To the west of the central plains, the topographical diversity of the great mountain chains creates unparalleled ecological heterogeneity. Rocky Mountain National Park, for example, includes 13 major vegetation zones, all within a few miles of each other (Stohlgren in prep.). In the prairie ecosystem at the foot of the mountains, prairie dogs perform a keystone role and directly or indirectly support 170 vertebrate species, including endangered black-footed ferrets (Miller et al. 1994). Other threatened species include mountain plovers, ferruginous hawks, and swift foxes. Partway up the slopes, old-growth forests are a key resource for many wildlife species such as caribou, wolverines, Albert's squirrels, fishers, northern goshawk, and several owls. Protected areas are home to several species that today occupy only a fraction of their former range, including grizzlies, wolves, caribou, bighorn sheep, and beaver. Although they occupy only a fraction of the landscape, mature aspen ecosystems play a key role for hundreds of birds and mammals, and are especially important for deer and elk (Stohlgren in prep.). Farther up the slopes, past the tree line, the alpine tundra zone is home to a host of plant species shared with arctic ecosystems (Billings 1988).
Global change brings an additional stress that, in the face of existing pressures, these ecosystem complexes are ill suited to withstand. Increasingly, protected areas are becoming islands of natural habitat in a sea of human development. Rocky Mountain National Park and the surrounding region is at the forefront of these impacts (Stohlgren et al. 1995) and provides a poignant example of what may be in store for other mountain protected areas of the Rockies, such as Montana's Glacier National Park and Banff and Jasper National Parks much farther to the north in Canada. A litany of change, so far mainly unrelated to climate, has resulted in the declines of a host of species for diverse reasons (see Stohlgren et al. 1995, Stohlgren in prep.). For example, prairie dog control programs throughout the west resulted in 98% reductions in their populations and the near extinction of black-footed ferrets. Currently, widespread declines in amphibian populations are being observed, including western toads, boreal toads, and leopard frogs.
A long history of fire suppression has caused steady declines in the habitat for species that depend on post-fire seral stages, such as the black-backed woodpecker and many understory plants. Populations of many old-growth specialists are declining, a net effect of logging in the region that has resulted in the loss of the large trees and their replacement with younger, faster growing trees. In contrast, elk have tended to benefit from reduced predation and hunting, but the steady loss of habitat outside the park has lead to the concentration of more and more animals into the park. Migratory routes are now cut off because of development, and the herd is stationary for the first time in history.
Coupled with these habitat changes are changes in the agressive exotic competitors that the native flora and fauna must face. More and more non-native species are invading the area, with serious effects. Introduced blister rust in concert with fire suppression appears to be irreversibly damaging white bark pine ecosystems, an important food resource for jays, grizzlies, and other animal species. Introduced moose are now invading the lush riparian zones of the park in increasing numbers. The effect of this population increase is unknown, but increases in moose populations have led to marked changes in plant communities elsewhere (Pastor et al. 1993, Peterson 1977, Peterson et al. 1984, Wedeles and Van Damme 1995).
These are only a subset of the problems that beset Rocky Mountain National Park, which include other, more subtle, effects of human activities (Stohlgren et al. 1995, Stohlgren in prep.). Observations and sophisticated mesoscale climate models show that land use outside the park is greatly influencing weather patterns within the park (Pielke et al. 1993). Agricultural development has changed the surface albedo and landscape "roughness" surrounding the park, resulting in increased intensities of summer storms within the park. Other local climate alterations appear to be resulting from this land-use change, including cooler and wetter summers at low elevations. These trends, together with warmer winters resulting from global climate change, will probably combine to increase fire risk.
In addition to pressures that arise from outside the park but drive change within it, there is global warming. As annual temperatures gradually increase, plants and animals will find themselves in a new environment, and a reassortment of the existing ecosystem mosaic will be the result. Just as models predict that arctic ecosystems will be at the forefront of the global impacts of global warming, so too will be montane and high alpine systems. In common with their arctic conspecifics farther north, alpine plants are able to withstand low temperatures and reproduce in the extreme cold of high altitudes. With sufficient warming, and the right precipitation conditions, plants currently at lower elevations could invade and displace high alpine species. If it occurs, this "migration" of ecosystems upslope is predicted to have devastating impacts on alpine ecosystems. Studies of the tree line in Rocky Mountain National Park reveal that the impact of the current long-term warming trend may already be under way. Evidence for global warming impacts can be seen in the fact that the stunted, wind-trimmed Engelmann spruce trees at the tree line (termed Krummholz vegetation) are growing at an abnormally high rate and are showing both lateral and height extension. These trees can be hundreds of years old and only a couple of feet high, yet now, warmer conditions (and possibly changes in snowpack) are causing them to sprout and grow like normal trees. The sparse tree community at the tree line is closing in and crowding out understory and alpine tundra plant species (Stohlgren et al. 1995).
Small changes in the amount of snowfall and the timing of snowmelt could have major implications for alpine flora. If warmer or drier conditions cause earlier snowmelt, this will change the competitive relationships between plants in the alpine zones. For example, studies in Colorado show that early snowmelt may initially increase the population of alpine snow buttercups by increasing the growing season, but that eventually will lead to more plants with large seeds in the population. Large seeds improve competitiveness under a stable climate but reduce the ability of plants to spread upslope in response to rapidly warming climate, so the net effect could be negative in the long term (Galen and Stanton, 1993). Another, similar impact has been identified for Delphinium nelsonii in dry subalpine meadows near Rocky Mountain National Park. Earlier snowmelt exposes the plants to colder spring temperatures, thus delaying flowering and reducing flower production. This impact cascades through the food web and reduces populations of bumblebees and broad-tailed hummingbirds, which depend on the delphiniums. The bumblebees and hummingbirds normally depend on nector from early-flowering delphiniums, and later in the season act as pollinators for other plants (Inouye and McGuire 1991).
Global warming is expected to bring other changes to already stressed park ecosystems. In Rocky Mountain National Park, as in other conifer-dominated systems, fire plays a key role in forest dynamics, and changes in fire frequencies have important implications for old-growth systems. Tree-ring studies of previous fires in the park demonstrate the strong link between climatic conditions and fire occurrence. During the extreme droughts that occur about once per century, nearly half of the entire montane zone burns. Periods of high variability in rainfall also tend to have more fires. Scenarios of global warming suggest increased occurrence of both threats: increased variability in climate conditions and increased frequencies of the once-a-century droughts. Biologists also suspect that increased disturbance rates in the park, wrought both by agents within the park and by atmospheric change, will lead to increased opportunities for invasions by non-native species (Stohlgren pers. com.). These invasions will in many cases displace native species before they become susceptible to the effects of changed climate per se.
The overall outlook for Rocky Mountain National Park in the coming decades is one of accelerating environmental degradation as a result of rapid population growth and land use change in the near vicinity, exacerbated by accelerating rates of global warming. The park has one of the leading climate change impact research programs, and along with those centered on other mountain protected areas, including Olympic, Glacier, Sequoia-Kings Canyon and Great Smoky Mountains National Parks, could become part of an invaluable network for monitoring change as well as a crucible for testing the integrated management strategies that will be needed to cope with environmental change in the next few years.