The ecology of the Rocky Mountains is diverse due to the effects of a variety of environmental factors. The Rocky Mountains are the major mountain range in western North America, running from the far north of British Columbia in Canada to New Mexico in the southwestern United States, climbing from the Great Plains at or below to peaks of over . Temperature and rainfall varies greatly also and thus the Rockies are home to a mixture of habitats including the alpine, subalpine and boreal habitats of the Northern Rocky Mountains in British Columbia and Alberta, the coniferous forests of Montana and Idaho, the wetlands and prairie where the Rockies meet the plains, a different mix of conifers on the Yellowstone Plateau in Wyoming, the montane forests of Utah, and in the high Rockies of Colorado and New Mexico, and finally the alpine tundra of the highest elevations.
These habitats are home to a great deal of wildlife from herbivores, such as elk, moose, mule deer, mountain goat and bighorn sheep, to predators like cougar, Canada lynx, bobcat, black bear, grizzly bear, gray wolf, coyote, fox, and wolverine, along with a great variety of small mammals, fish, reptiles and amphibians, numerous bird species, and tens of thousands of species of terrestrial and aquatic invertebrates and soil organisms.
Permanent human settlement of the Rocky Mountains has caused numerous species to decline in population, including species of trout, birds, and sheep. Gray wolves and grizzly bears were almost eliminated from the United States portion of the range, but are returning due to conservation measures.
Setting
The Rocky Mountains range in latitude between the Liard River in British Columbia (at 59° N) and the Rio Grande in New Mexico (at 35° N), and in height up to the highest peak, Mount Elbert at , taking in great valleys such as the Rocky Mountain Trench and San Luis Valley. Precipitation ranges from per year in the southern valleys to per year locally in the northern peaks. Average January temperatures can range from in Prince George, British Columbia to in Trinidad, Colorado.
Biotic zones Ecologists divide the Rocky Mountain into a number of biotic zones, defined by whether they can support trees, and the presence of one or more indicator species. Areas of the Rockies that do not support or have few trees include the prairie of the eastern foothills and the Alpine tundra. The foothill prairie grassland lies to the east of the Rockies where the mountains fall to meet the Great Plains at the Rocky Mountain Front (below roughly ). Alpine tundra meanwhile occurs in regions above the treeline, which varies from in New Mexico to at the northern end (near the Yukon).
The USGS defines ten forested zones in the Rocky Mountains. The more southern, warmer, drier zones are defined by the presence of pinyon pines/junipers, ponderosa pines, or oaks mixed with pines. The more northern, colder, wetter zones are defined by Douglas-firs, Cascadian species (such as western hemlock), lodgepole pines/quaking aspens, or firs mixed with spruce. Near the treeline, zones can consist of white pines (such as whitebark pine or bristlecone pine); or a mixture of white pine, fir, and spruce that appear as shrub-like krummholz. Finally, rivers and canyons are home to unique forest habitats even in the more arid parts of the mountain range.
Biotic zones and vegetation types in the Rocky Mountains can be explained by elevation, aspect, and precipitation. Clinton Merriam recognized that two-dimensional diagrams of elevation and aspect described plant community distribution in the Southern Rocky Mountains. Other ecologists generally embraced this two-dimensional view until the complexities of environmental gradients such as temperature, precipitation, solar radiation, wind, soils, and hydrology could be described and modeled. Peet provided the most complete description of 10 major forest community types, which are summarized here. Two nonforested vegetation types, plains and alpine tundra, described by Sims and Billings, are added. Because of the variations in latitude and precipitation along this huge mountain range, the elevations presented here are gross generalizations.
Extensive investigations have been made of the forests of the Rocky Mountains. Weber cautioned that the vegetation zones "overlap and telescope into each other considerably" in a landscape that is "always full of surprises." The resulting patchwork mosaic of vegetation types and disturbance regimes leads to a complex of side-by-side communities, wildlife habitats, and species distributions.
Plains The eastern side of the Rocky Mountains is bordered by mixed-grass prairie to the north and by short-grass prairie to the south. The prairie, some of which has been described as the Montana Valley and Foothill grasslands generally extends to elevations of . Dominant plants of the mixed grass prairie include little bluestem, needlegrasses, wheatgrasses, sand-reeds, and gramas, with dropseeds and cottonwoods in riparian zones. Short-grass prairie species include little bluestem, buffalo grass, western wheatgrass, sand dropseed, ringgrass, needle-and-thread, Junegrass, and galleta. Extensions of these vegetation types reach well into the Rocky Mountains along the valleys and on dry slopes. Plant species composition varies locally with changes in soil characteristics and topographic position—that is, from hilltops to valley bottoms.
Riparian and canyon forests Broad-leaved deciduous cottonwoods, alders, and willows, and aspens line streamsides and canyons. The herbaceous layer in riparian communities is often more diverse than upslope areas and adjacent forests. Riparian and canyon vegetation types are generally too thin or too small to be displayed on regional vegetation maps, but the habitat is extremely important in the arid West.
Pinyon-juniper woodland
In the southern Rocky Mountains, a transition occurs between about , where plains communities are accompanied by pinyon pines. Two-Needle Pinyons and singleleaf pinyons are found in western Utah, alligator junipers and Rocky Mountain junipers grow to the south, and Utah junipers grow to the north. Many shrubs and grasses of the plains occupy the gaps between tree outcrops. Heavy livestock grazing is associated with the spread of junipers (by reducing competition from grasses), and fire suppression is partly responsible for their continued dominance.
Ponderosa pine woodland
The appearance of ponderosa pine woodlands varies from scattered individuals in low-elevation or rocky areas to dense forests at higher elevations or on deeper soils. Although ponderosa pines dominate the biomass of this community, other tree species such as Douglas-fir and Rocky Mountain juniper, shrubs (for example, raspberries, big sagebrush, gooseberries, currants, bitterbrush), and herb layers (such as mountain muhly, sedges, and sagebrushes) can develop. Typical intervals between natural fires are less than 40 years in most ponderosa pine forests. Fires that recur every 10–20 years suppresses the growth of Douglas-fir saplings and encourages the growth of ponderosa pine.
In geological time, ponderosa pine ecosystems are relatively new to the foothills of the central Rocky Mountains. An even newer addition to the ecosystem, European-American settlers, devastated the ponderosa pine forests through logging for houses, fencing, firewood, mine timbers, and railroad ties, and with fire. The ponderosa pine forests were close to the developing population centers at the forest-prairie edge. The scale of the loss of ponderosa pine habitat is demonstrated best in several hundred paired photographs from the early 20th century and 1980s. However, nearly all the paired photographs also reveal that the most important feature of the ponderosa pine ecosystem is its resilience. Ponderosa pine seedlings establish quickly in disturbed sites. Research in the Front Range of Colorado shows a tenfold increase in ponderosa pine biomass since 1890 in many stands. This regeneration has restored habitat for many wildlife species but has also led to unnatural forest densities in many areas.
Pine-oak woodland In the southern Rocky Mountains, lower slopes of ponderosa pine communities can be accompanied by Gambel oaks, other oak species (for example, Emory oaks, silverleaf oaks, netleaf oaks), and shrubs (such as sumacs, buckbrushes, and mountain-mahoganies). In the absence of fire, the oak stands may be invaded by pines.
Douglas-fir forest Douglas-firs grow in a broad range from Mexico to British Columbia, generally from near lower treeline upward in elevation to spruce-fir forests. In Colorado, the species ranges from about and is often found in mixed stands with ponderosa pine, blue spruce, or lodgepole pine. Like ponderosa pine, Douglas-fir is tolerant of frequent, low-intensity surface fires. High-intensity fire intervals in Douglas-fir forests in Wyoming average 50–100 years.
Cascadian forest Several tree species commonly associated with the Cascade Mountains grow on the rain-swept western slopes of the northern Rocky Mountains. These include western hemlock, western redcedar, grand fir, mountain hemlock, and larches. These forests are subject to infrequent, high-intensity fires.
Montane seral forest
Lodgepole pine forests interspersed with stands of quaking aspens are fire-resilient forests that dominate the central and north-central Rocky Mountains. Usually found between in Colorado, lodgepole pines and aspens grow rapidly after fire in mostly even-aged stands. In the Canadian Rockies, the lodgepole pine/quaking aspen forests occur in the foothills, mixed with white spruce and balsam poplar. In the northern Canadian Rockies, black spruce also occurs in this biotic zone.
Intervals between fires typically range from 100 to 300 years. As evidenced by the fires in the Yellowstone National Park in 1988, lodgepole pine forests are rejuvenated by crown fires that replace tree stands. Aspen stands are keystone communities for hundreds of birds and mammals and are especially important forage for deer and elk.
Spruce-fir forest
The subalpine forests of the Rocky Mountains are characterized by spruces and firs and are floristically and structurally similar to the boreal conifer forests to the north. Dominant tree species in the Colorado Rocky Mountains subalpine forests include Engelmann spruce and subalpine fir, as well as lodgepole pine and the occasional Douglas-fir. In the Black Hills of South Dakota, white spruce replaces Engelmann spruce. Further north are the lodgepole pine, Engelmann spruce, and alpine fir mix of the Alberta Mountain forests above of elevation. Stand-replacing fires typically occur at 200- to 400-year intervals. Widespread insect outbreaks in spruce-fir forests occur more frequently. Recently, spruce bark beetles have begun to impact more old-growth spruce trees in these forests.
Subalpine white pine forest
On exposed, dry slopes at high elevations, subalpine white pine forests replace spruce-fir forests. Common species of the white pine forests include whitebark pine in the northern Rocky Mountains, limber pine in the central and north-central Rocky Mountains, and bristlecone pine in the southern Rocky Mountains. Typical intervals between fires range from 50 to 300 years. The white pines are tolerant of extreme environmental conditions and can be important postfire successional species. The whitebark pine is a keystone species in upper subalpine forests of the northern Rocky Mountains. However, the whitebark pine has been in decline due to white pine blister rust: whitebark pine mortality in some areas exceeds 90%. Entire forest vistas, like that at Avalanche Ridge near Yellowstone National Park’s east gate, are expanses of dead, gray whitebarks.
Treeline vegetation