Habitat and bedrock modify the relationship between plant and herbivore
species richness in a South-African savanna
Abstract
How grazing interacts with environmental conditions in determining grass
species richness and abundance in savanna is still insufficiently
understood. Quantifying the impact of herbivores on ecosystems can
provide some insights into these relationships. In Kruger National Park,
South-Africa, we recorded grass species and estimated their covers in 60
plots 50 × 50 m in size, selected to account for varying proximity to
water and nutrient availability. Thus, we located plots (i) near
perennial rivers, near seasonal rivers, and on crests that are distant
from all waterbodies, and (ii) on nutrient-rich basaltic and
nutrient-poor granitic bedrock. The presence and abundance of large
herbivores was recorded by 60 camera traps located in the same plots.
Grass cover decreased significantly with herbivore abundance and
differed between habitats and bedrock types, with plots at crests
showing the highest cover and plots near perennial rivers the lowest
grass cover; on basalts grasses reached higher cover than on granites.
The relationship between herbivore abundance and grass species richness
changed with the type of bedrock; it was positive on basalts but
negative at granite plots. Similarly, the relationship between herbivore
species richness and grass species richness was positive on crests and
on basalts, but negative near seasonal rivers and on granites; near
perennial rivers the relationship was non-significant. We suggest that
the positive relationship between herbivore richness or abundance and
grass species richness, which is more pronounced on basalts, is due to
herbivores supressing dominant grass species. This may increase
microsite heterogeneity and therefore support grass species richness. In
contrast, the decrease in grass species richness with herbivore species
richness at seasonal rivers indicates that the high impact over-rides
the resistance of some species to grazing and trampling.