Shaping the landscape: Fire–grazer interactions in an African Savanna
Ecological Society of America
Faculty of Computing, Health and Science
School of Natural Sciences / Centre for Ecosystem Management
The effects of fire–grazing interactions on grass communities are difficult to identify because fire and grazing influence each other on a landscape scale. Persistent heavy grazing can prevent the spread of fire by breaking up the grass layer. In contrast, frequent burning might inhibit the persistence of grazed patches by attracting grazers to the post-burn green flush. We explored the effect of burning on grazing activity, and the persistence of grazed patches, in a landscape-level experiment in a South African savanna.
We created 17 grazed patches by mowing grass in a 20 m diameter plot, with an adjacent un-mown control. We used dung counts as a measure of grazer visitation, and grass height as a measure of grazing intensity, at each of the sites over a year. Nearly all mowing treatments resulted in a rapid increase in grazing activity relative to controls (on average, 4–6 times more dung was found on mown sites). Subsequent fates of the grazed patches depended on their location with respect to fire. Burned areas drew animals off nearby unburned grazed patches, which then recovered lost biomass. Patches >1.5 km from burns remained grazed short. Frequent large fires might prevent areas of heavy grazing from persisting in the landscape, and thus limit the spread of grazing-adapted grasses.
Spatial information on fire frequencies in the park was used to explore the influence that the “magnet effect” of fire can have on grass communities. We mapped the distribution of tall, bunch grasslands and grazing-lawn grasslands using a 1999 Landsat TM satellite image. The extent of grazing lawns was directly related to fire return interval. Areas with a fire return of <4 years had less lawn grass than would be expected from the proportions of lawn grass in the park. A logistic regression analysis, which used various environmental variables known to influence grazing, showed fire history to be an important predictor of grazing-lawn distributions.
This work shows that, by influencing where, when, and for how long animals graze a patch, fire can influence the competitive balance between grazing-tolerant, and grazing-intolerant grass species and affect their distributions in the landscape. We discuss the implications of this research for the management of natural grazing systems and rangelands.