Purple loosestrife suppresses plant species colonization far more than broad-leaved cattail: experimental evidence with plant community implications

P>1. Dominant plant species, whether native or invasive, often change community composition and cause decreases in diversity. Still invasive species are considered more deleterious to communities than dominant natives, although evidence for this is surprisingly rare. We tested two hypotheses: (i) an exotic invasive species will have greater impacts at the community level than a dominant native and (ii) this deleterious impact will be exacerbated with eutrophication. 2. Both hypotheses were tested by evaluating colonizer success in large, well-replicated experimental monocultures of two dominant and widespread wetland species, invasive purple loosestrife (Lythrum salicaria) and native broad-leaved cattail (Typha latifolia). To facilitate comparisons, we standardized competitive effects by canopy biomass and by light availability beneath the canopy. The latter is a novel approach that accounts directly for resource reductions caused by community dominants. 3. Loosestrife was particularly detrimental to rare species and dramatically reduced colonizer success compared to cattail by nearly all of our metrics, including colonizer biomass (50.2% lower), species richness (34.2% lower), Shannon diversity (35.8% lower) and the proportion of mesocosms that were colonized (38.5% lower). Moreover, 15 of 16 uncommon species failed to colonize loosestrife communities. Graminoids fared poorly in loosestrife monocultures, but forb biomass (predominantly Sagittaria latifolia) was 3.5 times higher there. These results suggest that over time, plant communities under loosestrife canopies will contrast sharply with those under cattail. Contrary to our second hypothesis, fertilization did not exacerbate loosestrife's ability to suppress colonizers, relative to that of cattail. 4. Canopy biomass and light attenuation were similar for cattail and loosestrife, yet biomass explained little variation in colonizer success. Increasing light availability in the understorey increased colonizer richness and diversity only under cattail canopies, suggesting loosestrife suppresses colonization via below-ground competition while cattail does so via light reduction. 5. Synthesis. Ours is the first study to show that an invasive species suppresses colonizers much more than a dominant native and to identify contrasting mechanisms by which this may occur. Biomass-based comparisons of competitive effects may have limited utility for highly productive dominant species generally, thus our approach also offers a viable new alternative that could be applied broadly.