Beta diversity of herbivorous insects is coupled to high species and phylogenetic turnover of plant communities across short spatial scales in the Cape Floristic Region

Aim Insect distribution patterns can result from historical contingency (biogeography and dispersal limitation), abiotic filtering and biotic factors (ecological interactions and evolutionary associations). Here, we analyse turnover of plant and insect herbivore community composition at multiple spatial scales to tease apart these influences. While positive associations between plant and insect turnover across broad spatial scales could arise through any of these influences, strong association at very local scales is only likely if insect distributions are determined primarily by biotic factors (i.e. host specificity). Location The Cape Floristic Region (CFR), South Africa. Methods To characterize the relationship between spatial turnover in plant and insect composition in the CFR, communities of Restionaceae, a dominant family in the florally diverse CFR, and their associated herbivores were sampled using a spatially nested sampling design on three spatially separated mountain blocks with similar climates, thus controlling for broad abiotic influences. This allowed us to quantify insect and plant turnover, and their association, at multiple independent spatial scales. Redundancy analysis was used to determine the effects of plant on insect composition, controlling for geographical distance. Results Insect species turnover was significantly related to plant species and phylogenetic turnover at local, as well as broad, spatial scales, suggesting that insect distribution patterns are mainly structured by host specificity. Plant communities show near complete turnover at small spatial scales (i.e. communities situated 0.1–3 km apart), with insects mirroring this pattern. Further, insect turnover increased significantly with increasing geographical separation (e.g. between mountains), suggesting an additional influence of biogeographical factors on insect distributions in the CFR. Measured environmental and plant structural components had no influence on insect composition. Main conclusions High insect beta diversity positively associated with plant turnover at local scales suggests insect herbivore diversity patterns in the CFR are primarily structured by plant distribution patterns.