Grasslands and savannas occupy one eighth of the land surface on Earth, are economically important as grazing lands, and are highly impacted by human activities. They also have tremendous conservation value and support most remaining large native grazing animals in the world, particularly in Africa. A fundamental understanding of the factors most important for sustaining savanna grasslands, including their natural fire and grazing regimes, is required for their management and conservation. Scientists have questioned whether grasslands in different regions function similarly. Convergence in functioning would mean that ecological understanding and conservation strategies could be generalized for savanna grasslands worldwide. But contingent factors specific to a region, such as differences in evolutionary history or the types of grazers present, may lead to unique responses in some savanna grasslands. The goal of this research is to compare responses of key ecological processes to fire and grazing in North American and South African savanna grasslands. Scientists will take advantage of three ongoing, long-term experiments that manipulate fire return interval and the presence of large grazers. Responses of plant productivity, plant diversity and nutrient cycling will be compared, and the mechanisms underlying responses, such as differences in plant traits or evolutionary histories, will be assessed.
Comparative research elucidating the consequences of human alterations of fire and grazing regimes worldwide is needed to enable development of management strategies that can insure the sustainability of ecologically and economically important savanna grasslands. This research will provide the basic knowledge necessary to do so, as well as afford training and important global and cultural experiences for young scientists, graduate students and undergraduates. Finally, though collaborations that are already established, results will be made available to scientists and managers in South Africa.
In grasslands and savannas, human have changed the patterns of fire and grazing that have long been important in these ecosystems. In most savanna grasslands today, either all or most native large herbivores have been lost and historic fire regimes have been changed to either complete fire suppression or annual burning. Because both fire and grazing are important in these systems, our research project’s goal was to learn more about the consequences of the combined alteration of fire and grazing. This is particularly important for African savanna grasslands that still harbor diverse, large herbivore populations that exist nowhere else. Such knowledge is necessary for managing and conserving these ecologically and economically important ecosystems. For almost a decade, we have compared how savanna grasslands in North America (Konza Prairie Biological Station, Kansas, USA) and South Africa (Kruger National Park) respond to changes in fire frequency and the loss of grazers. Our experiments have shown that fire and grazing have similar effects on many processes, such as how much grass is produced each year, but also very divergent effects on the number and types of plants (the plant communities) at Konza and Kruger. The most striking difference was found in experiments where grazers were removed from the savanna grasslands. We observed dramatic changes in plant communities, in particular a large reduction in plant diversity, with the loss of grazers at Konza but very little change in Kruger. We conclude that the unexpected lack of response to removal of grazers in South Africa is due several important traits in the grasses that are most abundant at Kruger. The most important trait is that one of the most abundant grasses is unpalatable due to the presence of multiple aromatic chemical compounds produced in its leaves. Thus, grazers avoid this grass when possible, and thus these savanna grasslands do not respond strongly to the removal of grazers. In contrast, the most abundant grasses at Konza are palatable and are preferred as food by grazers. Here, when grazers are removed, rapid plant community and grass production responses occur. The comparisons between Konza and Kruger have been invaluable for enhancing our depth of understanding of both systems. In particular for Kruger, this knowledge is essential for determining how to best manage this savanna grassland in one of the world’s premier national park and the crown jewel of the South African National Parks system, with >1 million visitors each year.
