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.
Geological Time Table
| 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:
the vital factor
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?