Long-term change in the floristic composition and vegetation structure of Carnac Island, Western Australia.
This paper presents a synthesis of all available floristic and vegetational information for Carnac Island, which is 8 km offshore from Fremantle, south-west Western Australia. A comparison of lists of plant species was made for 1951, 1958-9, 1966-7, 1975-6 and 1995-6, and a comparison of vegetation, based on structural and floristic elements, for 1951, 1965, 1972, 1984 and 1995. Floristic composition (both native and exotic species) changed most dramatically in the period from 1975-6 to 1995-6, with a 37% reduction in number of 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 the flora has been the increase in annual exotic species since 1951. Immigration and extinction rates were greatest in the periods 1951 to 1958-9 and 1958-9 to 1966-7, respectively. Vegetation structure has also altered, involving a reduction in the height of dominant species from 3-4 m to 1 m as Acacia rostellifera and Olearia axillaris have declined in distribution. The weed species Mesembryanthemum crystallinum (first recorded 1975) and Malva parviflora (1958) now dominate the vegetation of half the island. Five factors are considered to have contributed to botanical 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 species richness and vegetation cover. Experimental studies are required to determine the strength of these interactions. Two weed species, Zantedeschia aethiopica (first recorded 1966) and Lycium ferocissimum (1992) have the potential to dominate the vegetation of the island.