Do priority effects benefit invasive plants more than native plants? An experiment with six grassland species

Invasive, non-native plant species often outcompete native species and reduce biodiversity. Invasive plants frequently begin growth before native plants, yet few studies have examined whether invasives win in competition partly by colonizing disturbed sites more quickly or by beginning growth earlier in the season than native plants (i.e. due to priority effects). We hypothesized that invasive plant species would benefit more from priority effects than would comparable native species and that earlier growth of invasive species would decrease plant biodiversity. To test this hypothesis, we grew three pairs of invasive and native plant species from three different functional groups/plant families (C-3 grasses/Poaceae, non-leguminous forbs/Asteraceae, and legumes/Fabaceae). We seeded each of the species 3 weeks before seeding the other five species into large pots in a greenhouse. Consistent with our hypothesis, we found much stronger priority effects with invasive than native species. Each invasive species formed a near-monocultures when seeded first (97.5 % of total biomass, on average) whereas native species did not similarly dominate (29.8 % of total biomass, on average). Similarly, Simpson's species diversity was 81 % higher when the initially sown species was native rather than invasive. The literature suggests that invasive species in the field often begin growth earlier in the spring than native species and that climate change may increasingly allow invasives to begin growth before native species, indicating invasive priority effects may become increasingly common.