Napier Shelton
based on an earlier work by Natt Dodge
NATURAL HISTORY SERIES
This book is a simple account of the natural history of Saguaro National Monument. It is intended to help you understand the relationships between land, climate, plants, wild animals, and man in the environment of a hot desert. While it includes brief profiles of many representative species, it is not intended to serve as a guide to the monument. It does indicate where the several distinctive natural communities exist, and when and where to look for certain plants and animals. For identification purposes, you will need field guides.
The present edition is my revision of the 1957 book by Natt Dodge, then naturalist for the Southwest Region of the National Park Service. The first five and the last two chapters are essentially new; the central chapters on plants and animals remain largely as written in the first edition.
The authors wish to thank former and present members of the monument staff for their help and companionship, in both field and office. We are particularly grateful to Chief Naturalist Harold T. Coss, Jr., who devoted much time and effort to obtaining many of the photographs and, with Park Biologist Warren F. Steenbergh, gave the manuscript a thorough scrutiny. The cooperation and hospitality of Superintendent Harold R. Jones have created the best possible climate for work on the revised edition. National Park Service geologist Robert H. Rose contributed in very great measure to the geological content. Finally, our thanks go to the many students of desert life on whose knowledge this book has been built, and to monument visitors who ask questions—for their concern gives hope for better relations between man and nature.
—N.S.
The Desert Scene, 1
The Sonoran Desert and the Monument, 3
Climate: The Vital Factor, 9
Rocks: Foundation and Soilmakers, 13
Plant and Animal Zones, 19
Desert Plants, 23
Succulents, 25
The Saguaro—Monarch of the Monument, 27
Other Common Cactuses, 33
Non-Succulents, 36
Perennials, 36
Ephemerals, 42
Major Plant Communities of Saguaro, 49
Plants of the Hills and Mountains, 55
Oak-Pine Woodland, 55
Ponderosa Pine Forest, 58
Animals and How They Survive, 61
Invertebrates, 62
Amphibians, 64
Reptiles, 64
Birds, 66
Mammals, 72
The Rhythms of Nature, 79
The Impact of Man, 83
Appendix, 87
Suggested Reading, 87
Common and Scientific Names of Plants, 89
Reptiles and Amphibians of the monument, 92
Birds of the monument, 94
Mammals of the monument, 97
Brittlebush display along Cactus Forest Drive.
Scattered through the wide, lonely Sonoran Desert, isolated mountain ranges raise jagged blue silhouettes against the sky. The high ones wear a crown of dark pines and a speckled mantle of oaks. Lapping against their feet is the desert sea, studded with the green masts of giant saguaro cactuses rising above a motley crew of tough, strange, often handsome subordinates.
On either side of the Santa Cruz Valley in southeastern Arizona, Saguaro National Monument embraces two of these ranges, with the desert lands at their feet. Although it was established primarily to preserve impressive stands of saguaros, it gains wholeness by including the mountains that shed the rocky soil on which the saguaros grow. The monument is in two sections, some 30 miles apart. The Rincon Mountain Section, east of Tucson, includes in its 99 square miles a spectrum of plantlife ranging from saguaro communities at the low elevations to a wet fir forest on the north slope of 8,666-foot Mica Mountain. The 24-square-mile Tucson Mountain Section, about 12 miles west of the city, has a denser stand of saguaros, on the lower slopes of a wild jumble of volcanic mountains dominated by 4,687-foot Wasson Peak.
Within the cactus forests and upon the mountain slopes, life in myriad forms goes on in a delicate and continuous adjustment to changing environments. Over millennia, mountains rise and crumble, species evolve and fade from the scene. Over days and weeks and years, populations of plants and animals rise and fall, in response to thousands of interactions which we are only beginning to understand. In the following pages, we will meet the main characters in this natural drama (including some of the most improbable in all of nature); we will investigate the environmental stage upon which they perform; and we will try to understand something of the play itself, including our own role in it.
The Saguaro Forest in the Tucson Mountain Section.
Saguaro National Monument is at the northeastern edge of the Sonoran Desert. Named for the state of Sonora, Mexico, in which the greater part of it lies, this, one of four major deserts in North America, is distinguished by differences in climate and vegetation. The Great Basin Desert, mainly in Nevada and Utah between the Rockies and the Sierra Nevada, has cold winters, sparse precipitation distributed fairly evenly throughout the year, and rather simple vegetation dominated by the low shrubs, sagebrush and saltbrush. Immediately to the south, in southern Nevada and southeastern California, is the Mohave Desert, with cool winters during which most of the year’s precipitation comes, and with plant cover consisting mostly of shrubs such as creosotebush. At higher elevations grows the Joshua-tree, a giant yucca. The Chihuahuan Desert, with cool winters and summer rainfall, covers the broad plateau of north-central Mexico, extending into southern New Mexico and west Texas. Its vegetation consists mostly of small cactuses, spiny shrubs, and succulent-leaved plants such as yuccas.
Lying between the Mohave and Chihuahuan deserts, the Sonoran Desert has both winter and summer rainfall, with spring and autumn droughts. Its mild winters and bi-seasonal rainfall encourage a vegetation far surpassing in lushness and variety that of the other deserts. From west to east, the land rises and precipitation increases. Yuma, in southwestern Arizona, lying at 141 feet above sea level, gets about 3 inches a year. Tucson, in southeastern Arizona, is at 2,400 feet elevation and averages 11 inches a year. In the western part, where plains are extensive and mountain ranges low and far apart, a very few small-leaved species such as creosotebush dominate. Toward the east, mountain ranges become more numerous, shedding material on which more kinds of plants, particularly paloverde, mesquite, and cactuses, assume leading roles in the vegetative cover. Both sections of the monument lie within the eastern, wetter, more diversified part of the Sonoran Desert known as the Arizona Upland. Organ Pipe Cactus National Monument, to the southwest, is in an area transitional between the Arizona upland and Colorado River lowland phases of the Sonoran Desert.
DESERT AREAS OF NORTH AMERICA
But even between the two sections of Saguaro National Monument there are differences. There is the obvious fact that the Tucson Mountains are much lower and smaller in mass than the Rincons. And there is the not-so-obvious fact that the Tucson Mountain Section extends to lower elevations (2,200 as opposed to 2,700 feet). These conditions are reflected in the somewhat warmer environment of the western section. This helps to explain why certain Sonoran Desert plants and animals here reach their northeastern limits; among them are ironwood, desert iguana, desert horned lizard, western shovel-nosed snake, sidewinder, desert kangaroo rat, and the Le Conte thrasher. The Rincon Mountain Section, by virtue of its higher elevations, has plants and animals—such as ponderosa pine, Steller’s jay, and whitetail deer—not found in the western section.
On the geologic time scale the Sonoran Desert, like all the earth’s present deserts, is a recent development. Some 50 million years ago tropical forests grew here, as they did over most of southern North America. As the Southwest gradually became drier, new species adapted to the new conditions evolved; and the general type of vegetation changed—with some retrogression during wetter periods—from forest to savanna (grasslands with scattered trees) to arid subtropical scrub (such as is now found in southern Sonora State), and finally to the plant communities of today’s Sonoran Desert. The last stage has occurred only during the past few million years.
The assemblage of species from which most of our present desert and lower mountain plants were derived is known as the Madro-Tertiary Flora, to denote its center (Sierra Madre in Mexico) and time (Tertiary period—one million to 65 million years ago) of early development. Reflecting these origins, a majority of the species growing below 6,000 feet elevation in the monument today are basically Mexican or Central or South American in their distribution.
On the higher mountains of the Sonoran Desert, however, exist species with an entirely different ancestry. Most of the plants of the pine and fir forests of these high places, like the plants of the Great Basin Desert, derive from the Arcto-Tertiary Flora, which dominated the northern part of our continent during Tertiary times. As climate changes, one or the other of these great plant assemblages will benefit at the expense of the other. If cooler, wetter periods return, as they did during the Pleistocene epoch, ponderosa pines, now found above 6,000 feet, may again grow nearly as low down as today’s cactus forests. But if the present long-term drying trend continues, the northern plants will eventually be squeezed off their mountaintops.
Animals, to some extent, reflect the same dichotomy of origins we have seen in the plants. Thus, on the mountaintops supporting pine and fir forests, a northern contingent of animals predominates, while on the lower slopes and desert the fauna has a Mexican character. The canyons and oak-pine forests of middle elevations in southeastern Arizona are particularly exciting, because in these biological islands, isolated by surrounding desert, live many “Mexican” species of animals found nowhere else in the United States. Naturally, mountain ranges nearest the border—such as the Huachucas and Chiricahuas—have the greatest numbers of Mexican specialties; but the Rincons have their share too.
The wonderful diversity of plant and animal life in Saguaro National Monument depends on a diversity of habitats. These in turn owe their existence to a widely varying triumvirate of environmental factors—climate, soil, and topography. To understand the biological interplay that goes on here, we first must know something of the environmental conditions that circumscribe it.
Plant communities on Mica Mountain.
|
Era |
Period |
Epoch |
Years before present time |
Major geological events |
|
Cenozoic |
Quaternary |
Recent |
10,000 |
Cascade Range and Sierra Nevada uplifted |
|
|
|
Pleistocene |
||
|
|
|
|
1,000,000 |
|
|
|
Tertiary |
Pliocene |
||
|
|
|
|
12,000,000 |
|
|
|
|
Miocene |
||
|
|
|
|
26,000,000 |
|
|
|
|
Oligocene |
||
|
|
|
|
38,000,000 |
|
|
|
|
Eocene |
||
|
|
|
|
54,000,000 |
Rocky Mountains formed |
|
|
Paleocene |
|||
|
|
|
|
65,000,000 |
|
|
Mesozoic |
Cretaceous |
|||
|
|
|
|
136,000,000 |
|
|
|
Jurassic |
|||
|
|
|
|
195,000,000 |
|
|
|
Triassic |
|||
|
|
|
|
250,000,000 |
Appalachian folding |
|
Paleozoic |
Permian |
|||
|
|
|
|
280,000,000 |
|
|
|
Pennsylvanian |
|||
|
|
|
|
320,000,000 |
|
|
|
Mississippian |
|||
|
|
|
|
345,000,000 |
|
|
|
Devonian |
|||
|
|
|
|
395,000,000 |
|
|
|
Silurian |
|||
|
|
|
|
440,000,000 |
|
|
|
Ordovician |
|||
|
|
|
|
500,000,000 |
|
|
|
Cambrian |
|||
|
|
|
|
570,000,000 |
|
|
Precambrian |
Summer thunderstorm in the Rincon Mountain Section. The Catalina Mountains rise in the background.
Climate is the chief arbiter of life on earth. Each plant and animal, including man, has tolerance limits for heat and water below or above which it dies. The Sonoran desert, with its low and uncertain rainfall and high summer temperatures, thus presents one of earth’s most taxing environments. For most desert plants and animals, the main problem presented by this climatic combination is a scarcity of water. Why, we might ask, is this part of the globe so dry?
Essentially, the dryness of the Sonoran and other southwestern deserts results from their geographic situation. First, these lands lie east of a persistent high-pressure cell over the eastern Pacific—part of a belt of high-pressure cells which encircles the earth at this latitude. High-pressure systems in the Northern Hemisphere have descending, warming, dry air in their middles and along their eastern sides. The air that flows eastward from the “East Pacific high” thus contains little moisture, and dictates desert conditions over much of the Southwest. Furthermore, the mountains that more or less ring these deserts intercept much of the meager incoming moisture, by forcing the air to rise and cool so that its moisture condenses and falls to the ground.
But during two periods of the year, the desert does receive rain. In winter, the East Pacific high shifts southward with the sun, reducing its effects on the Southwest. Now the low pressure storm systems, which at this season move eastward across the continent, can sometimes dip far enough south to bring rain to the Sonoran Desert. These rains are usually gentle, and cover large areas. Their clouds may cover the sky for several days at a time.
By April, the East Pacific high has returned far enough northward to resume its control of southwestern weather. Now the fore-summer drought sets in, intensified by gradually rising temperatures. Day after day the sun climbs higher in the sky, baking the earth and testing the endurance of desert life. Then, sometime in late June or early July, clouds begin forming in the afternoons over the mountains; finally, one day they build up to great thunderheads. The drought ends suddenly as rain pours down and lightning cracks the sky. Down the canyons and along the desert washes rages the water, brown with soil. In a short time, the storm is over, and the water disappears into the ground. Just 1 mile away, rain may not have fallen.
These intense, local summer rains, so different from the winter ones, have a different origin as well. They are spawned by the Atlantic’s Bermuda high, which has shifted northwestward and sent moist air from the Atlantic and Gulf of Mexico flowing off its western edge toward the deserts. This air rises quickly when it strikes hot, high mountains such as the Rincons, and its moisture condenses to fall as rain.
Some time in September the rains of summer stop, as the Bermuda High moves south again, removing its influence and returning the East Pacific high to a position of dominance. The sun and wind again wring moisture from the soil, but with diminishing effect. The thermometer no longer reaches the 100’s as it did in June, July, and August. In November or December, the winter rains begin. Water stays longer in the soil, because temperatures now range between 30° and 80°. Life in the desert begins a new cycle. This rainy period usually lasts into March.