Dispersal among patches is considered to be one of the major forces behind community assembly, linking communities into metacommunities. However, dispersal ability varies among species and can be highly context dependent. We compared dispersal ability between 18 native and 18 exotic plant species growing in monocultures and mixtures. Using an ongoing experiment comparing diverse native state plots to low diversity exotic species plots, we conducted seed additions to test whether these states were stable. Half of the plots received a seed mix, and half served as controls. Mixture plots received seeds with a factorial arrangement of seed mix (none or 18 species) x species dominance (native vs. exotic) x summer irrigation (128 mm, 0 mm). Monoculture plots received their corresponding species pair (native into exotic and vice versa) x summer irrigation. Seedling numbers were counted as they emerged over two growing seasons, and data were analyzed with generalized linear models with a negative binomial distribution.
Results/Conclusions
Seedling emergence was consistently much higher in exotic than native species in both mixtures and monocultures. Addition of seeds led to 2X to 4X greater emergence in exotic species invading native plots than when native species invaded exotic plots (P < 0.001). This result was consistent across the four sampling dates. Exotic species were able to invade and convert native species monocultures to exotic dominance much more commonly than the converse. Native mixtures that had high species diversity had lower exotic emergence compared to plots with low species diversity, suggesting that species diversity reduced exotic species invasion. Our results indicate two things: 1) that dispersal is a stronger force for exotic species than for native species in grassland communities, and 2) that low diversity in native communities will increase their probability of being converted to exotic species dominance.