The scientist who developed the concept of life zones, C. H. Merriam, based his theory on research he did in the Grand Canyon region in 1889 as head of the U.S. Biological Survey. He proposed that plant and animal communities change not only with latitude, but also with elevation.

The life zones, or biomes, identified by Merriam are the Lower Sonoran, Upper Sonoran, Transition, Canadian, Hudsonian, and Arctic-Alpine. Grand Canyon encompasses five out of six life zones Merriam identified, often likened to traveling from Mexico to Canada.

Merriam’s system is still used in the western U.S., though most scientists today think in terms of biologic communities rather than zones. Even so, plants and animals aren’t aware of the neat scientific boundaries assigned to them. The canyon acts as a barrier to some species and a corridor to others.

Orientation to the sun, rainfall amounts, differences in terrain and soils—these factors create a range of microclimates within biologic communities. Desert species extend higher on sunny ridges and slopes, while deep, shady draws allow higher elevation species to move downward.

The intermixture between neighboring biologic communities is referred to as an ecotone, where flora and fauna mingle in amazing variety. Approximately 1,800 species of plants grow in Grand Canyon, nearly half of the flora found in Arizona, which is considered one of the most botanically diverse states in the country. A dozen plants are endemic, found only in the Grand Canyon area.