Introduced forage species planted as monocultures often possess traits that promote persistence under intense grazing by domestic and native herbivores. Rapid maturation, secondary chemicals, and fungal associations are among the traits that enhance persistence and can facilitate dominance over native plants that are preferentially selected by native and domestic herbivores. Forages native to North American evolved under a fire-grazing interaction whereas many introduced forage plants evolved under heavy grazing pressure without the fire interaction. We believe that exotic forage species should be considered as special cases in invasion ecology because they are intended to persist under heavy grazing but also be useful in a forage context. We have focused on four functionally different introduced forage species, all invading native grasslands and all adapted to grazing through mechanisms that limit palatability. These species invade grazed grasslands because they have traits that increase their resistance to intense herbivory providing an advantage when animals selectively graze in mixed species communities.
Results/Conclusions When patch fires are applied, grazing animals select less at the species level and more at the patch level, reducing the effectiveness of the grazing resistance mechanisms. We have developed a conceptual framework, with evolutionary patterns of disturbance and grazing preference as the key to successfully limiting invasion of exotic forages in natural ecosystems.
