Effects of latitude on the competitive ability of native and invasive genotypes of Phragmites australis

One commonly cited mechanism for the success of invasive species is their superior competitive ability relative to that of native species. Although 88% (22 of 25) of the empirical studies support the prediction that the strength of competition for native species increases with latitude, no studies to date have compared the competitive ability of native and co-occurring invasive species across a broad latitudinal range. In a greenhouse, I investigated whether the competitive ability of North American native and European invasive haplotypes of Phragmites australis vary with latitude. Another widespread, non-indigenous haplotype, the Gulf Coast haplotype, was also included for assessment of competitive ability only. Competitive ability of each haplotype was evaluated against a standardized plant species, Spartina alterniflora, which is a common co-inhabitant of coastal marshes. The competitive ability (measured in terms of the proportional reduction in biomass of plants grown in the presence and absence of a potential competitor) of native haplotypes decreased with increasing latitude, whereas the competitive ability of invasive haplotypes showed no relationship with latitude. This study provided the first evidence that native and invasive species (or haplotypes) exhibit non-parallel gradients in competitive ability. Overall, the invasive haplotype was competitively superior to the native haplotype - biomass production of S. alterniflora was 19% lower when grown with the former than the latter haplotype. Moreover, in the presence of an interspecific competitor, the invasive haplotype produced 45% more aboveground biomass and 50% more belowground biomass than the native haplotype. Results also indicated that the Gulf Coast haplotype was not significantly different from either the native or invasive haplotypes in terms of competitive ability. Because the invasive haplotype appeared to have the greatest competitive advantage over the natives at northern latitudes, it may be more successful in this region of its invaded range. The results from this research indicate that novel and important findings are possible when the mechanisms concerning invasion success, such as biotic resistance and competition, are examined from a biogeographical perspective