Southern Africa is expected to experience a greater frequency of El Niño Southern Oscillation (ENSO) effects associated with increased global temperatures. ENSO effects in southern-Africa are associated with extreme drought conditions in the savanna and grassland biomes with strong impacts on the two dominant consumers of vegetative biomass in these systems (herbivores and fire). These consumers have contrasting impacts on grass and tree communities – selecting for different suites of vegetation traits associated with fire-tolerance and grazing/browsing-tolerance, respectively. Many savannas are influenced by both fire and herbivory, and landscapes can transition from being fire-dominated (black world) to herbivore-dominated (brown world) depending on past management activities. The processes driving these transitions and especially the effects that drought events have on them are not yet well understood, although they have important consequences for the fauna and flora. The Kruger National Park in South Africa has recently experienced a major drought event linked with ENSO. Prior to this drought, we setup a landscape scale experiment to determine community and functional responses to varying levels of fire and herbivory on grass community composition and function and monitored changes until one year post drought.
We found that concentrating grazers in specific areas of fire dominated grasslands shortened grass over long periods and ultimately eliminated fire. This change in the dominant consumer resulted in increases in palatable, stoloniferous grasses and grass species with low root to stem ratios. Areas that were repeatedly burnt with herbivores removed did not show the same shifts. The onset of the drought drove an almost complete loss of previously dominant fire-adapted grass species from all high intensity grazing treatments. Fire treatments experienced a decrease in one of the more dominant palatable grass species Themeda triandra during the drought with annual grasses increasing immediately after the drought broke. Contrary to expectations, the drought appears to have driven further functional segregation in grass communities within fire driven landscapes compared with those under heavy grazing. It appears that switches between black and brown world system states require disturbances that act to concentrate or disperse grazers in the broader landscape and are not largely impacted by short-term drought events.