Tipping points induced by palaeo-human impacts can explain presence of savannah in Malagasy and global systems where forest is expected

Abstract

Models aimed at understanding C 4 -savannah distribution for Australia, Africa and South America support transition to forest at high mean annual precipitation (MAP), and savannah grasslands of Madagascar have recently been reported to be similarly limited. Yet, when savannah/grassland presence data are plotted against MAP for the various ecosystems across the Malagasy Central Highlands, the relationship does not hold. Furthermore, it does not always hold in other sites on other continents. Instead, in high-rainfall savannahs, palaeo-human impacts appear to have selected a fire-adapted habitat, creating tipping points that allow savannah persistence despite high rainfall, suppressing forest return. We conducted the largest systematic literature review to date for global evidence of palaeo-human impacts in savannahs, and conclude that impacts are widespread and should be incorporated into models aimed at understanding savannah persistence at elevated precipitation, particularly as more palaeodata emerges. Building on existing studies, we refine the MAP savannah relationship at higher MAP. Palaeoanthropogenic impact can help explain inconsistencies in the savannah/forest boundary at higher MAP, and points to a key role for palaeoecology in understanding systems. Including these effects presents a crucial change to our understanding of factors determining global savannah distribution, supporting a human hand in much of their formation.