We call the Santa Catalina Mountains a "sky island" - a natural and appropriate metaphor for a mountain that rises from the desert floor and is surrounded by seas of grasses and scrub.

The term can be misleading, however, in an ecological context, said herpetologist Tom Van Devender.

The sky islands of Arizona, New Mexico and Mexico are not "insular," he said.

Unlike ocean islands, they do not have a dearth of plant and animal species, nor a wealth of unique ones.

They are incredibly diverse and they share species with each other, and particularly with the Sierra Madre of Mexico.

Van Devender counts 52 mountain ranges as part of what is called the Madrean Archipelago. The longtime researcher at the Arizona-Sonora Desert Museum now leads the Madrean Archipelago Biodiversity Assessment for the Sky Island Alliance.

In a paper prepared for the Madrean Archipelago conference last year, he tracked down the earliest uses of both terminologies:

• In a U.S.-Mexico boundary survey in the late 1800s, Lt. David Gaillard described the region as "bare, jagged mountains rising out of the plains like islands from the sea."

Nature writer Weldon Heald popularized the term in his 1967 book about the Chiricahua Mountains, titled "Sky Island."

• Van Devender traces the more recent coinage of "Madrean Archipelago" to two 1992 papers - one by herpetologist Charles H. Lowe and the other by botanists Peter S. Bennett and Michael R. Kunzmann.

The Madrean Archipelago is a string of sky islands, most with a north-south orientation, that provide a bridge between the Sierra Madre Occidental in Sonora and the Mogollon Rim in east-central Arizona.

Van Devender counts 16 sky islands in Arizona and 36 in Sonora.

The Sky Island Alliance has been sponsoring major expeditions of U.S. and Mexican researchers to many of the little-known but incredibly diverse mountains in Mexico and is compiling lists of their flora and fauna.

Diversity increases as you go south, Van Devender said.

Naturalist Steven McLaughlin, in a report prepared for an earlier Madrean Archipelago conference, estimated that, regionally, these mountains are home to as many as 2,800 species of flora.

In Southeastern Arizona's sky island ranges, the number is estimated at 2,100.

The mountains also host a diversity of birds, mammals, reptiles, insects and arachnids.

There are more than 120 mammal species, said John Koprowski, professor in the University of Arizona School of Natural Resources and Environment.

Your average sky island contains more diversity of mammals than a big, geographically restricted park such as Yellowstone, he said.

"For me, this is an incredible place to be," said Koprowski. "It's a little biodiversity laboratory."