The African savannas are vast expanses of terrestrial ecosystems that host an array of plants and animals and their interactions that have continued to fascinate ecologists and the public alike. Many important conceptual advances regarding plant-herbivore interactions, and particularly the role of grazing animals, have come from studies in these ecosystems including: accelerating turnover of key nutrients for grazers (McNaughton, Banyikwa & McNaughton, 1997); determining tree-grass cover (Sankaran et al., 2005) and even the relative impact of fire combustion on aboveground biomass (Bond & Keeley, 2005).
These unique systems combine complex ecological interactions with wide variation in soil fertility, and boast some of the most diverse large grazer diversity in the world. At the same time, the shifting spatial and temporal scales, which make climate, fire, and grazing impacts in these systems functionally important, also makes it difficult to decipher and to definitively attribute causality, especially in the face of human-induced climate change that is altering precipitation regimes in the region.
The Editor’s Choice article for this issue by Staver et al. (2018) is a new step forward in our understanding of how important these grazing animals are to plant response to low rainfall years. The authors demonstrate that herbivore movement has a dramatic effect on the vegetative response during drought years, evaluated over two and a half decades, from 1990 to 2015. The severe decline in grass biomass in drought years observed in this study was more dramatic than declines seen in many other regions of the world. Moreover, adjacent areas (grass refugia) that did not experience drought also showed marked reductions in biomass in drought years.
The authors attribute these differences in response to the fact that grazing animals that moved in the landscape exacerbated the physiological stress of the plants through increased consumption of biomass, and these animals became increasingly dependent on grass refugia to sustain their populations in low rainfall years. The study highlights that in order to understand the realized impacts of changing precipitation in these ecosystems, we must look further than the ecophysiological responses of the vegetation and recognize that how the buffalo (and other large grazers) roam is a key landscape attribute affecting ecosystem-level responses to climate variability in these savanna ecosystems.
Amy Austin, Senior Editor, Journal of Ecology
Read the full paper online: Grazer movements exacerbate grass declines during drought in an African savanna