Test of enemy release hypothesis with native and invasive genotypes of Phragmites australis

Enemy release hypothesis suggests that exotic species receive lower enemy pressure in the introduced range than in their native range which may result into a substantial increase in their population growth rate. The reduced enemy pressure in a novel range compared to co-occurring native species has been considered as one of the primary causes of biological invasion. We examined this hypothesis with Phragmites australis. This species has been a member of the wetlands of North America for millennia but an introduced Eurasian genotype has been invading the wetlands of North America in the past century.

We performed a common garden experiment in Louisiana to examine the plant palatability and defenses between native and exotic genotypes of P. australis. Plant defense (leaf toughness), chewing damages caused by a generalist herbivore, fall armyworm (Spodoptera frugiperda), and herbivore performance (change in weight biomass) were evaluated. Exotic plants produced tougher leaves than natives. Although survivorship rate of larvae did not differ between native and exotic genotypes, feeding damages and larval growth rates were substantially lower on exotic genotype than the native. These results suggest that escape from natural enemies may have contributed to the invasiveness of the Eurasian genotype in North America.