Abstract
Our understanding of the potential mechanisms driving the spread and naturalization of alien plant species has increased over the past decades, but specific knowledge on the factors contributing to their increased impact in the introduced range is still urgently needed. The native European plant Centaurea stoebe occurs as two cytotypes with different life histories (monocarpic diploids, allo-polycarpic tetraploids). However, only tetraploids have been found in its introduced range in North America, where C. stoebe has become a most prominent plant invader. Here, we focus on the ploidy level of C. stoebe and origin of neighbouring community in explaining the high impact during the invasion of new sites in the introduced range. We conducted a mesocosm experiment under open-field conditions with the diploid (EU2x) and tetraploid (EU4x) cytotype of Centaurea stoebe from its native European (EU) range, and with the invasive tetraploid (NA4x) cytotype from the introduced North American (NA) range in competition with EU (old) or NA (new) neighbouring plant communities. In the presence of competition, the biomass of EU neighbouring community was reduced to a comparable level by all three geo-cytotypes of C. stoebe. In contrast, the biomass of the NA neighbouring community was reduced beyond when competing with tetraploid, but not with diploid C. stoebe. The fact that the biomass of all three geo-cytotypes of C. stoebe was correlated with the biomass of the EU neighbouring community, but not with that of the NA neighbouring community suggests that different mechanisms underlie the competitive interactions between C. stoebe and its old vs. new neighbouring communities, such as competition for the same limiting resources at home vs competition through novel allelo-chemicals or differential resource uptake strategies in the introduced range. We therefore caution to simply use the ecosystem impact assessed at home to predict impact in the introduced range.
Highlights
Biological invasions are among the most significant components of current global environmental change, causing enormous economic and ecological losses [1, 2]
Increased impact has been explained by the evolution of increased competitive ability (EICA) hypothesis, which posits that invasive species might divert available energy and resources to compete with naïve neighbours when they arrive in the introduced range without their enemies [22]
The biomass of EU2x C. stoebe was lower than that of EU4x and NA4x C. stoebe (Tukey’s Honest Significant Difference (HSD), P = 0.002 & P = 0.01, respectively), while no difference was found between the two 4x C. stoebe (Tukey’s HSD, P = 0.39; Fig 2b)
Summary
Biological invasions are among the most significant components of current global environmental change, causing enormous economic and ecological losses [1, 2]. Hahn et al [37] found earlier flowering and increased seed mass in North American tetraploids compared with European tetraploids and higher seedling emergence of tetraploid NA compared to tetraploid EU C. stoebe Those results have been explained as a trade-off between growth and defence traits (evolution of increased competitive ability (EICA)-hypothesis) [30], suggesting post-introduction evolution in introduced tetraploid populations. We tested (1) whether the tetraploid C. stoebe are more competitive than the diploid C. stoebe, irrespective of the origin of the tetraploids and the origin of the neighbours (evidence for an inherently higher competitive ability, i.e. pre-adaptation), (2) whether tetraploid C. stoebe are competitive when growing with a species mixture consisting of new, naïve plant species from the introduced range (evidence for a context-dependent preadaptation) or (3) whether the NA tetraploids reveal a higher impact on resident communities in general or the NA community than the EU tetraploids (evidence for post-introduction evolution). Based on findings from an earlier pairwise competition study between tetraploid C. stoebe and individual EU vs. NA neighbours [38], we assessed (4) whether the two neighbouring communities vary in their relationship between the biomass of C. stoebe and that of the neighbouring communities, and (5) whether this relationship differs among the three geo-cytotypes
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