Neighbour tolerance, not suppression, provides competitive advantage to non‐native plants

Abstract

High competitive ability has often been invoked as a key determinant of invasion success and ecological impacts of non-native plants. Yet our understanding of the strategies that non-natives use to gain competitive dominance remains limited. Particularly, it remains unknown whether the two non-mutually exclusive competitive strategies, neighbour suppression and neighbour tolerance, are equally important for the competitive advantage of non-native plants. Here, we analyse data from 192 peer-reviewed studies on pairwise plant competition within a Bayesian multilevel meta-analytic framework and show that non-native plants outperform their native counterparts due to high tolerance of competition, as opposed to strong suppressive ability. Competitive tolerance ability of non-native plants was driven by neighbour's origin and was expressed in response to a heterospecific native but not heterospecific non-native neighbour. In contrast to natives, non-native species were not more suppressed by hetero- vs. conspecific neighbours, which was partially due to higher intensity of intraspecific competition among non-natives. Heterogeneity in the data was primarily associated with methodological differences among studies and not with phylogenetic relatedness among species. Altogether, our synthesis demonstrates that non-native plants are competitively distinct from native plants and challenges the common notion that neighbour suppression is the primary strategy for plant invasion success.

Common alien plants are more competitive than rare natives but not than common natives

“…On average, alien species were not more competitive than native species. Therefore, our results contradict those of two recent meta‐analyses which found that alien species were more competitive than native species (Kuebbing & Nunez ; Golivets & Wallin ). However, in the two meta‐analyses, approximately 90% of the alien species were invasive plants.…”

“…In contrast to the results of two recent meta‐analyses, which found that alien species experienced weaker strength of interspecific competition than native species (Kuebbing & Nunez ; Golivets & Wallin ), we found no significant difference in interspecific competition experienced by native and alien species. Possibly, although we measured interspecific competition with a more accurate approach (i.e.…”

“…More importantly, our results suggest that, although high intrinsic growth rates would sometimes come at the cost of strong intraspecific competition, they are the major driver of high competitive ability of common alien species, and thus might play an essential role in both invasion success and rarity. Finally, most pairwise competition experiments aimed to determine the strength of interspecific competition (Kuebbing & Nunez ; Golivets & Wallin ), which is insufficient to predict competitive outcomes as suggested by our results. Therefore, experiments examining intrinsic growth rates and intraspecific competition are urgently needed if we are to predict coexistence between native and alien species more definitively.…”

“…A primary hypothesis is that alien plant species are more competitive than native species (Baker ). Three meta‐analyses on hundreds of experiments support this hypothesis (Vilà & Weiner ; Kuebbing & Nunez ; Golivets & Wallin ). However, not all alien species become dominant (Williamson & Fitter ), and not all native species are vulnerable to competitive exclusion (Levine et al ).…”

“…It is often used as a surrogate for competitive outcomes (Gibson et al ). For example, Golivets & Wallin () and Kuebbing & Nunez () synthesized studies on the strength of interspecific competition. They concluded that alien species were more competitive than native species.…”