Thirty years of vegetation dynamics in response to a fluctuating rabbit population on sub-Antarctic Macquarie Island

Australia's sub-Antarctic Macquarie Island is presently undergoing one of the most ambitious vertebrate pest eradication programmes ever undertaken. The anticipated success of this programme will release the island's tundra-like vegetation from well over a century of grazing and disturbance from House Mouse (Mus musculus), Ship Rat (Rattus rattus) and most significantly European Rabbit (Oryctolagus cuniculus). This study describes results from 30 years of vegetation quadrat monitoring (prior to the most recent and comprehensive pest eradication programme) when lower level pest animal control programmes were underway. Plant species were assigned to one of five distinct functional plant groups: Indigenous short-lived perennials, Introduced short-lived perennials, Indigenous perennials rarely grazed by rabbits, Indigenous perennials occasionally grazed by rabbits and Indigenous long-lived perennials heavily grazed by rabbits, with one species, Agrostis magellanica, analysed as a sixth monospecific group. Results from monthly rabbit counts were used to compare changes in abundance of these six groups under different rabbit populations. It was found that there were three distinct phases of rabbit activity during the study period, indicated by (i) an initial very high count year in 1980-1981, followed by (ii) 20 years of low counts ending in 2001-2002 after which (iii) counts rose to medium/high until the commencement of the eradication programme in 2010-2011. Vegetation composition and progression were distinct for these three rabbit count phases. The first four of the plant functional groups decreased under lower count periods and increased in cover under higher rabbit count periods. Agrostis magellanica appears to respond primarily to interspecies competition and is disadvantaged under extended periods of low rabbit numbers. Indigenous long-lived perennials heavily grazed by rabbits, which includes the large tussocks and megaherbs, is inversely related to rabbit numbers. During the study period, there has also been an overall decline in plant species richness with average species count per quadrat falling by between 0.6 and 2.7 taxa. This study attempts to address the observed vegetation change from this long-term monitoring, to discuss other potential contributing factors and to use the results to predict likely future vegetation changes after eradication of vertebrate pests.