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).