Species richness and endemism in the Western Australian flora

ABSTRACT

Aim Estimates of endemic and non-endemic native vascular plant species in each of the three Western Australian Botanical Provinces were made by East in 1912 and Beard in 1969. The present paper contains an updated assessment of species endemism in the State.

Location Western Australia comprises one third of the continental Australian land mass. It extends from 13° to 35° S and 113° to 129° W.

Methods Western Australia is recognized as having three Botanical Provinces (Northern, Eremaean and South-West) each divided into a number of Botanical Districts. Updated statistics for number of species and species endemism in each Province are based on the Census of Western Australian Plants data base at the Western Australian Herbarium (Western Australian Herbarium, 1998 onwards).

Results The number of known species in Western Australia has risen steadily over the years but reputed endemism has declined in the Northern and Eremaean Provinces where cross-continental floras are common. Only the isolated South-West Province retains high rates of endemism (79%).

Main conclusions With 5710 native species, the South-West Province contains about the same number as the California Floristic Province which has a similar area. The Italian mediterranean zone also contains about this number but in a smaller area, while the much smaller Cape Floristic Region has almost twice as many native species. The percentage of endemic species is highest at the Cape, somewhat less in south-western Australia and less again in California. Italy, at 12.5%, has the lowest value. Apart from Italy, it is usual for endemism to reach high values in the largest plant families. In Western Australia, these mainly include woody sclerophyll shrubs and herbaceous perennials with special adaptations to environmental conditions. While those life forms are prominent in the Cape, that region differs in the great importance of herbaceous families and succulents, both of which are virtually absent from Western Australia. In California and Italy, most endemics are in families of annual, herbaceous perennial and soft shrub plants. It is suggested that the dominant factor shaping the South-West Province flora is the extreme poverty of the area's soils, a feature that emphasizes sclerophylly, favours habitat specialization and ensures relatively many local endemic species.