Changes in the cover of plant species associated with climate change and grazing pressure on the Macquarie Island coastal slopes, 1980–2009
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Climate change and alien species have affected the vegetation of subantarctic islands. Long-term monitoring of vegetation change on the steep coastal slopes of subantarctic Macquarie Island has allowed responses of plant species to various disturbance regimes to be well documented, although, until recently, the confounding effect of feral herbivore disturbance obscured any responses that might be attributed to climate change. The uncoupling of climate change from variation in feral rabbit numbers allowed us to test whether any plant species were increasing or decreasing on the coastal slopes of the island between 1980 and 2009, independent of rabbit grazing pressure. We used analysis of variance to test for differences in species cover classes between four measurement times on each of 101 quadrats in each of 1980/1981, 1995, 2003 and 2009. We had 54 quadrats on landslips and 47 elsewhere. Approximately two-thirds of the species with significant temporal change exhibited changes that could be expected from variation in rabbit grazing pressure. However, approximately one-third of the species increased in cover irrespective of grazing pressure. On landslips, variation in the cover of these increaser species was largely related to time in a linear mixed model, whereas elsewhere altitude and time were both important. The increase in both atmospheric dryness and episodic soil water-logging that has been described for the island since 1980 may best explain the increaser species.