Neutral interactions among non-native plants are common

Globalization has accelerated the establishment of non-native plant species, increasing the prevalence of multispecies co-invasions; yet interactions among co-invaders remain poorly understood. We combined extensive field surveys of 3039 plots with a global meta-analysis of 4415 observations from 122 experiments to quantify co-invasion prevalence, its effects on non-native plant performance, and the frequency of interaction types. Co-invasion was common in the surveys, occurring in 40% of the plots. Meta-analysis showed that co-invasion reduced non-native plant performance by 18% on average, indicating that invasional meltdown among plant species is not the norm. Most interactions were neutral (70%), whereas competition (18%) and facilitation (12%) were relatively uncommon. Thus, the widespread co-occurrence of non-native plants does not necessarily imply pervasive facilitation among co-invaders. Crucially, the mean competitive outcome was context-dependent. Interactions shifted from competition to facilitation with increasing non-native richness and intensified competition with natives. Furthermore, phylogenetic distance among co-invaders did not explain the interaction outcomes, whereas interactions among woody, nitrogen-fixing species, or invaders with greater SLA produced mean neutral effects. These findings expand our understanding of the invasional meltdown hypothesis by showing that co-invasion outcomes are context- and trait-dependent and offer a predictive framework for assessing and mitigating multispecies invasion risks.