Grass species richness: mixed effects of grazing and habitat
We found no relationship between herbivore abundance and grass species richness, which is in contrast with the results of McNaughton (1979) or Milchunas et al. (1988). However, when testing the effect of herbivore abundance or species richness without accounting for other predictors, we found a marginally significant unimodal relationship (p = 0.059 and p = 0.077 for herbivore abundance and species richness, respectively). Theoretically, the greatest grass species richness could be expected at intermediate grazing levels, when strong dominants are suppressed, and disturbances created by herbivores allow for early successional species also to occur (Thrash et al. 1993, Todd 2006). The positive effects of moderate herbivore pressure include the removal of biomass by grazing and other consequences of grazing animals’ presence (trampling, urinating, wallowing, digging, etc.) that collectively result in a mosaic of microhabitats providing niches for more species than in a uniform grassland. These other animal activities are concentrated mainly at water sources because animals come to drink, cool down, and also rest in the shade of gallery trees (Olff and Ritchie 1998, Bakker et al. 2006). Such activities bring about different effects on plants than just grazing. For example, trampling often leads to a shift from tall palatable species like Panicum maximum or Themeda triandrato annual small-statured species, such as Tragus berteronianus orAristida spp. that tolerate intensive soil disturbances and compaction, while intensive grazing itself promotes small-statured perennials like Cynodon dactylon or shrub-like statured grasses as Bothriochloa radicans or Cenchrus ciliaris . Further, the occurrence of some of the herbivores that we recorded in our plots is more or less restricted to the vicinity of water sources, such as hippo, buffalo, and waterbuck (Fig. 3). The patches with moderate grazing pressure can be inhabited by a range of grass species differing in their competitiveness, tolerance to disturbance or nutrient requirements.
While the gradient in herbivore abundances across all habitats largely followed the increase in water availability, with the lowest pressure on crests and highest at perennial rivers, the gradient in herbivore species richness is not primarily driven by habitat. With the exception of extremely poor and rich plots that are located at crests and perennial rivers, respectively, the herbivore species richness is more or less evenly spread across the gradient of water availability.