Fortuitous invasions into experimental wetland mesocosms: The effects of nitrogen enrichment and canopy-species composition

An experimental array of plant community mesocosms was planted in 2000 to test theories of competition between an invasive and a native plant common to North American wetlands.  We manipulated the species composition (Lythrum salicaria monocultures, Typha latifolia monocultures, and L. salicaria-T. latifolia mixtures), the level of herbivory (including herbivory by Galerucella calmariensis, a biological control agent on the invasive L. salicaria), and nitrogen supplementation. In the course of removing naturally-invading weeds from the mesocosms non-random patterns of species occurrences were observed, so in 2002 all weeds removed were sorted to species, dried, and weighed. Low fertility mesocosms and T. latifolia monocultures promoted greater invader biomass and higher species richness and evenness than did high fertility mesocosms and mesocosms that included the invasive L. salicaria. The effects of fertility and canopy species remained significant even after correcting for canopy species biomass as a covariate. This suggests that not only does the invasive L. salicaria have a greater effect on invading species than the native T. latifolia, but that it has a greater effect on a per gram biomass basis. Responses of the most frequently occurring invading species largely followed the same pattern.