Invasive success of exotic wild oat depends on nutrient availability and competition in temperate grasslands of southern Australia.
Aims: Invasion by exotic species commonly influences the structure and diversity of natural grasslands particularly in fertile soils. This study examined the effect of resource availability on intra- and interspecific competition between a native and an exotic grass, and provides mechanistic explanations for the successful invasion of exotic species in temperate grasslands of southern Australia. Methods: Frequently co-occurring exotic Avena barbata (wild oat) and native Rytidosperma caespitosum (wallaby grass) of temperate grasslands in southern Australia were grown with and without competition across a soil nutrient and moisture gradient in a glasshouse experiment. Results: Wallaby grass and wild oat showed similar growth responses to soil nutrient levels: both performed better at lower levels. Intra- and interspecific competition significantly reduced plant biomass and relative growth rate, but their effects depended on resource availability. Higher soil nutrient availability promoted the performance of the exotic grass and strengthened its competitive advantage over the native grass because of their different responses to the interaction of competition, soil nutrient and moisture. Moreover, changes in relative competition intensity suggested wallaby grass experienced stronger suppression from interspecific competition than wild oat in mixture which led to its decreasing abundance. Conclusions: Nutrient accumulation due to management for grazing combined with high rainfall during the wet season can reduce the interspecific competitive ability of native grass and favors exotic invasion in temperate grasslands of southern Australia. The ongoing climate changes may dramatically increase wild oat's prevalence and pose a great challenge on the restoration of native temperate grasslands in Australia.