Changes in Species Composition of Floras on Islets Near Perth, Western Australia

Abstract

Seventy-six aeolianite limestone islets were visited at least twice between 1975 and 1978, and their floras were recorded. 38% (twenty-nine of the islets) remained unvegetated; they had a smaller area and lower maximum elevation than islands that were vegetated. 55% (forty-two) of the islets showed no change in plant species composition, were smaller, and had fewer plant species than islands that did show change in species composition over time. Comparisons with the same islets sampled in the 1950s show that turnover of species is 3 times higher for long census intervals (20 years) than for short census intervals (1-3 years). For those islets censused in 1975, 1977 and 1978 there was a significant difference between 1975-77 and 1977-78 in the proportion of islands showing only extinctions or immigrations of species; this may be related to an increase in rainfall or a cyclone in 1978. Physically stable islands such as we studied seem to be more relevant in assessing the extent of plant species turnover on islands than cays, which tend to be physically unstable.