Non-native plant species are known to often outcompete native plants.  However, relatively little work has been completed to examine whether non-native plants may win in competition partly by colonizing disturbed sites more quickly than native plants or by beginning growth earlier in the season than native plants.  We hypothesized that non-native plants would benefit more from a priority effect than would native plants.  To test this hypothesis, we grew three non-native plant species (Trifolium pratense, Leucanthemum vulgare, and Dactylis glomerata) in combination with three native species that were closely related to the non-native species (Dalea purpurea, Ratibida pinnata, and Elymus canadensis, respectively).  In separate treatments we seeded each of the species three weeks before seeding the other five species in large pots in a greenhouse.  We also implemented one “control” treatment where all six species were seeded at the same time.   

Results/Conclusions

Consistent with our hypothesis, we found that non-native species sown before others produced over four times as much biomass as native species sown before others.  Each of the non-native species sown before others produced significantly more biomass than any native species, and the non-native species sown before others produced 97.6% of total pot biomass on average while the native species sown before others produced only 30.3%.  Even though native species did not produce a great deal of biomass when grown before others, they did reduce the biomass of non-native species by 45.8% relative to when all species were sown at the same time.  These results suggest that non-native species may win in competition partly by being able to grow quickly and take advantage of any priority effects which may occur.  In our study, non-native species germinated slightly more quickly than native species, but the differences in germination time were generally not dramatic.  However, we found a strong negative relationship between the day of germination and the weight of an individual at the end of the experiment.  This relationship was much more negative for non-native species than native species, which provides additional evidence for the importance of priority effects for non-native species.  In conclusion, our results show that non-native species have the potential for a strong benefit due to priority effects.  Further study will need to be completed in the field to examine how commonly non-native species germinate and grow before native species.