An inter-continental comparison of the effects of grazing and fire on savanna grassland plant community composition and ecosystem function

Background/Question/Methods

For millennia, herbivory and fire have shaped plant communities in mesic grassland ecosystems worldwide. These historical disturbance regimes are being altered (fire), and at times, completely eliminated (herbivory) from mesic grasslands. However, it is unclear if the effects of alterations in fire and loss of large herbivores will have similar or different effects on mesic grasslands with different large herbivore assemblages, e.g., North America with a single large herbivore and Southern Africa with a diversity of large herbivores. To assess the consequences of such alterations, we conducted a comparative experiment in which large herbivores were excluded for eight years from mesic savanna grassland sites burned at different frequencies in North America (Konza Prairie Biological Station) and South Africa (Kruger National Park). We hypothesized that the removal of a single grass-feeding herbivore from Konza would decrease plant community richness and shift community composition due to increased dominance by grasses. Similarly, we expected grass dominance to increase at Kruger with loss of large herbivores, but because large herbivores are more diverse at this study site - targeting both grasses and forbs - the changes due to large herbivore removal would be muted. 

Results/Conclusions

After eight years of large herbivore exclusion, the plant community at Konza changed drastically whereas little change was evident at Kruger. As predicted, removal of a single grazer (bison) at Konza caused reductions in richness and diversity occurring only one year after herbivore exclusion. These effects increased over time and were moderated by fire frequency, with the greatest reductions in richness and diversity in annually burned sites compared to a weaker, yet still significant, reduction without burning. In contrast, plant community structure at Kruger was relatively resistant to the removal of multiple types of large herbivores, regardless of fire frequency. We found that this divergence in response was largely due to differences in how herbivores impact dominance of the plant community. In both systems, increasing dominance results in a decrease in richness and diversity. However, only at Konza did the removal of herbivores increase dominance. In Kruger, herbivore removal did not impact dominance because the dominant caespitose grass is unpalatable. Thus, the diversity of large herbivores lost may be less important in determining plant community dynamics than the functional traits of the grasses that dominate mesic, disturbance-maintained, savanna grasslands.