Ungulate herbivory overrides rainfall impacts on herbaceous regrowth and residual biomass in a key resource area

• Herbivore exclosures utilized to quantify ungulates impact on herbaceous regrowth.
• Both ungulates and rainfall limits herbaceous regrowth.
• Ungulates impacts overrode rainfall.
• Ungulates reduced residual biomass that determines regrowth after defoliation.
• Increased frequency of defoliation lowers herbaceous regrowth during the dry season.

Key grazing lands that provide dry season forage to both resident and migrating ungulates may experience heavy grazing impacts during the dry season, thereby jeopardizing future forage productivity. In this study a herbivore exclosure experiment was used to quantify the effects of grazing by large ungulates on herbaceous regrowth and residual aboveground biomass in a fragmented key resource area; the fringe zone around Lake Naivasha, Kenya. Top-down control mechanisms were prevalent in both the dry and wet seasons suggesting the existence of a high resident herbivore density. Intense grazing significantly reduced residual biomass that in turn reduced plant regrowth. An increased frequency of defoliation reduced regrowth during the dry season demonstrating the negative effect resulting from high herbivore densities during the dry season. This study indicates that grazing exerts a higher control on regrowth than rainfall as heavily grazed residual biomass did not recover during the following wet season.