Functional trait differences and trait plasticity mediate biotic resistance to potential plant invaders

Biotic resistance represents an important natural barrier to potential invaders throughout the world, yet the underlying mechanisms that drive such resistance are still debated. In theory, native communities should repel both functionally similar invaders which compete for the same resources, and invaders which possess less competitive traits. However, environmental stress, trade-offs across vital rates and competition-induced plastic trait shifts may modify expected competitive outcomes, thereby influencing invasion dynamics. In order to test these theoretical links between trait distributions and biotic resistance, we performed a mesocosm experiment with 25 non-native ornamental species invading native plant communities. Each non-native species was grown with and without the native community under two watering treatments (regular and reduced). We measured biotic resistance as the difference in performance of non-native individuals grown with and without the community in terms of their survival, growth and reproduction. We quantified overall functional dissimilarity between non-native ornamental individuals and native communities based on the combination of plant height, specific leaf area and seed mass. Then, assuming each of these traits is also potentially linked to competitive ability, we measured the position of non-natives on trait hierarchies. While height is positively correlated with competitive ability for light interception, conservative leaf and seed characteristics provide greater tolerance to competition for other resources. Finally, we quantified plastic trait shifts of non-native individuals induced by competition. Indeed, the native community repelled functionally similar individuals by lowering the invader's survival rate. Simultaneously, shorter ornamental individuals with larger specific leaf areas were less tolerant to biotic resistance from the community across vital rates, although the effect of trait hierarchies often depended on watering conditions. Finally, non-natives responded to competition by shifting their traits. Most importantly, individuals with more competitive traits were able to overcome biotic resistance also through competition-induced plastic trait shifts. Synthesis. Our results highlight that both functional dissimilarity and trait hierarchies mediate biotic resistance to ornamental plant invaders. Nevertheless, environmental stress as well as opposing trends across vital rates are also influential. Furthermore, plastic trait shifts can reinforce potential invaders’ competitive superiority, determining a positive feedback.