Invaders responded more positively to soil biota than native or noninvasive introduced species, consistent with enemy escape

Understanding the mechanisms governing biological invasions has implications for population dynamics, biodiversity, and community assembly. The enemy escape hypothesis posits that escape from enemies such as herbivores and predators that were limiting in the native range helps explain rapid spread in the introduced range. While the enemy escape hypothesis has been widely tested aboveground, data limitations have prevented comparisons of belowground mechanisms for invasive and noninvasive introduced species, which limits our understanding of why only some introduced species become invasive. We assessed the role of soil biota in driving plant invasions in a phylogenetic meta-analysis, incorporating phylogeny in the error structure of the models, and comparing live and sterilized conditioned soils. We found 29 studies and 396 effect size estimates across 103 species that compared live and sterilized soils. We found general positive effects of soil biota for plants (0.099, 95% CI = 0.0266, 0.1714), consistent with a role of soil mutualists. The effect size of soil biota among invaders was 3.2x higher than for natives, the strength of effects was weaker for older conditioning species with a longer introduced history, and enemy escape was stronger for distant relatives. In addition, invasive species had a weaker allocation tradeoff than natives. By demonstrating that the net effect of soil biota is more positive for invasive than native and noninvasive introduced species, weakens over time since introduction, and strengthens as phylogenetic distance increasing, we provide mechanistic insights into the considerable role of soil biota in biological invasions, consistent with the predictions of the enemy escape hypothesis.