Alien plants alter the growth form ratio and structure of Australian grasslands

Abstract

AbstractQuestionsNiche complementarity is often invoked to explain co‐existence between native and alien plant species in grasslands. However, positive correlations between native and alien plant diversity observed in recent studies could mask the displacement of particular native species and functional groups or the negative effects of particular alien species. We asked: do alien species alter the species composition or proportions of growth forms in grasslands? Do particular alien species decrease native plant diversity?LocationSouth Australian grasslands.MethodsWe performed RDA ordination on growth form abundances or Hellinger‐transformed species abundances obtained from plot‐based surveys, constrained by alien species richness and cumulative cover. Control variables (climate, soil, land use and geographic space) were partialled out. We related individual alien species abundances to native richness, diversity and cover. We tested for functional differences between coexisting growth forms using trait hypervolumes.ResultsWhile alien richness and cover explained just 2% of variance in native species composition (control variables 17%), aliens explained 18% of variance in native growth form abundance (control variables 33%), and were associated with increased herb and grass cover. Few individual alien species were associated with strong negative or positive differences in native richness, diversity or cover: correlations followed a Gaussian distribution with near‐zero mean. Trait hypervolumes differed between native and alien herbs with an overlap of 0.44, indicating substantial, but not complete, functional differences.ConclusionsGiven environmental context, alien cover was a good predictor of native herb and grass abundance relative to woody growth forms, but not of species composition per se. While the direction of causality is equivocal, aliens may facilitate native grasses and herbs, and niche complementarity may be involved. Due to functional redundancy across the species pool, the resulting species composition appears to be spatially contingent. Importantly for management, we identify alien species associated with reduced native diversity.