Chilean needle grass (Nassella neesiana) in the native grasslands of south-eastern Australia: biodiversity effects, invasion drivers and impact mechanisms.

A study of the biodiversity impacts of Chilean needle grass Nassella neesiana (Trin. & Rupr.) Barkworth in temperate native grasslands of south-eastern Australia shows that native vascular plant richness (species m 2) is reduced in infested areas and decreases with increasing patch size, with forbs and one or more of the major native grasses most affected. Exotic plant species richness is similar inside and outside patches. There are also reductions in invertebrate species richness and population sizes. Major disturbance (removal/death of the dominant native grasses) enables N. neesiana invasion. Gaps of c. 1 m enable establishment. Native grasslands in gapfree condition are resistant to invasion. In areas with N. neesiana propagule pressure, increased senescence of Themeda triandra swards (due to absence of fire or grazing) is accompanied by invasion. N. neesiana depletes soil water in spring more than T. triandra, a mechanism that may explain the ongoing losses of native species. Historical aerial photos demonstrate that invasion is absent or very slow unless grasslands are in poor ecological condition, when linear expansion rates >5 m per year can be expected. Much of the plant diversity loss in invaded areas probably precedes invasion and is caused by degradation including T. triandra dieback, mowing and major soil disturbance.