Change in Undisturbed Vegetation on the Coastal Slopes of Subantarctic Macquarie Island, 1980-1995

Abstract

The vegetation of 30 undisturbed permanent quadrats on the steep coastal slopes of subantarctic Macquarie Island was recorded in 1980-81 and 1994-95, a period in which temperatures rose briefly then declined, precipitation increased and rabbit grazing pressure decreased. Previous investigators of the plant ecology of the island have suggested a successional sequence, in the absence of disturbance, towards total dominance of the two major plant species on the coastal slopes, the tall tussock-forming grass Poa foliosa and the megaherb Stilbocarpa polaris, with a concomitant reduction in the diversity of subordinate species. Our observations demonstrate a more complex reality. Dense tall tussock grassland became more open, resulting in an increase in quadrat species richness. The more open tussock grassland on the upper slopes became more dense, with a concomitant decrease in quadrat species richness. The large herb S. polaris changed little in its abundance over the period, perhaps reflecting a need for disturbance for its expansion. The opening of dense stands of P. foliosa may be part of a previously unrecorded endogenous successional process, while the closure of open stands of Poa could be a response to improved growth conditions, including relief from rabbit grazing and the relatively high temperatures in the initial years of monitoring, but may also be endogenous in origin.