The study region is a 9,000 sq. km rectangle that encompasses several major river drainage basins and is seen in its California context in Figure 3. It is one of the country’s most desirable places to live, and in 1990, had a population of about one million. The regional planning agencies forecast the population to increase by 500,000 by the year 2010, and it is expected to continue growing beyond that date. Marine Corps Base Camp Pendleton, one of the United States’ major military facilities, occupies 50,000 hectares with twenty-seven kilometers of coastline and is the largest unbuilt portion of land on the southern California coast. Camp Pendleton will also be more intensively used in the future as other military bases close.

This landscape has several major physiographic regions that range from coastal to mountain, and includes the Temecula Valley. As a result, the study area is one of the most biologically diverse environments in the continental United States. It includes a wide variety of habitat types including coastal lagoons and estuaries, coastal scrub areas, maritime-influenced chaparral and scrub communities, oak woodlands, coniferous mountain areas, and dry, hot, sparsely-vegetated deserts. Each of these supports a unique range of animal species. Within the region are over 200 plants and animals listed by federal or state agencies as endangered, threatened, or rare. These include the least Bell’s vireo, the coastal cactus wren, and the California gnatcatcher. In addition, a number of plants and animals are of local concern due to declining populations, such as the California cougar.

Most who fly over this area, think of the region simply as "open space" between two large cities. Even though it is the "far end" of three counties–Orange, Riverside, and San Diego–it is, in reality, one of the country’s fastest urbanizing regions. Land is very expensive, especially along the coast; the value of private land decreases rapidly with distance from the coast.

All of the region’s unbuilt private land is zoned or planned for development. The region has considerable amounts of public land, and the remaining land–belonging to several Native American nations–is exempt from public land use controls.