Keystone Species—Pima County
"Keystone Species are species who enrich ecosystem function in a unique and significant manner through their activities, and the effect is disproportionate to their numerical abundance. Their removal initiates changes in ecosystem structure and often loss of diversity. These keystones may be habitat modifiers (e.g. cottonwoods, rushes and ironwood), keystone predators (e.g. puma and coyote) or keystone herbivores (e.g. prairie dog and beaver)."
(This is a partial list.)