Early emergence and resource availability can competitively favour natives over a functionally similar invader

Invasive plant species can form dense populations across large tracts of land. Based on these observations of dominance, invaders are often described as competitively superior, despite little direct evidence of competitive interactions with natives. The few studies that have measured competitive interactions have tended to compare an invader to natives that are unlikely to be strong competitors because they are functionally different. In this study, we measured competitive interactions among an invasive grass and two Australian native grasses that are functionally similar and widely distributed. We conducted a pair-wise glasshouse experiment, where we manipulated both biotic factors (timing of establishment, neighbour identity and density) and abiotic factors (nutrients and timing of water supply). We found that the invader significantly suppressed the performance of the natives; but its suppression ability was contingent on resource levels, with pulsed water/low nutrients or continuous watering reducing its competitive effects. The native grasses were able to suppress the performance of the invader when given a 3-week head-start, suggesting the invader may be incapable of establishing unless it emerges first, including in its own understorey. These findings provide insight for restoration, as the competitive effect of a functionally similar invader may be reduced by altering abiotic and biotic conditions in favour of natives.