An invasive grass species has both local and broad-scale impacts on diversity: Potential mechanisms and implications

Questions: The impact of invasive plant species on native diversity varies with spatial scale, with some invaders leading to broad-scale diversity declines and others to only local declines. These discrepancies may reflect the invaders” capacity to reduce niche opportunities across spatial scales which can be associated with their functional traits. We investigated impact-scale relationships and trait-based mechanisms, in areas invaded by the exotic perennial grass species, Bothriochloa pertusa. We examine root traits specifically, as below-ground competition was considered particularly important to the success of this species. Location: Grassy “ironbark” woodlands of eastern Queensland, Australia. Methods: We examined plots with varying degrees of invasion by B. pertusa, at multiple spatial scales (up to 1,000 m2) and analysed changes to the species–area relationships (SARs) with increasing invader cover. Changes to SARs were assessed in relation to the invaders” effect on rare (low patch occupancy) and common species in the community. In a separate analysis within the same habitat, we collected root cores across a gradient of invader cover and analysed changes to community root traits that were considered important correlates of competition for space and nutrients. Results: Invasion-induced reductions in diversity were pervasive at all scales investigated, and this was associated with a proportionally greater effect on rare species in the community. In the separate root analysis, changes in community root traits with increasing invader cover were potentially indicative of more intense competition for resources rather than space. Conclusions: The observed regional-scale dominance of B. pertusa and associated declines in diversity warrant serious concern for the conservation of native plant communities and species in a region already at risk from other anthropogenic threats. Intense competition for below-ground resources is likely a contributing mechanism to the success of B. pertusa in this study system. Experimental examination of this and other mechanisms would help to validate these findings.