Species richness, turnover and equilibrium in insular floras near Perth, Western Australia

Abstract

In 1975 lists were made of the plant species occurring on 121 limestone islands, varying from 3 m2 to 160,900 m2 in area, west of Perth. Fifty-four of the islands supported no vascular plant species. An explanation of the different number of plant species found per island (S) was sought in terms of island area (A), elevation (E), and diversity of habitat types (H). Significant linear correlations were found between S and A, S and E, and S and H. The validity of neglecting islands with zero species number in regression analyses was evaluated and found to be wanting. Small islands (area < 5000 m2) tend to have impoverished floras; a study of the exceptions shows that the impoverishment is mainly due to exposure to sea spray.Inter-island distance plays a negligible part in determining the similarities of island floras. Comparisons of the present flora lists for 20 islands with those made by G. M. Storr in 1956 or 1959 show that species turnover has occurred. Although 12 islands show an increase in plant species number (as total plants, perennials, annuals and native species), this tendency is not significant. Annual species have higher extinction rates than perennial species and alien species higher ones than native species. Alien species also have higher immigration rates than native species. Floras of islands with cormorant colonies tend to have higher extinction rates than islands without. Islands with gull rookeries have higher immigration rates of alien species than islands without. Considering all 20 islands together there are no significant differences between extinction and immigration rates. These rates are independent of island area and isolation respectively. It is suggested that seabird colonies override the importance usually ascribed to island area and isolation in influencing these rates.