It has been almost common-sense knowledge that soils over a limestone or chalk bedrock usually harbour more diverse vegetation communities and in turn, more insect species. These are usually also very prone to endemism. However, published evidence for why this happens and the extent of it seems to be scant.
Regarding plants, a higher count on therophytic species seems to be associated with more acidic soils than carbonate-rich ones where there is a prevalence for perennials... why?
From a soils perspective, there can be quite a variation over limestone in soil acidity and sometimes texture in small localised areas ranging from alkaline to moderately acidic within a few metres where microclimate and OM content of the soil in crucial. In acidic soils these extremes in variation generally (NB there are always exceptions to the rule!) does not happen to such an extent. Such diversity in localised soil conditions gives a huge range of niches especially in karst conditions perfect for increasing diversity. Again, there are always exceptions, but this is my experience.