Biogeographical Aspects of Speciation in the Southwest Australian Flora

Abstract

The southwest of the Australian continent is a region of biogeographic interest, having an angiosperm flora remarkably rich in endemic species. It was estimated that 2450 (68%) of the 3600 species known in the region by the mid 1960s were endemics (12, 13, 107). Recent taxonomic research suggests that the actual level of endemism may approach 75-80%, since southwestern genera are being enlarged on average by 10-30% in current monographs (19, 28-30, 36, 38, 61, 62, 67, 72-74, 76, 112, 113, 122, 126, 154, 155). As pointed out by Hooker (70) and subsequent authors (34, 37, 38, 76, 123), the rich flora of the southwest is all the more remarkable in view of its low relief and subdued topography. The Great Plateau of Western Australia (89), an extensive deeply weathered landform that rarely exceeds 500 m in height, dominates much of the southwestern landscape (Figure 1). Areas of relatively high relief are confined to a series of small ranges along its southern margin and uplifted or resistant remnants of Tertiary plateau levels along its western margin. All of these small mountains and plateau remnants are less than 1000 m high, with the exception of a few peaks in the Stirling Range that rise to 1109 m. In contrast, other regions of the world rich in endemic species have much higher and more extensive mountain systems, e.g. South Africa (66, 149), California (5, 96, 132, 144, 150), Turkey and Greece (52, 148, 158), and some oceanic islands (26, 37).