Cattle and wildlife effects on persistence of a successional savanna landscape mosaic

Background/Question/Methods Landscape heterogeneity is an important and influential driver of ecological processes.  In semiarid ecosystems, heterogeneity may be especially important, commonly occurring in the form of nutrient-rich patches that support specialized vegetation and attract large herbivores (e.g., grazing lawns in the Serengeti).  Large herbivores have the potential to directly influence the longevity of patch vegetation through feedbacks on soil and plant community dynamics.  Here we focus on how large herbivores, both wild and domestic, may slow or speed vegetation succession in these patches. In wooded grassland ecosystem in Laikipia, Kenya, treeless glades derived from abandoned cattle corrals are long-term “hot spots” of increased nutrient levels, improved soil texture, palatable grasses, and herbivore use.  Glades persist on the landscape for several decades, undergoing succession from dominance by the grass Cynodon plectostachyus to dominance by the grass Pennisetum stramineum.  We used 1m x1m x 1m herbivore exclusion cages to test whether herbivory influences invasion of the late-successional Pennisetum grass species into stands of the early successional Cynodon grass species.  We then used a landscape-scale factorial experiment (KLEE, the Kenya Long-term Exclosure Experiment), to test how wildlife vs. cattle affect succession in glades.

Results/Conclusions The small-scale exclosure experiment indicated that large herbivores retard succession by suppressing invasion of the later successional Pennisetum species into Cynodon-dominated areas. The large-scale exclosure experiment showed that cover of the early successional Cynodon species was greater in glades accessible only to wildlife; this suggests that herbivore wildlife are responsible for ‘setting back’ succession at the glade scale, keeping vegetation in an earlier successional state that is more attractive to wildlife and livestock.  A growing body of evidence from other systems also suggests that herbivory modulates successional dynamics; here we have uniquely tested how the effects of cattle and wildlife may differ.  Through their effects on glade vegetation succession, wildlife behavior has direct implications for the longevity of the glade landscape mosaic.  This information is particularly relevant in these savanna landscapes where managing for the location and persistence of these “hot spots” may be useful to both livestock and wildlife managers.