High School
Teacher's Guide
to Craters of the Moon
Chapter Six:
Life on the Lavas: Preserving the Balance
A Harsh Environment
Chapter Six:
Life on the Lavas: Preserving the Balance
A Harsh Environment
Living things at Craters of the Moon must overcome daunting challenges if they are to survive. In addition to the scant soil on the cinder cones and lava flows, plants contend with considerable environmental stress. Summer brings blazing heat and winter frigid cold. In volcanic soils, the temperature beneath the surface quickly responds to changes in air temperature. Scientists have detected changes in soil temperature three inches below the surface within one hour of a change in air temperature. The marked difference in daytime and nighttime summer temperatures, if extreme, can shock a plant's root system. This coupled with the abnormally high surface temperatures reached as the black soil absorbs heat from the sun--more than 150 degrees--creates conditions potentially deadly for plants.
Average annual precipitation is only 17 inches, much of which occurs as snow. Rainwater and snow melt sink quickly into the porous lava and out of the reach of plant roots. By the summer growing season little moisture is available. The irregular cavities in the cinders partially offset this by temporarily retaining small amounts of water. The air is dry and southwesterly winds, averaging a daily 12 mph in the summer, further rob plants of precious moisture. All of these factors make it difficult for plants to establish themselves in this volcanic landscape.
Despite these barren conditions, Craters of the Moon is home to a surprising array of plants and animals: more than 300 plant, 20,000 insect, 2 amphibian, 8 reptile, 148 bird, and 44 mammal species! These living things have unique structural and behavioral adaptations which enable them to survive the environmental extremes of the lava desert.
Desert plants have two fundamental strategies for coping with lack of water -- evasion and resistance. Drought evaders avoid dry conditions by growing at times or in places which provide adequate moisture. Mosses and ferns, for example, escape drought by growing near persistent water supplies such as ice caves or in moist crevices on the lava flows. Aspen and Douglas-fir trees grow only on sheltered slopes or along streams. The tiny dwarf monkeyflower carries out its full life cycle during the few moist weeks in early summer, surviving as a seed the rest of the year.
Most of the plants in the monument, however, must be able to endure blistering heat, extreme drought, and desiccating wind. They resist rather than evade environmental extremes. Various structural features enable these plants to locate water and to conserve moisture. The tissues of sagebrush and antelope bitterbrush, for example, can withstand extreme dehydration without permanent cell damage. They are also able to extract water from very dry soils. Like many desert plants, the bitterbrush has tiny leaves, which expose less surface area to the drying heat and wind. White hairs on the leaves and stems of plants like scorpionweed reduce surface evaporation by inhibiting air flow, reflecting sunlight, and trapping moisture as it leaves the plant. Succulent plants such as pricklypear cactus can store water for later use. The showy blossom of the blazing star opens only at night rather than exposing itself to intense sunlight.
Animals at Craters of the Moon must cope with the same environmental extremes as the plants. Most are nocturnal and avoid heat and intense sunlight. Those that remain active during the day retreat to rocky burrows in the lava or other shady locations during the hottest hours. Some creatures have more sophisticated mechanisms for reducing body temperature. Jackrabbits, for example, radiate heat from their ears when air temperature climbs higher than body temperature. Blood flow through the large surface area of the ears dissipates body heat. Marmots take more extreme measures. When conditions become too hot and dry, they go into a state of deep stupor called estivation, during which their metabolism and other body functions drop for weeks at a time.
Animals living on the lava flows have essentially no access to water and must get their moisture directly from their food. This is true regardless of whether they are consuming plants, insects, or larger animals.
Two animals at Craters of the Moon have developed distinct subspecies which are darker in color than their relatives elsewhere, the pika and pocket mouse. This adaptation protects them from predators, since it makes them more difficult to see against the dark surface of the lava.
In addition to surviving the rigors of summer, each animal must have a strategy to survive the sub-zero temperatures, deep snow, and lack of food during the winter months. Migration and hibernation are both effective ways to escape extreme winter conditions. Most of the bird species present during the summer overwinter elsewhere; the western tanager, for example, spends its winters in Mexico and Central America. There are animals that migrate shorter distances on foot. The Craters of the Moon mule deer herd moves down slope onto the desert where the snow is not as deep during the winter. Ground squirrels, marmots, bears, and other animals spend the cold months in a state of deep sleep. Marmots, for example, experience a drop in body temperature from 98 degrees to just 34 degrees Fahrenheit and their heart rate plummets from 80 to just 4 beats per minute. This is a very effective way to conserve energy and survive on stored fat until warm weather returns.
There are animals, though, that are active in winter. Some, like the pocket gopher, mountain vole, and pika, actually remain beneath the snow layer. The small open space between the snow and the ground is warmer than the surface and affords some protection from predators. Pikas assure an adequate food supply for themselves by storing stacks of dried grasses collected during the summer in their rocky dens. Food storage is a strategy also adopted by red squirrels. They store limber pine cones near rocks and fallen logs all summer long, and dig through the snow to find this food cache.
It took thousands of years for this community of plants and animals adapted to the rigorous conditions at Craters of the Moon to become established. The formation of soil on the barren rock was the necessary first step in their colonization of the lava.
Soil formation proceeds in different ways on cinder cones and lava flows. On cinder cones, wind, water, and changing temperatures break large cinders into smaller and smaller pieces. Meanwhile, the wind carries in fine particles of dust and soil, which mix with the top layer of cinders and provide many of the components needed for plant growth. However, there is no nitrogen within newly deposited cinders, so only plants capable of getting nitrogen from some other source can grow. Certain algae and bacteria are able to absorb nitrogen directly from the atmosphere. This unique characteristic allows algae and bacteria to take hold on cinder fields. When they die, their bodies decompose and provide nitrogen for the plants that follow.
In contrast to loose cinder piles, lava flows have a hard skin of volcanic glass and an interior riddled with cracks and vesicles. Soil formation on the lava flow begins with the growth of lichens. Lichens are a combination of organisms: an algae that obtains nutrients by secreting an acid which dissolves rock, and a fungus that absorbs water and protects the algae from the drying effects of the sun. This mutually beneficial relationship provides everything needed for lichens to colonize bare rock.
Lichens break off small fragments of rock, which the wind blows into the many crevices of the lava flow. There, these particles combine with larger deposits of wind-borne dust and build rudimentary soil. Eventually enough soil builds up in the cracks for plants to sprout. Once plants begin to grow in the newly created soils of the cinder cones and lava flows, a process of ongoing change called succession has begun.
Succession is the orderly stage-by-stage replacement of one plant community by another. Each community in turn alters its environment by adding organic matter to soil, increasing its ability to hold water, and be providing shade. Eventually this results in conditions that favor the survival of a new group of plants. Succession proceeds differently on the lava flows than on the cinder cones, but the final "climax" community is identical.
Once enough soil has accumulated on cinders, small herbs such as dwarf buckwheat, scorpionweed, bitterroot, and dwarf monkeyflower begin to grow. As time progresses the remains from these plants mix with the cinders to produce a richer, deeper soil able to support more complex plants. Plant succession then occurs differently on north and south facing slopes. North-facing cinder slopes receive less direct sunlight and are sheltered from the prevailing winds. Therefore, snow stays longer, and these slopes remain cooler and wetter than southern exposures. These conditions initially favor limber pine and bitterbrush. As more organic matter accumulates, increasing water retention in the soil, Douglas-fir replaces the pine.
On south-facing slopes, bitterbrush and other shrubs replace the pioneer herb gardens as the soil slowly builds. Limber pines are interspersed with the shrubs, but disappear as the plant cover becomes more dense, since pine seedlings cannot survive in the shade of the bushes. By growing rapidly upward, bitterbrush can withstand partial burial by wind-drifted cinders. The upreaching branches intercept dust particles, gradually adding soil and creating conditions favorable for sagebrush and grasses to grow. As erosion begins to level out steep slopes and the soil deepens and matures, a sagebrush-grassland covers the cinders. Succession has proceeded from cinder garden herbs to bitterbrush/limber pine and finally to sagebrush-grassland.
Succession is essentially at a standstill on the surface and in the shallow cracks of fresh lava flows. It is in the wind-deposited soils of the deep crevices that lava flow succession begins. There plants such as tansybush, syringa, and bush rockspirea find protection from direct wind and sun. Dead leaves and stems from these plants, plus airborne debris, accumulate and the crevices eventually fill in, creating conditions like those in the shallow cracks. More drought-resistant species replace the original deep crevice plants. These new plants flourish, creating a layer of soil around them. Only when the crevices and cracks have filled with soil can a complete plant cover develop. Wide expanses of lava then gather a layer of soil in which extensive tracts of sagebrush-grasses thrive. Thus, succession on the lava flows proceeds from deep crevice drought evaders to shallow crevice drought resisters to sagebrush-grassland.
Each successional plant community has its own set of animal inhabitants. Barren lava flows conceal the nests of rock wrens, a grayish-brown bird sometimes seen bobbing up and down among the rocks. They raise their young in the a'a lava where they can retreat into cool crevices in the heat of the day. Great-horned owls find the high ledges in lava tubes or cinder cone cliffs ideal sites to nest, protected from attack by other birds. Pikas and yellow-bellied marmots also live in lava outcrops.
Limber pine stands host animals that feed on pine nuts, like the yellow pine chipmunk and the larger golden-mantled ground squirrel. The Lewis' woodpecker and Clark's nutcracker both use dead pines to build their nests, and nutcrackers are highly dependent on pine nuts for survival.
Douglas-fir groves shelter animals not normally found in the high desert: chickadees, red-breasted nuthatches, brown creepers, and golden-crowned kinglets and forest mammals like red squirrels and porcupines.
The sagebrush-grassland community holds the greatest abundance of animal life. Pronghorns and coyotes occasionally wander through, and the northern harrier, American kestrel, and sage thrasher are all abundant. Sage grouse move to the foothills just north of the monument in springtime. In winter they return to the sagebrush flats to escape deep winter snows. The sage grouse has feathered toes which enable it to walk on top of the snow and feed on an unlimited supply of sagebrush leaves.
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