Diverse Ecological Strategies Increase Invasion Resistance in an Experimental Grassland Restoration

Understanding how the characteristics of native plant communities influence invasion is a pressing question, with implications for theory and management. For decades, the primary native community characteristic used in tests of biotic resistance was species richness. However, previous studies have demonstrated that evolutionary history and functional traits shape the invasion process, as ecological theory predicts. Theoretically, restoration projects would benefit from designing seed mixtures around maximizing resistance to invasion. However, there is little empirical evidence on the importance of evolutionary diversity for management and still less guidance for practitioners on effective application of ecological theories. We empirically tested how several native community characteristics (phylogenetic diversity, functional diversity, phylogenetic relatedness, and mean trait values) affected the survival of three introduced invasive species. We explored this question in experimentally restored 15-species prairie plots with three levels of phylogenetic diversity and two levels of functional diversity. Our experiment also included monocultures of all native species, which were also experimentally invaded. We found evidence that phylogenetic diversity conferred biotic resistance against one invasive species, contributing to reduced biomass in models explaining up to 10% of variance. Tall species better suppressed invaders, with height explaining up to 27% of variation in invader biomass. Surprisingly, we found patterns in leaf and seed traits linked to invasion resistance which were associated with both conservative and resource-acquisitive strategies. We also found evidence in both the diversity and monoculture plots that invaders were more successful with more closely related native species. Taken together, our results indicate that invasion resistance emerges from nuanced interactions between phylogenetic diversity, functional traits, and community composition, rather than from any single community characteristic. Our results underscore the complexity of biotic resistance and suggest that practitioners should prioritize phylogenetic diversity and strategic species selection when designing restoration plantings to enhance invasion resistance.