The distribution, structure and function of mesic savanna grasslands are strongly driven by fire regimes, grazing by large herbivores, and their interactions. There is evidence to suggest, however, that fire and grazing influence savanna grassland structure and function differently in South Africa (SA) compared to North America (NA). These differences have been attributed to the contingent factors of greater biome age, longer evolutionary history with fire and grazing, reduced soil fertility, and greater diversity of plants and large herbivores in SA.
An alternative hypothesis is that differences in methods and approaches used to study these systems have led to differing perspectives on the role of these drivers. If the impacts of shared ecosystem drivers truly differ between NA and SA, this calls into question the generality of our understanding of these ecosystems and our ability to forecast how changes in key drivers will affect savanna grasslands globally. Since 2005, we have conducted an explicitly comparative research program to determine the degree of convergence in ecosystem (productivity, N and C cycling) and plant community (composition, diversity, dynamics) responses to fire and grazing in SA and NA. After 6 years of study, initial support has been found for convergence at the ecosystem level and divergence at the community level in response to alterations in both fire regimes and grazing. However, there have also been two unexpected findings (1) the ways in which fire and grazing interact differed between NA and SA, and (2) the rate of change in communities when grazers were removed was much greater in NA than in SA. Future questions to be addressed include (Q1) Will community structure and composition differ with altered fire regimes in the absence of grazing in SA and will the effects of altered fire regimes be different from those observed in NA? (Q2) What are the determinants of the different rates of change and how will these determinants influence future trajectories of change? Finally, (Q3) Will the different rates and trajectories of community change be mirrored by responses in ecosystem function over time? We are answering these and other questions through a large herbivore exclusion study established within the context of long-term (25-50+ yr) experimental manipulations of fire frequency at the Konza Prairie Biological Station (KPBS) in NA and the Kruger National Park (KNP) in SA. We are using a suite of core studies and measurements (plant community composition, ANPP, and herbivore abundance and distribution) to address our questions.
Our research is designed to address an important question about our understanding of savanna grasslands globally: Is our knowledge of fire and grazing sufficiently general to enable us to make accurate predictions of how these ecosystems will respond to changes in these drivers over time? These questions are important to answer not only because fire regimes and large herbivore abundance and diversity are being altered in savanna grasslands globally. Although much has been learned about relationships between community diversity and ecosystem function over the past decades, how differing rates and trajectories of change in communities influence ecosystem function is largely unknown. This understanding is highly relevant for predicting how ecosystems worldwide will respond to multiple drivers of global change.