Evolution in steppe with few large, hooved mammals

Mack, Richard; Thompson, John

Communities are organized through both the proximate and evolutionary constraints that shape the structure of populations and the interaction among species. Proximate constraints can be manipulated experimentally to assess the relative influence of a component species on the overall structure of a community, as in analyses of keystone (Paine 1969) or dominant species. These analyses are instructive in detecting how current selection pressures influence populations. Responses of species to current interspecific interactions, however, are not made in an evolutionary vacuum, but rather reflect the history of prior selection pressures that have molded the species' traits. Overall structure of some communities in turn may be influenced to a large extent by a few traits evolving in the most abundant species in response to continuous strong selection pressure. Assessment of the effect of these particular traits or suites of traits on community organization is difficult on an experimental basis as it necessitates removal of traits within species rather than species themselves (e.g., Windle and Franz 1979). Grasses with their basal intercalary meristems are cited commonly as having evolved with large herbivorous mammals (Osborn 1910; Love 1959; Barnard and Frankel 1964). Yet temperate grasslands and specifically steppes differ greatly in the extent of habitation by large mammals throughout the Neogene and hence the plants in these grasslands differ in the extent to which they have traits adapted to large mammalian grazers. In this paper we consider the structure of steppes on either side of the Rocky Mountains in an attempt to understand how differences in the history of acquisition of mammal-selected traits by plants and interactions with mammalian grazers can influence overall grassland structure.