The San Diego of popular imagination is a lotus land of sun and surfers, a ruggedly beautiful region where Mexican barrios coexist with mountain villages and deserts sprawl as far as the eye can see. Functionally, if not officially, its reaches extend all the way to Tijuana, Mexico, giving it an unusual international distinction, as well as a heterogeneous population of nearly five million people. Covering more than 4,200 square miles, San Diego County encompasses a variety of ecologies, including the beach and coastal strip, lowlands and mesas, foothills, mountains, and desert terrain. In...