Changes in vegetation on Macquarie Island coastal slopes, 1980-1990

The thesis examines various aspects of vegetation dynamics and change on subantarctic Macquarie Island over the ten years 1980 to 1990. The island is basically an elongated plateau surrounded by steep coastal slopes 100-250 meters in height. The major vegetation type on these slopes consists of tall tussock grassland dominated by the tussock-forming grass Poa foliosa, with the tall herb Stilbocarpa polaris an important accompanying species. The behaviour and distribution of these two species in the coastal slope vegetation under various disturbance conditions through time, forms the major focus of this study. The longitudinal observations of the study (involving longterm re-sampling through time) also allow the testing of an existing model of revegetation patterns on disturbed coastal slope sites which was based on a set of latitudinal observations (once-off \spatial\" sampling of time-sequence data from a series of differently aged sites). These latitudinal observations were taken at the beginning of the 1980-1990 study period. The timing of the study coincides with the first ten years after myxomatosis was first introduced into the feral rabbit population in 1978- 79 substantially reducing rabbit numbers. The effects of grazing on the coastal slope vegetation since the introduction of rabbits in the 1850's appears to have been substantial although until recently little documented. Landslipping is also a common disturbance factor affecting the coastal slope vegetation. Changes in the vegetation of 27 sites on areas affected by landslipping rabbit grazing or a combination of both were studied over the 10-year period by monitoring of fixed plots. This study was complemented by a broadscale study of vegetation change over a five kilometer section of the coastal slopes through comparison of aerial photographs taken at the beginning and the end of the study period and by a study of changes in a series of paired quadrats in relatively undisturbed vegetation over the same period. A study of seasonal growth and dieback patterns of Stilbocarpa polaris over a twelve month period and analysis of a series of observations on grazing patterns prior to 1980 provided additional insights into behaviour of Stilbocarpa polaris and effects of grazing on the coastal slopes. Through these means the effects of the various disturbance factors on the vegetation dynamics can be inferred. Since 1980 there has been an overall increase in abundance of Poa foliosa and Stilbocarpa polaris especially the former on the coastal slopes. This can be linked to the substantial reduction of rabbit numbers after 1978-79 to a level amenable to control but not eradication by myxomatosis. It could also possibly be linked to climatic warming. Vegetation succession on areas disturbed by landslipping or rabbit grazing will eventually progress to a Poa foliosa-dominated vegetation with Stilbocarpa polaris as a co-dominant or subsidiary 4.ominant in the absence of further grazing disturbance and/ or slope instability. Continuation of selective grazing on disturbed areas on the coastal slopes however will suppress the development of Poa foliosa and to a lesser extent Stilbocarpa polaris and an increase of grazing pressure will favour the development and maintenance of short grassland/herbfield vegetation. The conclusions from the longitudinal study largely verify those of the latitudinal study although minor modifications are justified by the new data."