Are problem invaders bigger and more fecund in the introduced versus native range?

Background/Question/Methods

Plant invasions have detrimental effects on ecosystem structure and function, and are expensive to manage.  Roughly one in ten exotic plants becomes invasive, but the underlying mechanisms that contribute to invasiveness are not well known. The evolution of enhanced competitive ability hypotheses (EICA) predicts that the escape from enemies allows for a greater allocation to growth, reproduction and competitive ability in the introduced range. In an effort to better understand the relationship between plant size and fecundity and invasive success, we compared growth and reproduction of six plant species in their native (Turkey) and exotic (Montana, USA) ranges.  Three species were chosen (Bromus tectorum, Hypericum perforatum, Potentilla recta) that represent high-impact invaders recognized as noxious weeds in Montana, while the other three (Carduus nutans, Poa bulbosa, and Rumex acetosella) were chosen because they are not recognized as problematic species in the area.  Five to ten replicate populations for each species were collected in each range for analysis of shoot biomass and seed production.   We predicted based on EICA that successful invaders would be bigger and produce more seeds in the introduced range relative exotic plants that fail to invade. 

Results/Conclusions

Of the problem invaders, B. tectorum grew larger and produced more seeds in Montana, while H. perforatum and P. recta grew larger in Turkey.  Conversely, of the species that are not problematic in the introduced range, C. nutans grew larger and produced more viable seeds in Montana, P. bulbosa did not differ in size but produced more seeds in Montana, and R. acetosella showed no difference in biomass or seed production.  For our six species, we found no consistent relationship between invader impact and plant size or fecundity between ranges.  Only one high impact species showed evidence of increased growth or fecundity, while two low-impact species were larger and or more fecund in the introduced range.  EICA might help to explain invasion success of some species, possibly including cheatgrass, but it does not appear to provide a general hypothesis for explaining invasion outcomes.

*Co-authors contributed equally and the results presented here are part of their theses.