Invasive plant richness and abundance synergistically amplify co-invasion success: Experimental evidence for invasional meltdown

Co-invasion by multiple invasive species is becoming increasingly prevalent, yet the role of interactions among invasive species in shaping invasion success remains poorly understood. While invader abundance is known to facilitate establishment, the combined effects of invasive plant richness and abundance on invasion outcomes have rarely been tested experimentally. We established a fully factorial mesocosm experiment manipulating four levels of invasive plant richness (0, 1, 2, or 3 species) and abundance (0, 12, 24, 48 individuals) using three Asteraceae invaders—Erigeron canadensis, Erigeron annuus, and Bidens pilosa—within native plant communities. We found facilitative and neutral interactions among invaders generated non-additive (reversal) negative effects on native biomass, which intensified with increasing invader richness. Higher richness facilitated invasion success by modifying selection effects among invasive species—primarily through size-mediated mechanisms—that enhanced invasive biomass and strengthened suppression of native plants. Invasive richness altered soil nitrogen pools without promoting invasion success, but was associated with changes in soil microbial biomass carbon and litter decomposition that coincided with reduced native performance. Elevated invader abundance further promoted invasion success and interacted synergistically with richness to amplify invasion impacts, indicating that co-invasion severity may intensify under future scenarios of increasing propagule pressure. Overall, our findings provide experimental evidence for the invasional meltdown hypothesis and demonstrate how invasive plant richness and abundance jointly enhance invasion success. Our results further suggest that early management targeting low-abundance invaders may disrupt the synergistic effect between invasive plant richness and abundance and mitigate the impacts of multispecies co-invasion.