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

Invasion of natural ecosystems by weeds is often associated with increasing levels of disturbance. However, there is substantial evidence that cessati on of disturbance can also lead to plant invasions, especially of woody spe cies. In some coastal grasslands in south-eastern Australia, removal of gra zing and burning appears to have triggered invasion by the indigenous shrub , Acacia sophorae. To quantify the impacts of such invasions on plant diver sity, 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 m ost rapid expansion from the late 1970s when grazing was removed. Vegetatio n sampling demonstrated a strong negative correlation between Acacia cover and plant species richness. A linear regression between the species richnes s of each quadrat and the number of growth rings on the largest. Acacia bra nch showed an average loss of 7.5 species (38% of species) after 10 years o f Acacia invasion and 15 species (76%) after 20 years. Long-term invasion f avoured just a small group of shade tolerant, rhizomic grasses and sedges. Without active ecosystem management, grassland plant diversity should conti nue to decline and the nationally threatened herb, Thesium australe, is lik ely to disappear. An adaptive management program should be implemented prom ptly to assess the effects of different management regimes on Acacia abunda nce and plant diversity at ENP. (C) 2000 Elsevier Science Ltd. All rights r eserved.