Evolution in Steppe with Few Large, Hooved Mammals

Abstract

The morphology of rhizomatous and caespitose grasses reflects the two extremes to which perennial grasses have evolved at least in partial response to continuous high versus low selection pressure by large congregating mammals. In North America steppe of the Bouteloua Province east of the Rockies is dominated by a mix of mainly rhizomatous C3 and C4 grasses which have long been associated with large herds of Bison and more recently with cattle. Introduction of cattle into these grasslands had much less effect on community structure than did livestock introduction into steppe of the Agropyron Province west of the Rockies which lacked large herds of mammals throughout the Holocene (and perhaps earlier). The underlying cause of native ungulate sparseness may have been related to the moisture cycle of the Prevailing Westerlies, which may have largely excluded C4 species, thereby severely controlling Bison numbers. In these communities both the dominant C3 caespitose grasses and the prominent cryptogam layer were soon destroyed by domestic ungulates and replaced largely by alien winter annuals. The relative changes in these two Provinces over the past 200 yr illustrate the importance in plants of herbivore-adapted traits in generating the overall physiognomy of some steppes and the resiliency of those grasslands to the introduction of novel selection pressure.