Seed density and priority effects outweigh limiting similarity and diversity in contributing biotic resistance to invasion

Despite numerous ecological theories proposed to explain plant invasion dynamics, few studies have evaluated multiple mechanisms simultaneously within a single experimental framework. We conducted a greenhouse pot experiment to quantify the relative importance of five ecological factors – limiting similarity, diversity effects, priority effects, seed density, and soil carbon enrichment – on the invasion success of Hypochaeris radicata . Our experimental design tested competitive interactions between simultaneously establishing species rather than resistance from mature communities, representing realistic post-disturbance or post-control scenarios. Nine native species and five species mixtures were grown with the invader under controlled conditions, with treatments manipulating functional similarity (three functional groups based on longevity and woodiness), diversity (monocultures vs. four-species mixtures), arrival timing (3-week intervals), seed density (native:invasive ratios from 1:1 to 10:1), and soil carbon (sawdust amendments). Using structural equation modeling (SEM), we found that seed density ( β = 0.487) and priority effects ( β = 0.427) were the strongest determinants of invasion resistance. While functional similarity ( β = 0.115) and diversity ( β = 0.240) significantly contributed to invasion resistance, these effects were substantially weaker than those of seed density and priority effects. Soil carbon enrichment showed only marginal effects on invasion resistance ( β = −0.081). Together, these factors explained 56.6% of the variation in competitive resistance. Our findings challenge the conventional emphasis on limiting similarity and diversity in restoring native communities resistant to invasion. Ecological restoration strategies should prioritize early establishment of natives at high seed densities rather than focusing primarily on functional trait matching or diversity enhancement.