Effects of invasion by the indigenous shrub Acacia sophorae on plant composition of coastal grasslands in south-eastern Australia

Abstract

Invasion of natural ecosystems by weeds is often associated with increasing levels of disturbance. However, there is substantial evidence that cessation of disturbance can also lead to plant invasions, especially of woody species. In some coastal grasslands in south-eastern Australia, removal of grazing and burning appears to have triggered invasion by the indigenous shrub, Acacia sophorae. To quantify the impacts of such invasions on plant diversity, this study focussed on the coastal grasslands in Eurobodalla National Park, NSW. Analysis of aerial photographs indicated that 96 ha of the 479 ha study area (20%) was invaded by Acacia between 1944 and 1994, with the most rapid expansion from the late 1970s when grazing was removed. Vegetation sampling demonstrated a strong negative correlation between Acacia cover and plant species richness. A linear regression between the species richness of each quadrat and the number of growth rings on the largest Acacia branch showed an average loss of 7.5 species (38% of species) after 10 years of Acacia invasion and 15 species (76%) after 20 years. Long-term invasion favoured just a small group of shade tolerant, rhizomic grasses and sedges. Without active ecosystem management, grassland plant diversity should continue to decline and the nationally threatened herb, Thesium australe, is likely to disappear. An adaptive management program should be implemented promptly to assess the effects of different management regimes on Acacia abundance and plant diversity at ENP.