Floristic Change on Heron Island, a Coral Cay in the Capricorn-Bunker Group, Great Barrier Reef

Abstract

Floristic change on islands is a matter of considerable theoretical and practical conservation interest. The vascular flora of Heron Island, a small coral cay on the Great Barrier Reef, Australia was, therefore, surveyed at 3 month intervals from September 1990 until December 1993. During this period 57 species were reported for the island, with the number of species present at one time varying from 40 to 53. Six alien species and two native species were reported from the island for the first time. Direct gradient analysis demonstrated a similarity between spring-summer samples and autumn-winter samples, with the pairs of samples being different from each other. Neither the total number of species present at one time nor the proportion of alien species in the flora has increased since 1984, suggesting that the flora has reached equilibrium with the disturbance caused by human activity on the island. A method for estimating the annual rate of species turnover (I₁) is outlined, and I₁ is calculated for Heron Island and four other islands in the Capricorn-Bunker Group. The low I₁ reported for Heron Island is further evidence for floristic equilibrium being approached on this highly disturbed site.