Long-term change in the floristic composition and vegetation structure of Carnac Island, Western Australia

Aim To document changes in the floristic composition and vegetation structu re of Carnac Island during a period of 40 years. This paper presents a synt hesis of all available floristic and vegetational information. Location Carnac Island is 8 km offshore from Fremantle, south-west Western Australia. Methods Comparison of lists of plant species for 1951, 1958/9, 1966/7, 1975 /6 and 1995-6. Comparison of vegetation, based on structural and floristic elements, for 1951, 1965, 1972, 1984 and 1995. Results Floristic composition (both native and exotic species) changed most dramatically in the period 1975/6-1995/6, with a 37% reduction in number o f plant species. The number of annual and perennial native species present in 1995/6 was most similar to that in 1951. The most remarkable change in t he flora has been the increase in annual exotic species since 1951. Immigra tion and extinction rates were greatest in the periods 1951-58/9 and 1958/9 -1966/7, respectively. Vegetation structure has also altered, involving a r eduction in height of dominant species from 3-4 m to 1 m as Acacia rostelli fera and Olearia axillaris have declined in distribution. The weed species Mesembryanthemum crystallinum (first recorded 1975) and Malva parviflora (1 958) now dominate the vegetation of half the island. Main conclusions Five factors are considered to have contributed to botanic al change: nesting seabird populations, eradication of the rabbit in 1969, drought, increased salt-load from occasional cyclones in summer or autumn, and competition from increasing dominance of several weed species. Several of these factors have operated in opposing ways with respect to plant speci es richness and vegetation cover: Experimental studies are required to dete rmine the strength of these interactions. Two weed species, Zantedeschia ae thiopica (first recorded 1966) and Lycium ferocissimum (1992) have the pote ntial to dominate the vegetation of the island.