THE FLORA OF GREAT BASIN MOUNTAIN RANGES: DIVERSITY, SOURCES, AND DISPERSAL ECOLOGY

The high elevation floras of 9 mountainous "mainlands" (3 in the Sierra-Cascade system and 6 in the High Plateau-Wasatch-Teton system) and 15 isolated mountain "islands" in the Intermountain Region have been analyzed. Mainland floras support more species per unit area and show a smaller increase in diversity as area is increased than islands. In this respect, the isolated mountains behave as true islands. The number of endemics is low on the islands (never exceeding 5 percent of any flora), however; and the island floras are overwhelmingly dominated by species with no apparent modifications for long-range dispersal. Furthermore, the eastern mainland has exerted a far greater influence on the flora and the vegetation of the islands than has the western mainland, despite the fact that the former is downwind of the islands. Thus, evidence from endemics, dispersal ecology, and sources of the floras suggests that the isolated mountains have not acquired their full floras by long-range dispersal. We conclude that although the floras of the islands have many insular characteristics, they were less isolated in the relatively recent past than at the present. The island floras do not appear to be in equilibrium in the sense that immigrations equal extinctions.