Abstract
Invasive non-native plants challenge ecosystems restoration, and understanding the factors that determine the establishment of invasive plants is crucial to improve restoration outcomes. However, the drivers of invasibility of plant communities are not sufficiently clear, and combined effects are not understood. Therefore, we investigated the contribution of the main drivers of invasion success during early phases of restoration, i.e., biotic resistance, invasive propagule pressure, and environmental fluctuations. We compared the contribution of these drivers in a series of mesocosms experiments using designed grasslands as a model system, and Solidago gigantea as invasive model species. Two grassland communities were designed according to competitive trait hierarchies with different sowing patterns, reflecting variation in biotic resistance. We then manipulated invader propagule pressure and applied different scenarios of environmental fluctuation, i.e., flood, heat, and N fertilization. Invasive biomass was considered as proxy for invasion success, while native biomass represented restoration success. There were consistent effects of biotic resistance to S. gigantea invasion via competitive trait hierarchies in the three experiments. Communities dominated by species with high-competition traits were more resistant regardless of environmental fluctuation. Clumped seeding of the native community reduced invasibility, whereas high non-native propagule density increased invasion. The effects of environmental fluctuation were less consistent and context-dependent, thus playing a secondary role when compared to biotic drivers of invasion. Restoration initiatives on grasslands impacted by invasive plants should consider biotic resistance of the restored community as a key driver and the importance of controlling further arrivals of invasive species during community assembly.
Highlights
Given the negative impact of invasive non-native plants on degraded ecosystems, restoration activities that constrain the spread of such species and promote the colonization of native ones are increasingly needed (Bakker and Wilson 2004)
We found strong evidence of biotic resistance conferred by trait hierarchies when the propagule pressure of the invasive species was manipulated, in addition to environmental fluctuations
We showed that biotic resistance based on hierarchies of traits related to competition has a consistent effect on invasion outcomes in early grassland establishment
Summary
Given the negative impact of invasive non-native plants on degraded ecosystems, restoration activities that constrain the spread of such species and promote the colonization of native ones are increasingly needed (Bakker and Wilson 2004). In the case of grasslands, where the design and reintroduction of native seed mixtures is common restoration practice (Kiehl et al 2010), most evidence shows that revegetation can decrease the performance of invasive plants, the magnitude of such effect is variable (Schuster et al 2018). The biotic resistance of a resident community implies a reduction of invasibility mainly through competition (Levine et al 2004). There is little evidence that species interactions can repel invaders completely, some resident communities reduce the establishment, abundance, and fitness of invaders to a certain extent (Levine et al 2004). The understanding of biotic resistance as driven by interspecific competitive interactions has been addressed by various hypotheses, based on diversity effects and the saturation of niche space (Elton 1958; Hooper et al 2005), limiting trait similarity (Funk et al 2008), phylogenetic relatedness (Yannelli et al 2017a), or trait hierarchies, with the focus on the identity and value of fitness-related traits that define which species are most likely to invade successfully (Kunstler et al 2012)
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