Exploring the potential for climatic factors, herbivory, and co-occurring vegetation to shape performance in native and introduced populations of Verbascum thapsus

Abstract

Biogeographic data describing performance differences in native and introduced populations of invasive species are increasingly coming to light, revealing that introduced populations often perform better than their native conspecifics. However, this pattern is not universal, nor is it well studied in species that fall on the more “benign” end of the invasion spectrum. Furthermore, performance data are infrequently linked with variation in key environmental factors experienced by populations in each range, making it difficult to assess which factors are typically associated with shifts in performance. Here we assessed performance in native and introduced populations of Verbascum thapsus (common mullein), an herbaceous biennial that was initially introduced to the eastern US from Europe, but which has subsequently expanded its range into semi-arid mountainous regions of the western US, where it appears to be more problematic. Indeed, we found that introduced populations were larger than native populations, with over half of them comprising more than 500 individuals, a size seldom achieved in the native range. We further explored the role that abiotic factors (latitude, elevation, and precipitation) might serve in shaping performance in European and western US populations, and quantified variation in two biotic factors relevant to invasion: herbivory, and the potential for competition from co-occurring vegetation (as well as its inverse, the availability of bare ground). When the influence of abiotic factors was not considered, introduced mullein performed better than native mullein in terms of plant density and plant size (i.e., number of leaves and area covered by the basal rosette). When the influence of abiotic factors was statistically taken into account, the difference in the density of native and introduced populations remained strong, while the difference in number of leaves was reduced, though it remained significant. In contrast, controlling for abiotic factors reversed the pattern for plant area such that plants in introduced populations performed less well than natives. These results suggest that the difference in climate experienced by native and introduced populations is an important driver of mullein performance only for plant area. Thus, increased performance in western US population likely hinges in part on shifts in biotic factors. Indeed, we found a reduction in the prevalence of several herbivore guilds on introduced relative to native mullein, accompanied by a significant decrease in chewing damage in introduced populations. We also found differences in the potential for competition: cover of vegetation is significantly higher in native mullein populations than in introduced populations, and increasing cover of vegetation is associated with declining performance (i.e., density) of native populations but not introduced populations. In sum, the introduced populations performed better than the native populations in several respects; thus, although mullein is considered a relatively ‘benign’ introduced species, it has the potential to differentially impact resident communities in its native and introduced ranges. Additionally, despite the disparity in abiotic conditions experienced by native and introduced populations, these factors do not appear to consistently drive differences in performance. Instead, evidence suggests that enemy escape and shifts in the competitive regime may facilitate mullein invasion. We use our data to propose hypotheses to be tested experimentally.