Dissecting the plant-insect diversity relationship in the Cape.

It has been argued that insect diversity in the Cape is disproportionately low, considering the unusually high plant diversity in this region. Recent studies have shown that this is not the case, but the precise mechanisms linking plant diversity and insect diversity in the Cape are still poorly understood. Here we use a dated genus-level phylogenetic tree of the Cape plants to assess how plant phylogenetic diversity compares with taxonomic diversity at various levels in predicting insect diversity. We find that plant phylogenetic diversity (PD) is a better predictor of insect species diversity that plant species diversity, but the number of plant genera is overall as good a predictor as PD, and much easier to calculate. The relationship is strongest between biomes, suggesting that the relationship between plant diversity and insect diversity is to a large extent indirect, both variables being driven by the same abiotic factors and possibly by common diversification, immigration and extinction histories. However, a direct relationship between plant diversity and insect diversity can be detected at fine scales, at least within certain biomes. Diversity accumulation curves also indicate that the way plant phylogenetic diversity and the number of plant genera increase over spatial scales is most similar to that for insect species; plant species show a greater increase at large spatial scales due to high numbers of local endemics.