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

Abstract

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.

Linking Darwin’s naturalisation hypothesis and Elton’s diversity–invasibility hypothesis in experimental grassland communities

“…For each trait, the hFDist was calculated as the trait value of the alien individual minus the trait mean of the six native individuals in a pot, and the aFDist was the mean Euclidean trait distance between the alien individual and the six native individuals. We calculated both hFDist and aFDist under the assumption that hFDist may capture competitive inequalities and aFDist may capture niche differences between alien species and native residents (see also Conti et al, ). This is based on the idea that competitive inequalities are directional whereas niche differences are directionless (Chesson, ).…”

Similarities between invaders and native species: Moving past Darwin's naturalization conundrum

“…Regardless of how the differences and dissimilarities between invaders and the resident community are calculated, their role in invasion may be complicated by the fact that invasion is a multistage process (Theoharides & Dukes, ) and different aspects of the plant assembly can impose separate effects on seed establishment and species growth (Bennett, Riibak, Kook, et al., ). Invaders that are phylogenetically distant or functionally dissimilar from the resident community may experience greater biotic resistance when initially establishing, but grow better and have greater competitive effects on the resident community once established (Conti et al., ; Li, Guo, et al., ; Ma et al., ). These results suggest that limiting similarity and niche differences may determine initial establishment, but that competitive hierarchies and fitness differences likely determine invader spread and impacts on the resident community.…”

“…Opposing effects of similarity on invasion success, coupled with reciprocal effects of the invader on the functional structure of the community, make it difficult to infer the relationship between similarity and invasion from observational studies that lack a temporal component (Bennett et al., ; Li, Cadotte, et al., ). It may be possible to identify specific trait relationships that are associated with biotic effects on invader establishment or growth, such as successful invaders being taller or having greater specific leaf area than species in the resident community (Conti et al., ), although many of these relationships are likely to differ as a function of the environment (Carboni et al., ). Observational studies that test how these specific trait relationships differ with the environment may be better able to develop hypotheses about how similarity affects invasion, yet experimental studies would still be required to test the veracity of such hypotheses.…”

“…Many traits could be used to explore the relationship between similarity and invasion, but the important traits are likely study‐specific because the traits important for passing the environmental and biotic filters depend on the local environment and resident community composition (Bennett & Pärtel, ; Carboni et al., ; Conti et al., ; Feng et al., ). As there is growing evidence that invasive species respond to the same ecological processes as native species (Lemoine, Burkepile, & Parker, ), modelling the trait structure within the resident community and species pool may aid in the identification of appropriate traits for models of the similarity‐invasion relationship (Bennett & Pärtel, ; Pearson, Ortega, Eren, & Hierro, ).…”

“…Similarly, trait plasticity may enhance competitive responses if species can shift their root traits to avoid competition belowground or their leaf traits to competitively suppress their neighbors (Bennett, Riibak, Tamme, et al., ). Similar effects of trait plasticity can also affect biotic resistance experienced by invaders (Conti et al., ), although its importance may depend on the characteristics of the invading species (Liao et al., ) and the traits that exhibit plasticity (Godoy, Valladares, & Castro‐Díez, ). Regardless of its importance, incorporating trait plasticity in similarity–invasion models will likely remain challenging unless plasticity is treated as a trait.…”