The ecoregion is characterized by dramatic vertical zonation of vegetation and asssociated fauna. This zonation is a consequence of abrupt elevational gradients between flatlands and mountains. Topographic relief is quite dramatic; for example, the Bighorn Mountains rise 2,794 m over the surrounding lowlands (Knight 1994). The range of biotic zones is greater in the higher mountains (e.g., Wind River and Teton Ranges in Wyoming; Madison Range in Montana). Secondary climatic effects of topographic relief (e.g., rain-shadow effects, exposure to or shelter from prevailing winds, and thermal inversions) likewise influence zonation (Peet 1988).
The Black Hills unit of the ecoregion, being the lowest in elevation and having relatively gentle topographic relief, exhibits the least amount of zonal variation. This unit has distinctive floristic diversity, however, containing flora representative of Great Basin, Eastern Deciduous, Boreal, Rocky Mountain, and Southern Great Plains (Knight 1994).
The dominant vegetation type in the ecoregion is coniferous forest. Küchler (1985) classifies the potential vegetation type as Douglas-fir / spruce-fir forest, which would be dominated by Engleman spruce (Picea englemannii), subalpine fir (Abies lasiocarpa), and Douglas-fir (Psuedotsuga menziesii). As Peet (1988) points out, however, most forests in the Rocky Mountains "are in some stage of recovery from prior disturbance . . . climax stands [are] less common than seral communities." Thus, instead of one or all of the expected fir species, large areas of the ecoregion are dominated by lodgepole pine. To some extent, the preponderance of lodgepole pine reflects the greatly altered (via fire suppression) disturbance regime in the ecoregion (Knight 1994). Whitebark pine (Pinus albicaulis) is an important species at the upper treeline / krummholz zone.
In addition to expansive conifer forests, the ecoregion contains several other vegetation communities. Mountain meadows, foothill grasslands, riparian woodlands, and upper treeline / alpine communities exist throughout the ecoregion. In the Yellowstone unit, unique biotic communities occur in association with geothermal features (e.g., geysers, hot springs), due to the micro-environments created by varying chemical compositions and relatively warm temperatures (Knight 1994).
Relative to other Rocky Mountain ecoregions, the South Central Rockies is dry, experiencing a predominantly continental climate. Summers are brief and winters long and cold. Significant precipitation occurs in the higher elevations, typically as snow.
Fire, snow avalanches, major seismic disturbances, and wind are major disturbance patterns in this ecoregion. Prevailing winds alter distribution and morphology of tree species at higher elevations, while periodic "blowdown" events can topple several hundred hectares of mature forest at a time. These blown-down areas in turn can fuel stand-replacing fires during dry seasons. Herbivory is also a significant influence, particularly on aspen and riparian willow communities (Knight 1994).
Habitat Loss and Degradation
Logging, hard-rock mining, oil and gas development, and recreational-residential construction are all major anthropogenic threats to the ecoregion. Domestic livestock grazing and spread of exotic species are altering species compositions. Burgeoning recreational use of remote areas is also affecting the ecoregion.
Remaining Blocks of Intact Habitat
Many of the ecoregion's mountain ranges are still relatively intact, though most have been altered somewhat by historic mining, logging, grazing, and fire suppression. The mountains in and immediately adjacent to Yellowstone NP have seen only minor human influence in the last century.
The following are largely intact:
•The Frank Church Wilderness - central Idaho
•the Lemhi and Lost River Ranges - eastern Idaho
•the Beaverhead - southwestern Montana
•Anaconda-Pintler - southwestern Montana
•Pioneer - southwestern Montana
•Tobacco Root - southwestern Montana
•Snowcrest - southwestern Montana
•Centennial - southwestern Montana
•Madison Ranges - southwestern Montana
The following are mostly intact:
•The Bridger - south-central Montana
•Big Belt - south-central Montana
•Little Belt - south-central Montana
•Crazy Mountains - south-central Montana
But these areas show the effects of being isolated ranges near large population centers. Mountains to the south of YNP, including the Wind River, Wyoming, and Salt River Ranges, are rugged, remote, and relatively intact.
Degree of Fragmentation
Intensive development in valley bottoms, combined with existing transportation corridors, is beginning to disrupt connectivity within the ecoregion. Low elevation development (often in ecoregions 57, 75, and 77) has eliminated winter range or blocked off migratory routes for ungulates. Massive clearcuts, particularly on the Targhee National Forest, have similarly caused fragmentation. Development of the high alpine environment has been limited to date, with only four major downhill ski resorts throughout the entire area.
Degree of Protection
The degree of protection in the ecoregion is fairly high, relative to some other Rocky Mountain ecoregions. Parks like Yellowstone and Grand Teton embrace a broad elevational and climatic gradient. Combined with adjacent wilderness areas, they form large blocks of protected habitat for many species. More work, however, still needs to be done to meet the needs of wider ranging species like grizzly bears.
Types and Severity of Threats
Indiscriminate logging, and especially associated roadbuilding, are major problems. Existing road networks in nonwilderness areas are quite dense and contribute to an overall loss of habitat security. Rapid development of low elevation areas is another threat, although concentrated mainly in other ecoregions. Mortality to grizzly bears and possibly to wolves through ungulate hunters in the fall is unacceptably high and could well be making the difference between a growing and a declining grizzly population in Greater Yellowstone.
Suite of Priority Activities to Enhance Biodiversity Conservation
•Identifying and maintaining critical linkage habitats within the ecoregion and among other ecoregions is vital.
•No more forest roads should be built, and seasonal closures and obliteration projects should be implemented to reduce motorized access.
•Management of visitor/recreationist behavior in parks and wildernesses should be increased. YNP should consider abandoning and obliterating part of its extensive paved road network.
Alliance for the Wild Rockies
Craighead Environmental Research Institute
Craighead Wildlife-Wildlands Institute
Defenders of Wildlife
The Ecology Center
The Great Bear Foundation Greater Yellowstone Coalition
Montana Wilderness Association
Northern Rockies Conservation Coop
Northern Rockies Conservation Coop
The Wilderness Society, Northern Rockies Regional Office
The Wildlands Project
The Yellowstone Institute
Wyoming Outdoor Council
Yellowstone Ecosystem Studies
Relationship to other classification schemes
This ecoregion closely matches Omernik's Middle Rockies, with the addition of the Frank Church and mountain ranges to the south and east of that area. It corresponds roughly to Bailey's M331A, D, E, and J. Bailey does not include the Black Hills as part of this ecoregion. Küchler classifies the area as 11, 14, 16, 45, and 49.