Wednesday, August 5, 2009

PS 54-143: Large ungulate herbivory and ANPP in South African and North American savanna grasslands

David L. Hoover1, Greg Buis1, Annikki Chamberlain, Richard W.S. Fynn2, Deron E. Burkepile3, Tadj Schreck4, J.M. Blair5, Melinda D. Smith6, Scott L. Collins7, and Alan K. Knapp1. (1) Colorado State University, (2) University of KwaZulu-Natal, (3) Florida International University, (4) University of California, Irvine, (5) Kansas State University, (6) Yale University, (7) University of New Mexico


The structure and function of savanna grassland ecosystems are influenced by three primary drivers: large ungulate herbivory, fire and climate variability.  While North American and South African savanna grasslands share these, some past research has indicated that their responses to them differ.  The goal of this project was to determine if herbivory and fire regime affects aboveground net primary production similarly in Kruger National Park (KNP), South Africa and Konza Prairie (KNZ) in Kansas.  Permanent and moveable grazing enclosures were established in 2007 in a long-term (20-50 years) experiment in which fire was manipulated to achieve a range of fire regimes (annual, 3-4 year and unburned).  Large ungulate treatments included herds of single herbivores such as bison (KNZ) or Cape buffalo (KNP) and herds of diverse herbivores with more than 20 species of large grazers and browsers (KNP).  There were three main questions this project addressed.  First, how does grazing under various fire regimes affect ANPP?  Secondly, how does productivity respond to multiple and single large ungulate as well as grazing exclusion?  Finally, are the responses in ANPP to fire and grazing convergent or divergent between North American and South African savanna grasslands?


During the 2007 growing season, there were primarily divergent responses to fire and grazing between the North American and South African sites.  At KNP, ANPP was the highest in the unburned plots, with little differences between the annual and intermediate burns for single, multiple and no grazing treatments.  At KNZ, there was no difference in ANPP among burning treatments.  At KNP, grazing by multiple and single herbivores increased productivity in the annual burned treatments by 39% and 52%, respectively.  In contrast, there was no significant response in ANPP to grazing at KNZ.  Overall, these results suggest that ANPP in South Africa savanna grasslands is much more responsive to herbivory, which is consistent with a much longer evolutionary history of plant-grazer interactions. Moreover, the reduction in ANPP response to herbivory in South Africa as herbivore diversity declined suggests that the historic loss of grazer diversity in North American savanna grasslands may also have affected this ecosystem’s response to grazing.