Grass species richness: interaction of habitat and bedrock
The effect of herbivore abundance on grass species richness interacted with that of bedrock: with increasing herbivore abundance, grass species richness increased on basalts but decreased on granites. The positive relationship on basalts may be explained by herbivores reducing the dominance of strong grass competitors, thus favouring the coexistence of more grass species (McNaughton 1979, Jacobs and Naiman 2008). On granites with lower grass cover and slightly higher (although non-significant) grass species richness, large herbivore pressure seems to lead to the loss of some species, hence a decrease in grass species richness.
The herbivore species richness interacted significantly with that of habitat and bedrock. In the case of bedrock, grass species richness increased with grazer species richness on basalts in a way similar to herbivore abundance; in fact, both these measures of the herbivore effect were positively correlated (r = 0.54, see Fig. S1). Similar trends could be partially attributed to greater herbivore pressure (as indicated by the above-mentioned mutual correlation). On nutrient-rich soils on basalts, there is a more intense competition among grass species, in particular at crests where the herbivore pressure is lower than by the rivers, leading to the dominance of fewer species, a pattern similar to what we observed in a previous study for all herbs (Hejda et al. 2022).
In terms of habitats, grass species richness increased with herbivore species richness at crests, probably because grazing there suppresses strong dominants, but decreased with herbivore species richness at seasonal rivers, which may be explained by a greater herbivore impact that over-rides the endurance of some species to grazing and trampling, leading to the decrease of grass species richness (McNaughton 1979, Milchunas et al. 1988). Besides the increase in absolute grass cover towards dry crests, it is also the relative cover of grasses that becomes increasingly important. Hejda et al. (2022) reported grasses to account for 83% of plant cover on the crest, with corresponding values of 68% at seasonal rivers and 50% at perennial rivers. Given that the relative cover of grasses on basalt exceeded that on granite (74% and 60%, respectively), basalt crests represent the environment where grasses gain the greatest advantage of all habitat/bedrock combinations we examined. The specific features of the clayey soils on basalt, with the quick water runoff and rapid desiccation, make it, at the same time, the most extreme environment for many forbs; this provides more niches for grass species, resulting in their greater species richness. Dye and Spear (1982) also found considerable interannual variation in grass biomass on clayey soils, probably in relation to precipitation variation, with large declines in biomass in dry years. Sandy soils were, in contrast, more stable in production of grass biomass, likely due to a carry-over effect of soil moisture stored in the subsoil from one year to another. Large moisture fluctuations in clayey soils may aggravate the establishment and survival of some species, leading to the persistence of fewer specialized species, such as Panicum coloratum or Bothriochloa radicans, that are typical of clayey soils, and, in turn, in lower species richness (Dye and Spear 1982).