Plant compositional change over time increases with rainfall in Serengeti grasslands

Previous work from the Plains of Serengeti National Park (SNP) suggested that plant species composition (i.e. the composite measure of species' identities and abundances) is unchanged on the temporal scale of decades ( Belsky 1985 ), raising the possibility that African grazing ecosystems may be resistant to climate change. In this study, grassland plant composition was measured three times between 2000 and 2007 at eight permanent sites spanning the SNP environmental gradient. Confirming previous work, plots in the Serengeti Plains were relatively stable with respect to plant composition through time. In contrast, sites in all other regions of the ecosystem showed greater change in plant species composition during the study. Three factors, annual rainfall, dry season rainfall, and fire frequency, were linearly correlated (all r>0.73) with the amount of composition change at the sites. A path model, which accounted for the strong correlation between annual and dry season rainfall and for the known effect of annual rainfall on fire frequency, was used to elucidate which factors were associated with composition change and which were spuriously correlated. The path model demonstrated that change in plant composition was positively related to annual rainfall, but not dry season rainfall or fire frequency. However, dry season rainfall was positively associated with changes in plant species identity, one component of compositional change, across sites over time. Because climate models predict both increased wet season and decreased dry season rainfall in East Africa, these findings raise uncertainty about how these grasslands will respond to future changes. This study and others ( Belsky 1985 ) suggests that plant assemblages adapted to low rainfall in Serengeti may exhibit compositionally stability, while the same may not be true in regions of higher average rainfall and larger species pools.