Contrasting wildlife and livestock impacts on plant biomass dynamics inside and outside the Serengeti National Park, Tanzania

Abstract

African savannahs represent one of the world's most productive ecosystems and one of the last vestiges of diverse wild herbivore populations. Accompanying human population growth, livestock is replacing wildlife as the dominant herbivore. To understand the impact of this shift in herbivore assemblage, we established a network of exclosures across a rainfall gradient contrasting pastoral and wildlife management around the Serengeti National Park, Tanzania. Small-scale exclosures and open plots were established underneath leguminous and non-leguminous trees and outside canopies to account for the landscape structure. Every month herbaceous biomass was estimated non-destructively in 128 plots using a calibrated pasture disc. For each site, monthly herbivore dung surveys were used to estimate the herbivore assemblage. Additionally, plant community composition was surveyed and root biomass determined via ingrowth cores. After the first year, aboveground biomass was most strongly associated with plant species composition with greater production for communities in the drier region. On average, the greatest aboveground biomass occurred inside exclosures underneath leguminous trees. Meanwhile, root biomass production was highest in the wetter region suggesting a shift of investment from belowground than aboveground across the rainfall gradient. Herbivore assemblage varied in space and time, but did not consistently influence aboveground biomass accrual. Thus, our results suggest plant biomass production across wild and domestic herbivore assemblages relates mainly to plant species composition and these species' adaption to climate variability. We discuss our results in the context of a changing savannah landscape involving people, wildlife and climate seasonality.