The effects of fire and quokkas (Setonix brachyurus) on the vegetation of Rottnest Island, Western Australia

Three different plant communities have dominated the vegetation of Rottnest Island over the past two centuries; low forest, Acacia rostellifera scrub, and sclerophyllous heath. In 1997 a fire burnt 90 ha of heathland and provided an opportunity to examine the relationships between dominant vegetation types, fire and grazing by quokkas (Setonix brachyurus). Our study and the literature indicated four major findings. • Burnt heath recovered slowly if grazed. If not grazed, heath regenerated rapidly to become dense, tall and weed-free compared with the surrounding heathland, and the number of native species increased. • Where there was no grazing, all three vegetation communities were able to become established immediately after the fire. Four and a half years after the fire, Acacia thickets dominated in some areas and reached 3-4 m in height, overshadowing a group of self-seeded Melaleuca seedlings and outcompeting heath species. It is anticipated that the Melaleuca will emerge and dominate when the shorter-lived Acacia declines, and that heath will continue to occupy open areas. • The three dominant vegetation communities can be regarded as stable state alternatives. Heath will dominate if there is heavy grazing, as the young trees and Acacia are palatable and so cannot regenerate. Low forest dominates if there are only occasional widespread fires. Fire triggers the release of seed stored in tree canopies, and also temporarily reduces the local quokka population, giving seedlings a chance to become established. Acacia dominates if there are frequent fires and little grazing. Frequent fires stimulate Acacia to sucker from underground parts, while competition from tree species disappears, as trees are killed by fire, and if a second fire occurs before young trees set seed, they are eliminated locally. • It appears that occasional widespread hot fires, presumably started by lightning, are likely to have occurred on Rottnest Island over the millennia before European settlement for Melaleuca/Callitris forest to have persisted.