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.