The impact of herbivore exclusion on forb diversity: Comparing species and functional responses during a drought

Rainfall and herbivory shape savannah herbaceous communities, but these disturbances are being altered globally. To assess potential consequences of such alterations, we evaluated herbivore effects on species and functional diversity during an episodic drought in a sodic savannah using data collected from long-term herbivore exclosures in the Kruger National Park, South Africa. Herbaceous life forms are rarely acknowledged as distinct functional entities. Moreover, the functional ecology of forbs remains elusive. Here, we present disturbances–responses by forbs separately from grasses. We hypothesised that combinations of intense utilisation and drought would be associated with low diversity and high dominance at species and functional levels for both life forms. Contrary to our hypothesis, low forb and grass diversity was associated with long-term herbivore exclusion, which exceeded expected undesirable effects of intense utilisation and drought. Grasses responded less sensitively, suggesting that forbs respond dynamically to changes in herbivore assemblage when these alterations are combined with drought. Consistent with patterns in savannah systems, forbs contributed significantly to species and functional trait diversity. High forb diversity is suggested to enhance resilience of this nutrient-rich ecosystem against declines in its functioning when subjected to drought and alterations in herbivory.