Divergence among species with "good competitor" and "good cultivator" strategies promotes asymmetric facilitation among co-invaders.

Abstract

Facilitative interactions among co-invaders may lead to invasional meltdown, accelerating non-native species accumulation and exacerbating ecological impacts over time. However, it remains unclear why certain non-native combinations promote facilitation while others do not, and may even constrain invasions. To address this question, we examine six invasive species in the Asteraceae family along two strategic dimensions: competitiveness and capacity to cultivate invader-promoting microbial communities. We then create experimental combinations to mix "good competitors" and "good cultivators" to varying degrees to form a "strategic divergence" gradient. We hypothesize greater strategic divergences generate more intense facilitations, whereas similar strategies generate inhibitions. Strategic divergence correlates with facilitation, but interactions are asymmetric: strong competitive suppressors of natives benefit from co-invasions with weaker competitors that cultivate favorable microbial environments but the performance of the latter are generally suppressed by the strong competitors. Metagenomic sequencing further indicates that good cultivators may promote facilitation by repelling pathogens (Ascomycota) and deterring microbes that might be exclusively beneficial for natives (Proteobacteria, Firmicutes, and Planctomycetota). Our results provide empirical evidence for the importance of strategic divergence among invasive species and offer a mechanistic basis for predicting which combinations of co-invading species might generate facilitation and which might result in inhibition.