The role of propagule pressure in invasion success of introduced Phragmites australis in North America

Phragmites australis (common reed) is a highly successful grass species found around the world in both its native and introduced ranges. In North America both native and introduced strains of Phragmites have been identified. Introduced Phragmites australis (subsp. australis) is a highly aggressive invader while in most cases the native Phragmites subspecies (Phragmites australis americanus) is not.  Phragmites reproduces both sexually and clonally with the latter generally considered the primary mode since seed production and viability are notoriously both low and variable. However, establishment of new populations can be initiated via seed or rhizome and genetic variation between clones suggests that sexual recombination is not uncommon. Furthermore, preliminary data resulting from hand cross-pollination in a greenhouse study suggests that interbreeding between the native and introduced strains of Phragmites may be occurring. Therefore, I suggest that while introduced Phragmites is undoubtedly successful because it is an aggressive competitor that spreads rapidly through vegetative growth, its success may also be due to the fact that it can exert pressure via three propagule types: rhizome fragments that spread naturally and with human assistance, dispersal of seed, and through interbreeding due to wind pollination. This reproductive plasticity combined with a broad environmental range of tolerance gives Phragmites a significant advantage over competitors and has contributed to its wide invasion success throughout North America.