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Mammals of Colorado

Denver Museum of Nature & Science and University Press of Colorado

Environments of Colorado

Mammals are a familiar and important component of Earth's biodiversity. Biodiversity is the kinds of organisms and their genetic and ecological relationships — an evolutionary and ecological phenomenon in space and time (E. Wilson 1992). The mammalian fauna of Colorado is a fascinating piece of that whole. To understand the diversity of mammals we need to have a perspective of the ecosphere more generally. Such a perspective is the purpose of this chapter, with a focus on environments of Colorado.

Colorado is known for its scenic beauty — from majestic mountain peaks and rushing white rivers tumbling down dark canyons, to red-rock deserts and ceaselessly shifting sand dunes, to the expansive sweep of the short-grass prairie. Grandeur is wherever we stop to appreciate it, at every scale, from canyons carved in crystalline rocks 2 billion years old, to bold peaks sculpted by the glaciers of the last Ice Age, to last night's furtive trail of a mouse across the snow. We humans appreciate ecological patterns and processes as beautiful or intriguing; to the rest of the mammalian fauna the evolving landscape represents opportunity, and native mammals respond accordingly. Thus, to understand the distribution and abundance of mammals and the details of their daily lives we must first understand the resource base, the mosaic of Colorado's environments in space and time.

Geography

From the standpoint of political geography, Colorado is simple: it is roughly rectangular (if we neglect some minor old surveyors' errors and the fact that Earth is spherical), measuring approximately 607 km by 444 km (377 by 276 mi.) and encompassing some 270,000 km2 (104,000 sq. mi.). Colorado lies between approximately 102° and 109° west longitude and 37° and 41° north latitude, and is subdivided into 64 counties (Map 1-1). A few of the counties are nearly natural, ecological units (e.g., Jackson, Grand, and Park counties encompass North, Middle, and South parks, respectively), but most are simply political artifacts with rectilinear boundaries.

From the standpoints of physical and biological geography, Colorado is anything but simple. The marvelous complexity of the scenery is the subject of this chapter, which describes environments of Colorado from several interrelated points of view. Geologic history and materials underlie environmental patterns. Physiography is the shape of the land, reflecting hundreds of millions of years of landscape evolution. Patterns of drainage reflect and produce the landforms. Vegetation integrates climate and geologic parent material in the development of soils. Plants and animals, fungi and microbes interact as biotic communities, integrated by symbioses, and they interact with the physical environment in ceaseless cycles of materials powered by a flow of solar energy. We observe — and seek to understand — an ecological whole of extraordinary complexity. But let us begin simply, with a little history.

Geology and Landforms

Colorado straddles the "backbone" of North America, the Rocky Mountains. From the mountain front, the Great Plains extend eastward toward the Missouri River. To the west lie canyons and plateaus of the Colorado Plateau and the Wyoming Basin. The juxtaposition of these major physiographic regions affects temperature, precipitation, wind patterns, and drainage.

Colorado is the highest of the United States, with a mean elevation of 2,070 m (6,800 ft.). The lowest point is 1,021 m (3,350 ft.), east of Holly, Prowers County, where the Arkansas River exits the state, and the highest point is 4,399 m (14,433 ft.), the summit of Mount Elbert, Lake County, at the top of the Arkansas watershed. Because of these varied conditions, species richness is high.

Physiographers divide most of Colorado among three "provinces" (see Fenneman 1931): the Southern Rocky Mountains, the Great Plains, and the Colorado Plateau. Northwestern Colorado is on the periphery of two additional provinces: the Middle Rocky Mountains and the Wyoming Basin. For an excellent summary of Colorado's landforms and their development, see A. Benedict (2008).

The present-day Southern Rocky Mountains arose in a long-term event called the Laramide Orogeny, beginning some 72 million years ago, in the Late Cretaceous Period. Prior to that time (during the Mesozoic Era, the "Age of Reptiles") Colorado occupied a low-lying area, alternately covered by shallow seas or exposed as deserts and flood-plains. With the rise of the Rockies, Mesozoic and older sediments were broken, bent, and tilted on end, resulting in the familiar hogback ridges and such features as Boulder's Flatirons, the spectacular Garden of the Gods, Loveland's Devil's Backbone, and the Grand Hogback. Streams heading in the newly uplifted mountains eroded the rocks, spreading the bits out in a deep "mantle" eastward across the mid-continent.

In Miocene to Pliocene times, about 5 million years ago, broad, domal regional uplift occurred, "raising the roof of the Rockies" by nearly a mile. Mountain ranges were exhumed from their mantle of Tertiary debris and today's "Fourteeners" reached their greatest elevations, only to face the inexorable processes of weathering we see today — the daily destructive march of rain and snow, wind and calm, freeze and thaw.

There is nothing simple about the Southern Rockies of Colorado (see Map 1-2), but we may think of the basic structure as two great ridges of granitic rocks arrayed in parallel lines oriented roughly north-south. The eastern series begins north of the Cache la Poudre River as the lower eastern Laramie Range and the higher western Medicine Bow Range. The Front Range extends from the Poudre to the Arkansas, ending in the Rampart Range (which includes Pikes Peak). The Wet Mountains are an independent range south of the Arkansas River.

A western chain of granitic mountains begins in southern Wyoming as the Sierra Madre (called the Park Range in Colorado); continues south as the Gore, Ten Mile, Mosquito, and Sawatch ranges; and then jogs a bit to the east to continue south into New Mexico as the spectacular ridge of the Sangre de Cristos. Between the granitic ridges are structural basins. North and Middle parks occupy a single structural basin, subdivided by the volcanic Rabbit Ears Mountains. South Park occupies a separate basin. West of the Park Range, the Wyoming Basin is continuous with much of southwestern Wyoming.

The San Luis Valley lies west of the main ranges of the Southern Rockies proper, but it looks like one of the parks, because it is surrounded by mountains, the Sangre de Cristos to the east and the younger, volcanic San Juans to the west. A range of volcanic hills marks the southern border of the San Luis Valley, roughly the southern border of Colorado. On the east side of the San Luis Valley is the spectacular Great Sand Dunes National Park and Preserve. Valley of the Dunes (Rozinski et al. 2005) provides a moving appreciation of El Valle in visual images and compelling prose. Every corner of Colorado deserves such treatment.

The main ranges of the Rockies represent uplifted Precambrian rocks and folded Paleozoic and Mesozoic sediments. Adjacent ranges like the San Juans were produced by Cenozoic volcanic activity. Features like the White River and Uncompahgre plateaus are independent uplifts. Grand and Battlement mesas are built of sedimentary rocks, protected by caps of resistant lava. J. Chronic and Chronic (1972), Matthews et al. (2003), and H. Chronic and Williams (2002) provided accessible introductions to the geology of Colorado; A. Benedict (2008) described the mountains in intimate detail; and Cairns et al. (2002) described the Rockies in the context of the ongoing human transformation of the region.

The eastern two-fifths of Colorado lies in the Great Plains Physiographic Province. When the Rockies rose, erosion and sedimentation clothed the area to the east with the pieces. For millions of years, this alluvium covered nearly all of eastern Colorado. In the Pliocene, and especially during the Pleistocene ice ages (with their high precipitation), the "Tertiary mantle" was largely eroded away and carried out of the state. Today it is preserved mostly on the High Plains, a nearly flat landscape interrupted occasionally by sandhills and eroded along stream courses to form canyons, cliffs, and escarpments. Between the High Plains and the mountain front lies the Colorado Piedmont. There the Tertiary mantle has been largely removed, exposing Mesozoic shales, limestones, and sandstones as hogbacks, low rolling hills, and canyons. Remnants of the Tertiary formations can be seen along the northern border of eastern Colorado. The dramatic Pawnee Buttes andthe escarpment of the Peetz Table suggest just how much material has been removed from the Colorado Piedmont. The divide between the Platte and Arkansas rivers is a remnant upland, providing an eastward extension of ecosystems of the foothills. The generally forested divide imposes a filter-barrier to north-south movement of many smaller mammals between the valleys of the master streams of the plains (D. Armstrong 1972, 1996).

At the foot of the mountains, sedimentary units generally dip steeply eastward, forming a great debris-filled trough, the Denver Basin. The southern rim of the basin is marked by roughlands south of the Arkansas River, which greatly complicate the ecology of southeastern Colorado, providing habitat for a number of species of Mexican affinities. Indeed, the very name of the physiographic region, the Raton Section, bespeaks its distinctive mammalian fauna, which includes several species of "ratones," woodrats, whose dens are conspicuous features of the landscape.

The Colorado Plateau is a world-renowned showplace for the effects of erosion on flat-lying sedimentary rocks. Add to that the complications of a history of volcanism nearby, and the result is a landscape of remarkable ecological diversity. The country is typified by mesas and plateaus dissected by canyons. These include the Book and Roan plateaus, the Piceance Basin, and lava-capped Grand and Battlement mesas. Farther south, the uplifted Uncompahgre Plateau and isolated peaks like Ute Mountain are conspicuous. Mesa Verde is a major highland near the southern boundary of the state.

The boundaries between physiographic provinces are often visible in ecological patterns. The transition from the Great Plains to the Southern Rockies on the Eastern Slope is especially dramatic, with the Front and Rampart ranges rising 2,400 m (nearly 8,000 ft.) in less than 30 km (18 mi.). Further, spectacular river canyons often mark gateways from the Rockies to adjacent physiographic provinces: Northgate Canyon on the North Platte, Glen-wood Canyon on the Colorado, South Platte Canyon, the Royal Gorge of the Arkansas, the Black Canyon of the Gunnison, and the Big Thompson and Poudre canyons. These and numerous lesser canyons and gulches provide corridors for movements of the biota, their south-facing slopes providing microclimates especially favorable for southwestern species.

Watersheds

Rivers carve landscapes and support moist corridors of opportunity for living things. The influences of rivers are especially striking in the semiarid West. We cannot appreciate natural landscapes of Colorado — or much of human history — without knowing something of the patterns of drainage, the hydrography, of the state.

Colorado lies astride the Continental Divide. Water that falls to the west of the Divide ends up in the Sea of Cortez (Gulf of California). Waters of the Eastern Slope are destined for the Gulf of Mexico, via the Missouri-Mississippi system. The San Luis Valley is partly an internal drainage basin, but the Rio Grande flows through the southern part of the valley on its way to the Gulf of Mexico, having gathered its headwaters in the high San Juans.

The Continental Divide is a fundamental geographic fact in Colorado. The main ridge of the Rockies intercepts moisture coming from the Pacific Ocean. Air is forced up, hence cooled, and its water vapor condenses, falling in the mountains as rain or snow. The Eastern Slope is, therefore, in a "rain shadow." The Western Slope has about one-third of the land area of Colorado but receives more than two-thirds of the precipitation. However, because only about 11 percent of the state's human population lives on the Western Slope, ambitious efforts have been made for more than a century to move water — the lifeblood of agriculture and urban and industrial development — to the Eastern Slope, the center of Colorado's human population. The actual amount of diversion varies from one year to the next and the pattern is complex, but as an example, the amount of Western Slope water diverted annually to northeastern Colorado typically supplements the native flow of the South Platte River by about one-quarter.

Transmountain water diversion has greatly modified environments of Colorado. The South Platte and Arkansas valleys, which nineteenth-century explorer Stephen Long called the "Great American Desert," have been transformed into rich agricultural regions, expanding habitat for a number of species of mammals, including relative newcomers like fox squirrels, raccoons, and opossums. Also, the tunnels through which diverted water flows sometimes provide roosting habitat for bats.

Several of the major rivers of western interior North America originate in the Colorado Rockies. Indeed, the only sizable river that flows into the state is the Green, which heads in the Wind River Mountains of western Wyoming. The master stream of the Western Slope is the Colorado River. The Yampa and White rivers drain northwestern Colorado before they join the Green. The mainstem Colorado — once called the Grand River — drains Middle Park and the western side of Rocky Mountain National Park and then joins the Gunnison at Grand Junction, flowing thence westward into Utah where it is joined by the Green en route to the Grand Canyon in Arizona. Southwestern Colorado is drained by several tributaries of the Colorado, namely the San Miguel, Dolores, and San Juan rivers and their tributaries, all born as mountain snow in the San Juans and destined for a muddy end in Mexico's Sea of Cortez.

The North Platte River heads in North Park and drains much of eastern Wyoming before joining the South Platte in Nebraska. The South Platte and its tributaries drain the Front Range and South Park. The Arkansas River heads in the Rampart, Sawatch, and Mosquito ranges. The High Plains of eastern Colorado give rise to the Republican and Arikaree rivers. The Cimarron heads in New Mexico and drains extreme southeastern Colorado.

Climates and Climate Change

Mammals are endotherms; that is, they maintain a high and constant body temperature by elegantly controlled production and retention of metabolic heat. Endothermy partially liberates mammals from the direct influence of climate, but climate still is an important influence on mammals, affecting individual lives, populations, and broad patterns of the distribution of species.

Weather in Colorado is highly variable from place to place, season to season, and moment to moment, all part of a broader changing climate. We can sketch only the broadest outlines of the pattern of climate. The Southern Rockies are the dominant influence. Other important factors are latitude, elevation, exposure, local topography, and location relative to storm tracks and prevailing winds. Far from the moderating effects of oceans (roughly 1,100 km [690 mi.] from the Gulf Coast, 1,600 km [994 mi.] from the West Coast, and 3,200 km [1,988 mi.] from the East Coast), the state has a "continental climate." Colorado's climate is temperate and semiarid overall, with low relative humidity and temperatures that show wide variation at all elevations. For example, annual precipitation ranges from more than 100 cm (39.4 in.) in some parts of the San Juan Mountains to less than 25 cm (9.8 in.) in parts of the San Luis Valley, just 80 km (about 50 mi.) away. The frost-free season in the Grand Valley averages 189 days; at Silverton it averages 14 days. The difference in mean annual temperature between Lamar and the summit of Pikes Peak (only 200 km [124 mi.] to the west but 3,200 m [10,500 ft.] higher in elevation) is about 20°C (approximately 35°F).

The Great Plains are typified by low precipitation, high winds, and low humidity. Summer daytime temperatures, although frequently hot, only occasionally and locally exceed 38°C (100°F). Winters have relatively warm periods interrupted by Arctic air masses sweeping down from Canada. Precipitation declines along an east-west gradient from an annual mean of about 45 cm (18 in.) along the Kansas and Nebraska borders to about 30–35 cm (12–14 in.) at the foot of the mountains.

Winters near the foothills are milder than on the plains or in the mountains, and the Front Range corridor, from Fort Collins south to Pueblo, supports more than 80 percent of the state's human population. Denver, at an elevation of about 1,610 m (5,280 ft.), has a mean annual precipitation of about 35 cm (13.8 in.) and a mean annual temperature of 10°C (50°F).

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Excerpted from Mammals of Colorado by David M. Armstrong, James P. Fitzgerald, Carron A. Meaney. Copyright © 2011 Denver Museum of Nature & Science. Excerpted by permission of Denver Museum of Nature & Science and University Press of Colorado.
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