Abstract
Identifying the mechanisms that result in a “high impact” invasive species can be difficult. Coexistence theory suggests that detrimental invasive species can be better predicted by incorporating both niche differences and fitness differences than examining niche overlap alone. Specifically, detrimental invasive species should take up shared limited resources more efficiently than their neighbouring resident species. While there is clear evidence that invasive Phragmites australis is successfully displacing resident species, there remains few field studies that attempt to quantify the niche overlap and fitness difference between P. australis and the species it is displacing in the field. We measured differences in photosynthetic performance (carbon assimilation rate, δ13C, photosynthetic water use efficiency, biomass, light compensation point, light saturation point), canopy height and interception of photosynthetically active radiation, and niche overlap between P. australis and three resident freshwater wetland species (Calamagrostis canadensis, Carex aquatilis, and Typha spp.) growing with or without aboveground interspecific competition. Invasive P. australis intercepted more photosynthetically active radiation, had higher photosynthetic water use efficiency, a higher average light saturation point, and had a larger niche region compared to resident species. Resident plant species showed a significant decrease in photosynthetic performance when growing in competition with P. australis and had a high probability of overlap onto the niche space of P. australis. These results provide evidence that the ability of P. australis to reduce the availability of a required resource and more efficiently use it over the growing season, while exhibiting high niche overlap with resident species, likely contributes directly to its success in North American freshwater wetlands.
Highlights
Identifying the mechanisms that confer an advantage to an introduced species can be challenging
From PAR levels 200 μmol s-1 m-2 to 1500 μmol s-1 m-2 carbon assimilation rates were higher in resident species growing without above-ground competition with P. australis compared to those growing with competition (Fig. 1A)
While there was no interaction between species and treatment, all three resident species had higher carbon assimilation rates at 1500 μmol s-1 m-2 PAR when they were growing without competition
Summary
Identifying the mechanisms that confer an advantage to an introduced species can be challenging. The complexities of biological communities can make it difficult to predict the effects of invasive species using a unified framework Both community assembly (Pearson et al 2018) and coexistence theory (e.g., Chesson 2000) have provided structures to guide invasion ecology that account for the complexities of biological interactions. In the community assembly framework, invasive species might “break the rules” of the resident community by having rare traits that benefit them relative to community-specific conditions (Pearson et al 2018) This can include traits that allow them to better take up resources or cope with limited resources (Gioria and Osborne 2014). An introduced species with a large fitness difference and high niche overlap with the resident community is more efficient at drawing down shared resources and more likely to establish and result in negative ecological effects on resident species (MacDougall et al 2009)
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