Field-based competitive impacts between invaders and natives at varying resource supply

1. One line of thinking is that highly successful plant invaders achieve dominance in recipient communities via competitive superiority over natives. In contrast, it has been proposed that exotic species attain dominance not by competitive prowess but due to their colonizing abilities after disturbance. Interestingly, there have been relatively few attempts to quantify competitive effects of invaders on natives (and vice versa ) in field settings. 2. We created monocultures of ten native perennial species and after two seasons of establishment we either left assemblages uninvaded or invaded them with seeds from one of three potent exotics; spotted knapweed ( Centaurea maculosa ), Dalmatian toadflax ( Lineria dalmatica ) and sulfur cinquefoil ( Potentilla recta ). Invasion was crossed with a supplemental water treatment to determine how increased resource supply might alter competitive outcomes. We also compared traits of exotics and natives to determine whether differences in height, lateral spread, shoot : root ratio and specific leaf area might shed light on competitive effects. 3. Exotics suppressed native biomass by an average of 51%. Water addition had no effect on invasibility or the competitive impact of exotics on natives. Although exotics decreased native biomass, invasion increased total above-ground biomass. Spotted knapweed was the most successful invader, and because of its greater abundance, it had the largest competitive impacts on natives. Spotted knapweed invasion also reduced plant-available soil nitrogen, water and light. 4. The majority of native species did not differ in their resistance to weed invasion, although Achillea millefolium and Festuca idahoensis monocultures were more resistant to invasion than monocultures of Antennaria rosea . Exotic biomass was either unaffected by competition with natives (knapweed) or was moderately suppressed by natives (cinquefoil and toadflax). Native species did not differ significantly in their competitive impacts on exotics and resource supply only influenced the competitive impacts of natives on spotted knapweed. Knapweed and toadflax, in particular, differed from all the natives in combined growth and leaf traits. 5. Synthesis . Our results show that exotics exert strong competitive dominance over individual native species with competitive effects relatively unaltered by increased resource supply. These effects occurred in undisturbed assemblages, suggesting that invader-native competitive outcomes can drive invasion dynamics for the species studied.