partitioning showed that rapid changes in the relative abundances of individual species resulted in surprisingly small changes in diversity across scales. Our novel analytical method reveals how strong ecological effects at different hierarchical levels can counteract each other, resulting in weak effects on diversity across broad spatial scales.