STEPPE-TUNDRA TRANSITION - A HERBIVORE-DRIVEN BIOME SHIFT AT THE END OF THE PLEISTOCENE

A simulation model, recent experiments, and the literature provide con sistent evidence that megafauna extinctions caused by human hunting co uld have played as great a role as climate in shifting from a vegetati on mosaic with abundant grass-dominated steppe to a mosaic dominated b y moss tundra in Beringia at the end of the Pleistocene. General circu lation models suggest that the Pleistocene environment of Beringia was colder than at the present with broadly similar wind patterns and pre cipitation but wetter soils. These and other observations suggest that the steppelike vegetation and dry soils of Beringia in the late Pleis tocene were not a direct consequence of an arid macroclimate. Tramplin g and grazing by mammalian grazers in tundra cause a shift in dominanc e from mosses to grasses. Grasses reduce soil moisture more effectivel y than mosses through high rates of evapotranspiration. Results of a s imulation model based on plant competition for water and light and pla nt sensitivity to grazers and nutrient supply predict that either of t wo vegetation types, grass-dominated steppe or moss-dominated tundra, could exist in Beringia under both current and Pleistocene climates. T he model suggests that moss-dominated tundra is favored when grazing i s reduced below levels that are in equilibrium with climate and vegeta tion. Together these results indicate that mammalian grazers have a su fficiently large effect on vegetation and soil moisture that their ext inction could have contributed substantially to the shift from predomi nance of steppe to tundra at the Pleistocene-Holocene boundary. Our hy pothesis suggests a mechanism by which the steppe ecosystem could be r estored to portions of its former range. We also suggest that mammalia n impacts on vegetation are sufficiently large that future vegetation cannot be predicted from climate scenarios without considering the rol e of mammals.