The Serengeti is ground-zero for nature’s struggle between life and death (consider, for instance, wildebeest versus lions). A new study in Journal of Ecology follows the early life struggles of one of Serengeti’s lesser known, but widely occurring, organisms: Acacia trees.
The research focuses on a long-standing ecological puzzle: what prevents savannahs from turning into forests? According to Dr. Tom Morrison, a research associate at the Institute of Biodiversity, Animal Health and Comparative Medicine at the University of Glasgow who led this study, “Savannahs occur because only a few young trees survive to become big trees. So the question is really: why do so few young trees survive to become saplings? The answer, it seems, is largely because of competition with grasses.”
In a 2-year experiment, Morrison and colleagues at Wake Forest University and University of Georgia in the U.S. transplanted ~2400 seeds and ~1200 seedlings of two native Acacia species across the Serengeti. Besides manipulating herbivory, rainfall and soil moisture, the researchers also removed grasses in some plots. “Experimental science like this is rare in the Serengeti because it is such a challenging environment,” says co-author Professor Michael Anderson. “This study was, shall we say, non-trivial in execution. Imagine trying to keep a million wildebeest from eating, and stomping on, vegetation across an area the size of Wales.”
The study shows that when trees germinate alongside grasses, the young trees struggle to survive their first year, regardless of other, potentially favorable conditions such as high water input and fertile soils. The fact that grasses are such a dominant factor in the lives of young trees was surprising, says Morrison, given the many other potential causes of death in the Serengeti. For instance, the Serengeti supports nearly 2 million large plant-eating herbivores and has extensive fires that burn vegetation on an annual basis.
A demographic model showed that in the presences of grasses, the best possible conditions for trees (no herbivores, high rainfall) only yielded about a 3% survival after 18 months, whereas removing grasses increased survival nearly 10-fold. “Herbivory is obviously bad news for trees and rainfall good news for seed germination, but we were stunned by the relative strength of the effect of grasses on young trees,” says Morrison.
The authors suggest that these results are important for understanding the processes by which savannas change over time. For instance, large areas of arid land in Eastern and Southern Africa have become increasing bushy in the last several decades. This “bush encroachment” is hugely problematic to livestock owners, as it reduces the amount of food that is available for their animals. Once established, this woody vegetation is difficult to remove. Surprisingly, the key to controlling it in the first place appears to be grass.
Press release written and prepared by Thomas Morrison, Research Associate, Institute of Biodiversity Animal Health and Comparative Medicine, University of Glasgow.