Biodiversity impacts of Chilean needle grass Nassella neesiana on Australia’s indigenous grasslands

The exotic invasive Chilean needle grass Nassella. neesiana (Trin. & Rupr.) Barkworth has\ud been recognised as a major threat to biodiversity in the endangered natural grasslands of\ud south-eastern Australia. Research was undertaken in Canberra and Melbourne grasslands to\ud examine mechanisms by which N. neesiana invades native grasslands, the impacts of\ud invasion on vascular plant and invertebrate biodiversity and mechanisms of impact. Time\ud series analysis of historical aerial photographs demonstrated that invasion was absent or very\ud slow when infestations abutted healthy grassland, but grasslands in poor condition\ud experienced linear infestation expansion rates >5 m per year. A field experiment\ud demonstrated that major disturbance (death of the native grasses) was required for N.\ud neesiana recruitment from panicle seeds and that intact grassland was not invaded, even with\ud high propagule pressure. Gaps of c. 1 m (as opposed to 10-30 cm) were required for\ud establishment. Pin transect sampling demonstrated that increased senescence of Themeda\ud triandra swards correlated with greater invasion. Nassella neesiana was found to deplete soil\ud water in spring compared to T. triandra, a likely mechanism of biodiversity impact. Areas\ud occupied by N. neesiana in three disparate grasslands were found to have significantly\ud reduced native vascular plant diversity (species m-2). Diversity decreased with increasing\ud size of the N. neesiana patch. Forbs were the most affected group but one or more dominant\ud grasses were absent at the smallest patch sizes. Exotic plant diversity was similar inside and\ud outside patches. Analysis of sweep net samples determined that invertebrate populations and\ud species richness were significantly reduced in N. neesiana grassland, although numerous\ud native insect species consume the plant. Much of the loss of diversity in invaded areas\ud probably precedes invasion and is caused by anthropogenic disturbances including T.\ud triandra senescence dieback, mowing and major soil disturbance. Native grasslands in good\ud condition are resistant to invasion