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.