One of the pleasures of walking or driving in the mountains lies in seeing everywhere evidence of the dynamic processes that shape the natural world. For example, driving north from Durango the road traverses stratum upon stratum of sandstones, shale, limestone, and volcanic deposits, each laid down under a different set of circumstances. This layer of sandstone represents a dune field tens of thousands of square miles in extent; those shales represent mudflats of a shallow inland sea; those red cliffs, a coastal plain in a tropical environment; the rimrock, the petrified sands of an ancient beach. The valley itself has been shaped by a river of ice that ran through hillsides clothed in spruce and fir and an alpine meadow where pinon and juniper now grow. To eyes that are open to the history of the planet, it is clear that life faces a ceaseless challenge to adapt to changing conditions. The living organisms found today along the San Juan Skyway are constantly responding to the endless sequence of changes. Life is still adjusting to the glacial retreat and to the arrival of humans, of European settlers and their livestock, and of modern technology. Travelers along the Skyway can observe the current state of the process and understand the factors that have helped create the biological communities visible from the roadway.

Fifteen thousand years ago, the climate of Durango was not greatly different from that seen today at the crest of Molas Pass (Pielou 1991). Musk oxen (Ovibos moschatus) grazed at times on the barren flats at the foot of the glacier, and alpine meadows bloomed on the hillsides above the great tongues of ice flowing out of the huge icefield at the headwaters of the Animas, Dolores, San Miguel, and Uncompahgre Rivers. Very likely there were no amphibians or reptiles in or around the Animas Valley then. As the glaciers melted back and temperatures warmed toward their present levels, species dispersed upstream from lower elevations. Many of the Arctic and northern Rocky Mountain animals that inhabited the area at the height of the glaciation became locally extinct; today only the pika (Ochotona princeps) and white-tailed ptarmigan remain to represent the glacial fauna. The changing of the guard is still continuing, in part because it may take thousands of years for species to expand their ranges and in part because the course of the climatic change fluctuates back and forth over periods of months, years, and centuries.

Many species living along the San Juan Skyway have very specific habitat requirements and are found only where these requirements are met. For example, cattails (Typha latifolia) and marsh wrens rarely live far from permanent water. Other species have such broad tolerances to physical factors that they could establish themselves at nearly any site if other species were not present. A mountain bluebird, for instance, seems equally at home on a sagebrush flat, in an area that has been clear-cut in the subalpine forest, or on a sunny alpine fellfield (a slope covered with large boulders). It can live and breed in any open place that has insects and other foods and a well-defended hole for a nesting site. A tansy aster (Machaerantbera spp.) thrives on heavy soils, sandy loams, and abandoned roadways consisting mainly of cobbles. It would probably cover any well-drained upland site, from sage desert to high pass, were no other species of plant present. Its actual distribution is limited to recently disturbed ground in sunny locations below timberline. An annual, it soon gives way to longer-lived species. Still other forms, such as lichens and the club moss (Selaginella spp.), can live on bare rock where no flowering plant could survive. And some organisms are completely dependent on the presence of another; the Abert's squirrel (Sciurus aberti), for example, never persists for long away from the branches of the ponderosa pine (Pinus ponderosa). Both physical and biotic factors can be important determinants of a species' range. Seldom does a single factor define an organism's distribution; more often the aggregate effect of many interacting factors sets the limits. We now should examine some of the major limiting factors in the San Juan Mountains.

PHYSICAL LIMITING FACTORS

The most powerful physical limiting factors are temperature and moisture. These vary greatly across the landscape, especially in relation to changes in elevation and topography. As moist air masses move inland from the Pacific Ocean and Gulf of Mexico, they strike the San Juan Mountains and are pushed upwards into the colder reaches of the atmosphere, where the air's water-holding capacity decreases and a large portion of the moisture falls as snow or rain. This augmented precipitation in the high country is the primary source of water for all human activities in the region, and it also supports extensive forests across the middle slopes of the mountains.

Average temperatures progressively decrease with increasing elevation. As a result of this gradient in precipitation and temperature, plant and animal species and biotic communities gradually change with altitude. A trip from the foothills to the top of one of the high peaks, representing a climb from 6,000 to 13,000 feet (1,800-4,000 m), is equivalent in many respects to a journey from southern Colorado to the edge of the Arctic Ocean in northern Canada. One experiences a similar pattern of climate changes and a comparable progression of vegetation zones by climbing either in latitude or in elevation. As a general rule of thumb, 1,000 feet (300 m) of elevational gain is comparable to 300 miles of northward travel.

At a certain point along this elevational gradient, the relative importance of the two limiting physical factors, temperature and moisture, changes. The threshold occurs roughly between 7,600 and 8,200 feet (2,300 and 2,500 m) in the San Juans. At lower elevations, scarce rainfall combined with hot summers makes water the principal limiting factor. Indeed, most terrestrial organisms in this semiarid zone have made remarkable adaptations to overcome or circumvent water stress. At higher elevations, water is still important, but acquiring enough heat energy to complete development and reproduction becomes ever more difficult. The depth and persistence of winter snowpack increase dramatically in this elevational zone; organisms must be able to cope with deep snow for several months each year.

Two additional factors - soil characteristics, traceable to the nature of the parent rocks, and topography - greatly accentuate the differences in water stress experienced at all elevations and often account for abrupt transitions from one type of biotic community to another. Several more or less distinctive zones of vegetation are apparent in the San Juan Mountains. Each zone lies within its own range of elevation, where conditions are suitable for growth of its component species. However, a given vegetation zone usually extends to lower elevations on north-facing slopes, where cooler and wetter conditions prevail, and on types of rock, such as sandstone, that form soils having high moisture-holding capacity. Similarly, each zone usually is found at higher elevations on south-facing slopes and on soils (such as those formed from Shale) having low capacity to retain moisture. North-facing slopes are generally cooler and moister than south-facing slopes because the sun strikes the latter more directly most of the year. Valley bottoms at any elevation tend to be relatively cool and moist and support a distinctive assemblage of plants and animals that need more water than is available on the drier slopes and ridges.

A good spot to examine the effects of soil characteristics on the vegetation of an area is between Mancos and Dolores along Colorado 145, a few miles off the main highway. The land south of the road lies on Mancos Shale; the area north lies atop Dakota Sandstone. Ponderosa pine (Pinus ponderosa) dominates to the north, pinon (Pinus edulis) and juniper (Juniperus spp.) to the south. The biological differences in these communities are almost entirely due to the different soils of the two areas, which lie at the same altitude and share the same rainfall and temperature patterns.

Water moves through the sandy soils derived from the Dakota Sandstone much more rapidly than through the heavy clay soils arising from the Mancos Shale. A summer shower of a third of an inch (8 mm) will wet the sandy soil about 3 inches (75 mm) deep in twenty-four hours. Some of the moisture will move by gravity to even lower depths over time. The same amount of rain on Mancos soils will penetrate to less than half that depth, because the water will be tightly held by the clay particles. Much of the moisture in the surface soil will evaporate within days, and the deep layers will remain dry throughout much of the summer. The environments of the roots of plants in the two soil types thus differ radically even though the rainfall at the surface is nearly the same. Such effects contribute to the abrupt transitions from one community of organisms to another.

The steepness of a slope also is important for the vegetation. In many steep areas, especially at high elevations, snow avalanches occur frequently. Distorted aspens and stunted willows that can tolerate this kind of disturbance are commonly found growing in avalanche paths, although they are quite uncommon in the less disturbed areas nearby.

BIOLOGICAL LIMITING FACTORS

The presence and abundance of a species in a particular locality usually depends primarily on physical limiting factors. Most amphibians and reptiles, for instance, are only found within particular ranges in elevation, which suggests that temperature is the most important limiting factor for these animals. For many mammals and other groups of plants and animals, however, the presence or absence of other species may be more important limiting factors than temperature and moisture. For example, the valley pocket gopher (Thomomys bottae) occupies the low valleys and cuestas in the southern end of the Skyway area, leaving mounds and serpentine cylinders of soil at the surface as it mines for roots and living space. At some elevation in each major valley, however, possession of the soil passes to the northern pocket gopher (Thomomys talpoides), whose range extends even onto the thin soils of the alpine zone. There is almost no overlap in the ranges of these two species: it is almost as though a political border equivalent to the national boundaries of two countries had been established. No major change in climate or soil at the edge of their respective ranges can explain why one gopher species gives way to the other. The two gophers have nearly identical habits and requirements, but it would seem that there is room for just one of the two species at any one site.

Similarly, montane voles (Microtus montanus) occur almost exclusively in the tall tussocks of fescue grass (Festuca spp.) in mountain parks such as Big Bear Park, visible on Missionary Ridge from the pullout where U.S. Highway 550 crosses the railroad tracks at Shalona. Long-tailed voles (Microtus longicaudus) also inhabit the parks in the absence of the mountain vole, but they are restricted to clearcuts, aspen groves, and shrubby grasslands when the montane vole is present.

Sometimes the competition is between native species and species introduced by humans. Domestic sheep carry lungworms that are frequently fatal when they infest young Rocky Mountain bighorn sheep (Ovis canadensis). Therefore, the bighorns are largely absent from much of their former range now used as pastures for domestic sheep.

In contrast to the competitive interactions described above, cooperative, mutually dependent relationships exist between many pairs of species - neither can survive without the other. The yucca plant (Yucca baccata) and the yucca moth are good examples of this kind of symbiotic relationship. The female moth gathers pollen from yucca flowers, forms it into a ball, and carefully inserts it into another flower, thereby assuring pollination. Then she places just the right number of eggs into the ovary of the flower such that her offspring will be able to consume some but not all of the developing seeds. Yuccas rarely set seed without the moth to pollinate their flowers, and the moth can reproduce only in yucca flowers. Many kinds of flowering plants and insects have similar, mutually beneficial relationships.

ADAPTATIONS OF PLANTS AND ANIMALS TO LIMITING FACTORS

Plants and animals exhibit a variety of remarkable adaptations that enable them to cope with the kinds of limiting factors we have been discussing. Adaptations in animals may be illustrated via three contrasting species of amphibians.

Western spadefoot toads (Scaphiopus multiplicatus) inhabit many of the valleys below 7,000 feet (2,100 m), although they are seldom seen except following heavy summer thunderstorms. Their biggest problem in this semidesert environment is coping with the limited and unreliable supplies of water for breeding. They solve the problem by reproducing whenever water does become available and by having very rapid development so the young can mature before the temporary ponds dry up. After a heavy rainstorm, the 2- to 3-inch-long (6-8 cm), smoothskinned, olive-drab animals make their way to temporary ponds, earthen tanks, and ditches. The females find mates by following the grating calls of the males. The eggs may hatch in fewer than three days, and the tadpoles grow quickly. The fastest-growing individuals assume predatory roles and consume their pondmates, thus growing even more rapidly on the easily assimilated diet of spadefoot flesh. Tadpoles can be ready for metamorphosis (the change into adult form) in as little as three weeks' time, having reached a length of 1 to 1.25 inches (2.5-3 cm). They move away from the pond as their parents already have done. Each one digs into the soil, coming to the surface to feed only when atmospheric humidity permits. When desiccation threatens, they dig even deeper and become dormant. Spadefoots are active less than 20 percent of the year and may remain dormant months or even years while they grow, develop eggs and sperm for future mating opportunities, and wait for favorable conditions to allow them to journey to the puddle where they were reared. They probably are limited to areas below 7,000 feet (2,100 m) because ponds at higher elevations do not provide the high temperatures required for their rapid development.

The life history of the tiger salamander (Ambystoma tigrinum) illustrates a more generalized adaptation to water limitations. The tiger salamander ranges from the basins and foothills to above timberline. It copes with the low temperatures at high elevations by extending its period of development. This adaptation restricts it to fairly permanent ponds and pools, in contrast to the spadefoot, which breeds in ephemeral ponds. Metamorphosis of the larval salamanders into the adult stage is triggered by oxygen stress, which elicits secretion of the iodine- containing hormone thyroxine. Some ponds are naturally deficient in iodine, and metamorphosis may be delayed indefinitely. Likewise, oxygen-rich water in deep ponds at high elevations may not stimulate the thyroid, allowing individuals to remain in the larval or juvenile state. These perpetually immature salamanders can grow larger than adults and may even reproduce while retaining their larval characteristics. However, they are unable to leave the water. The individuals that do metamorphose into adults can leave the pond and often wander great distances from the breeding site before descending into a burrow in soft earth.

A third type of adaptation in amphibians is seen in the boreal chorus frogs (Pseudacris triseriata). These little terrestrial creatures attain lengths of around 1.5 inches (35 mm) and weights of less than 3 grams. They breed in temporary pools, seasonal marshy ponds, and cattail swamps and forage in nearby meadows and grasslands. Probably the most abundant amphibians in Colorado and among the most widely distributed, chorus frogs breed from above 11,900 feet (3,600 m) to below 5,000 feet (1,500 m) in the San Juan region. They solve the problem of low temperatures at high elevations by selecting heat-trapping ponds, foraging actively only during the warmest part of the day, and maturing while still at a small size. Breeding commences soon after the thaw, typically before the last snow and ice have disappeared from the marsh. The ponds favored by chorus frogs at high altitudes cool overnight to near freezing and often develop a thin covering of ice early each morning, but the water usually warms to between 77 and 86 degrees F (25-30ºC) between noon and sundown. At low elevations chorus frogs are mainly active around dusk and dawn, and their breeding usually coincides with the season of high river water (or, nowadays, with irrigation flows).

Mammals also must cope with water and temperature limitations. Deer mice (Peromyscus maniculatus) are found almost everywhere along the San Juan Skyway. Indeed, campers must keep their stores of bread, grains, and snacks well protected or wake to find that the mice have transported amazing quantities of food into their own larders. This animal does not truly hibernate but depends on the food it has stockpiled to carry it through the winter. In years of heavy snow, populations may decline remarkably, presumably because long winters exhaust their food reserves. Probably some local populations are eliminated in exceptionally stormy years; the species' presence is soon restored by immigration from a population surviving nearby, usually at a lower elevation.

Lowland animals generally cope with water stress by avoiding the heat of the sun, by burrowing to find more humid conditions underground, by restricting their activities to evening or nighttime, or by making remarkable physiological adaptations for conserving water. The big ears of the black-tailed jackrabbit (Lepus californicus), for example, permit it to transfer heat to the air by convection rather than evaporation; it also recovers much of the water evaporated from its air passages and lungs through condensation onto the cooled surfaces of its nasal cavities and subsequent resorption. Most remarkable, perhaps, is that its body temperature falls several degrees below optimum in the cool early morning, thus offsetting heat gain as it heats up later during the day and avoiding the evaporative water loss that would otherwise be required.

All the organisms in the lowlands have equally effective strategies for conserving water, an absolute necessity in an and environment where ground temperatures routinely exceed 100 degrees F (40ºC) for considerable periods each day during the warm season and where no rain may fall for weeks on end. Some of the larger animals and birds may be able to travel considerable distances to the few permanent streams or springs or to depressions in the rocky outcrops that hold water for extended periods after rains, but most lowland resident animals must rely on their food to make up any deficit in their water balance. Lizards, for example, obtain a small quantity of water with every insect ingested, and their kidneys excrete wastes in a form requiring almost no water. Pocket mice (Perognathus spp.) and kangaroo rats (Dipodomys spp.) consume carbohydrate-rich foods or select seeds rich in oils and fats, which yield metabolic water as they are broken down inside the cells of the body. Pocket mice also have a behavioral ploy for coping with drought: for example, in periods of both low food supply and drought, the pocket mouse spends a portion of its time in a state of torpor, conserving energy and breathing so slowly that almost no water is lost through its lungs. It also may spend less time searching for food on the exposed surface of the ground and thus stretches its resources until the hard times pass.

THE ROLE OF DISTURBANCE

The broad patterns in distribution of species and the zones of vegetation primarily reflect the underlying patterns of geology, topography, soil, and climate. Superimposed upon these broad patterns are others brought about by past disturbances (for example, fires, avalanches, or rock slides) and interactions among species (for example, the effects of browsing elk or of tree-killing bark beetles). The boundaries between communities along a gradient of elevation are usually gradual; one type of vegetation blends into the next. Boundaries created by disturbances often are very sharp, however, as are boundaries between communities on two very different kinds of soil or between those on a steep north-facing slope and a steep south-facing slope.

Many kinds of disturbances are natural processes that have shaped the vegetation of the San Juan Mountains for thousands of years. Though they usually entail the death or injury of many individual plants, they may actually enhance the overall biological diversity of the area by creating new kinds of habitats and temporarily reducing the dominance of the most successful species, which otherwise might crowd out competitors. Consequently, most natural disturbances should be regarded not as disasters but as integral parts of the ecological system.

Humans also disturb the vegetation, of course, sometimes in ways that mimic the natural disturbances to which the system is well adapted. Thus, some level of grazing by livestock, cutting of timber, and hunting of game, if wisely planned and well regulated, may be no more deleterious to the integrity and stability of the San Juan Mountain ecosystems than the natural disturbances to which it has long been subjected. Unfortunately, many human activities in these mountains during the last century have been far greater in intensity and duration than natural disturbances, and the scars left by these activities are still evident. Nevertheless, in traveling the San Juan Skyway, one usually is struck less by the changes wrought by humans than by the remarkable diversity, resilience, and beauty of the natural landscape.

BIODIVERSITY IN THE SAN JUAN MOUNTAINS

The presence of the Rocky Mountain Cordillera, of which the San Juan Mountains are an impressive part, creates gradients in local temperature and annual precipitation. The range's high altitudes allow organisms adapted to conditions of the far north to extend their ranges to much lower latitudes than would otherwise be possible, bringing them into juxtaposition with organisms inhabiting and regions of the southwest. The species of the two regions mix along the climatic gradients in the mountains.

In general, diversity decreases as elevation increases. A small deviation from the overall trend occurs in the forests at middle elevations, which support a variety of arboreal species, such as Abert's squirrel, chickaree, and pine marten (Martes americana). A number of widespread species penetrate into the alpine zone while maintaining strong populations at low elevations. In fact, the majority of mammals encountered on the highest peaks belong to species equally at home on the and plateaus and grasslands. However, very few of the northern species extend into lower elevations in the San Juans.

A dramatic example of the process of subtraction and diminished diversity is provided by the amphibians and reptiles. A semidesert area just 25 miles southwest of Cortez harbors twelve species of lizards, at least ten species of snakes, and at least six species of amphibians, all native, an assemblage approaching the diversity of that great herpetofaunal center, the Sonoran Desert. By the time one has climbed to the top of Hesperus Hill or entered Rico, however, a determined search of the slopes and valleys turns up only two kinds of lizards, no more than two kinds of snakes, and perhaps four kinds of amphibians. All the reptiles would drop out of the list with another rise of 2,000 feet (600 in) in elevation, and only two amphibians reach to the treeline in a few locations. No reptiles or amphibians reach the summits of the fourteeners (i.e., 14,000-foot-high peaks).

Although the foothills and basins may appear barren and inhospitable compared to the green uplands, they harbor the majority of species of plants and animals to be found in this comer of Colorado. The diversity on any one acre tends to be low, but every rise and draw, ridge and canyon, outcrop and flat provides significantly varied conditions, permitting the region to support a great variety of species of both plants and animals. Apparently temperature is a more significant limiting factor for most vertebrate species than water, as the high elevations have ample water but low temperatures. Total land area also limits species diversity, with more species usually present in a larger area if all other factors are constant. The land area above 10,000 feet (3,000 m) is only one one-hundredth the area below that elevation, which further reduces the number of high-elevation species in the San Juans relative to lowelevation species.

MAJOR VEGETATION ZONES IN THE SAN JUAN MOUNTAINS

Moving from the semiarid basins and foothills to the frigid summits of the high peaks, one progresses through a sequence of vegetation zones. The lowest elevations in the basins consist of greasewood-shadscale shrub-steppe and Great Basin sagebrush shrub-steppe (Kuchler 1964). Moving higher up into the foothills, one passes through pinon-juniper woodland, the mountain shrub community, and ponderosa pine-oak-Douglas fir forest.

Above the foothills, one encounters mixed conifer forests, aspen groves, and mountain parks or meadows on the middle slopes of the mountains, above which stand the spruce-fir forests in the subalpine environment just below timberline. The highest peaks and ridges are capped with alpine meadows and fellfields lying above timberline.

Narrow strips of riparian vegetation slice through the vegetation zones of the foothills and mountains along streams and moist canyon bottoms. Riparian communities also thrive around lakes, ponds, and other wetlands.

REFERENCES

Kuchler, A. W., 1964. Potential natural vegetation of the conterminous United States. American Geographical Society Special Publication 36, 116 pp.

Pielou, E. C., 199 1. After the ice age: The return of life to glaciated North America. Chicago: University of Chicago Press, 366 pp.