Competitive ability, not tolerance, may explain success of invasive plants over natives

Abstract

When entering a new community, introduced species leave behind members of their native community while simultaneously forming novel biotic interactions. Escape from enemies during the process of introduction has long been hypothesized to drive the increased performance of invasive species. However, recent studies and quantitative syntheses find that invaders often receive similar, or even more, damage from enemies than do native species. Therefore, invasives may be those more tolerant to enemy damage, or those able to maintain competitive ability in light of enemy damage. Here, we investigate whether tolerance and competitive ability could contribute to invasive plant success. We determined whether invasive plants were more competitive than native or noninvasive exotic species in both the presence and absence of simulated herbivory. We found competition and herbivory additively reduced individual performance, and affected the performance of native, invasive, and noninvasive exotic species’ to the same degree. However, invasives exerted stronger competitive effects on an abundant native species (Elymus canadensis) in both the presence and absence of herbivory. Therefore, while invasive species responded similarly to competition and simulated herbivory, their competitive effects on natives may contribute to their success in their introduced range.

Appendix: Flower number analysis

Methods

At the end of the experiment we measured plant performance metrics, including height (cm) from the soil surface to apical meristem, aboveground biomass (g), and flower number. To determine whether our treatments influenced plant performance, we tested the effects of simulated herbivory and competition on plant biomass and height with mixed model ANOVA using the lmer function, and flower number with the glmer function, in the lme4 package in R (v. 1.1-7, Bates et al. 2015). To test treatment effects, our model included plant biomass (g), plant height (cm), or flower number as the response variable and clipping (clipped, unclipped), competition (competitor present, absent), status (native, noninvasive exotic, invasive), family (Asteraceae, Poaceae, Fabaceae), and all possible interactions as fixed predictor variables.

Flower number data was analyzed using the Poisson distribution, and because only a small number of individuals flowered during the course of the experiment, we analyzed only data for those individuals and species that flowered. To test significance fixed and random effects for flower number, we used Chi squared tests.

Results

No native species flowered during the experiment (Fig. 4a), and only noninvasive exotic Centaurea cyanus, Sonchus oleraceus, and Bromus hordeaceus, and invasive Lotus corniculatus, Melilotus officinalis, and Poa compressa flowered; only one individual of M. officinalis and P. compressa produced any flowers. Flower number depended on the interaction between status, clipping, and the competition treatment (Table 4); invasive species in unclipped competition pots produced significantly more flowers than did exotic species where either competition or clipping treatments were applied (Fig. 4a). This pattern was driven by invasive L. corniculatus, which produced significantly more flowers when grown in competition and without clipping compared to the control (Fig. 4b).

Fig. 4

Flower number data for native, noninvasive exotic, and invasive plants that flowered during the course of the experiment. Graph a displays data by status, while graph b displays data by species. Different colored bars represent the clipping and competition treatments. Bars indicate mean ± SE. Means with different letters are significantly different (P ≤ 0.05) based on post hoc contrasts

Table 4 Results from mixed model analysis of variance (ANOVA) showing the effects of status, family, clipping, and competition on experimental plant flower number