A paleoecologic perspective on past plant invasions in Yellowstone

The role of climate and natural disturbance in the past provides a context for understanding present and future changes in biota. The vegetation histo ry of the Yellowstone region. like that of North America as a whole, is lar gely one of plant invasions and extinctions in response to changes in clima te and environment. When Holocene plant migrations are examined on multiple spatial and temporal scales, several generalities are apparent. First, at a continental and regional scale, plant migration patterns followed the dir ection of climate change, whereas at local scales plant colonization was go verned by site-specific conditions and possibly by biotic interactions. Sec ond, species were individualistic in their response to climate change, and, as their ranges shifted across the landscape, existing communities were di smantled and new ones were formed. individual species met little resistance from existing communities. Third, rates of species invasion were astonishi ngly, rapid, suggesting that rare long-distance dispersal events were criti cal. Fourth, fire during periods of climate change was an important catalys t in allowing the invasion of new species, but it is unlikely that a single fire event triggered irreversible vegetation change. Regional climate and biotic changes in response to projected increases in a tmospheric CO2 in the next century suggest an even more complex picture tha n in the past. Model simulations portray changes in temperature and precipi tation in the Yellowstone region that have not occurred in the last 20,000 years. Likewise, projected changes in species ranges, including latitudinal , longitudinal, and elevational shifts, require faster rates than anything observed in the fossil record. Increased fire occurrence may help maintain sonic native taxa but promote the decline of others. Thus, future condition s are likely to create evermore opportunities for exotic species to invade and establish within the Yellowstone region.