Dominance can derive from a number of processes ranging from trait-based (born, made) to neutral (stochastic disturbance, propagule pressure), which are difficult to distinguish once dominance has been attained. This is especially true for successful invaders, where trait-based mechanisms are often assumed but sometimes difficult to confirm. Here, I used a global home-and-away study to untangle dominance mechanisms for Dactylis glomerata, an agronomic grass from Eurasia that now occurs worldwide. Dactylis is one of a number of globally pernicious agronomic invasive grasses that threaten the structure and function of native grasslands, with both trait-based and human-mediated neutral mechanisms possibly explaining success. I used common gardens to quantify differences among home and away populations, and between Dactylis and the native species in the invaded ranges (North and South America, New Zealand). I then used pair-wise competition and soil feedback trials to determine the ecological relevance of these differences, if any, for interactions with native species.

Results/Conclusions

The common garden trials found few significant differences between home and away Dactylis populations. Away populations were not polyploids, did not allocate carbon and nitrogen differently in their foliage, were not better at drawing down soil moisture, nor exhibited more ruderal-based life history strategies. Compared to functionally similar native grasses, however, Dactylis home and away populations exhibited significant trait differences relating to faster growth and greater biomass production. Competition trials supported this finding. There were significant differences in performance and competitive ability among away populations, suggesting different introductory histories and local habitat suitability. In conclusion, my work supports trait-based explanations for invader success but with no profound differences in away performance, as would be expected if factors such as enemy escape or polyploidy were prevalent. Instead, the most likely explanation for dominance derives from human activity. The species appears to have been intentionally chosen from its home range because of traits promoting high growth and production, and subsequently introduced en masse globally in association with cultivation. Although some high profile invaders may show substantial differences compared with their home populations, my results indicate that these differences are not necessary for invader success.