High native species diversity generally suppresses non-native invasions, but many ecosystems are now characterized by non-native assemblages that vary in species diversity. How this non-native species diversity affects subsequent invaders and its environmental dependence remain unclear. Here, we conducted a plant-soil feedback experiment to investigate how non-native plant species diversity affects the growth of subsequent non-native plants, the role of soil microbes in this process, and the dependence of these patterns on drought. We found that under well-watered conditions, the biomass of subsequent invaders was higher with soil inocula generated by high non-native diversity, which was associated with higher arbuscular mycorrhizal fungal richness. However, under drought conditions, the biomass of subsequent invaders did not depend on soil inocula generated by non-native diversity. Our study reveals soil microbial legacies likely contribute to the commonly observed co-occurrence of multiple non-native species in nature and the importance of environmental conditions for these effects.

Non-native plants are typically released from specialist enemies in new ranges, but continue to be attacked by generalists, but whether they shift relative allocation to constitutive or induced defenses is unknown. We compared herbivory on co-occurring native and non-native species and also constitutive and induced defenses. Non-natives suffered less damage than natives and constitutive defenses of non-natives was lower than that of native congeners, whereas induced defense was the opposite. The strength of constitutive defenses for a species was correlated with the intensity of herbivory experienced, for non-natives, whereas induced defenses showed the reverse. The defenses of natives were not related to herbivory pressure. Finally, the strength of induced defenses correlated positively with growth, suggesting a novel mechanism for the evolution of increased competitive ability. These results expand our understanding of fundamental tradeoffs in constitutive and induced defenses and provide novel insight into how herbivory pressure affects defense allocation.