Non-native Phragmites australis (European common reed) is rapidly spreading throughout North American wetlands. It can alter the ecological function of wetlands by creating expansive, dense monocultures which lead to a reduction in floral diversity, decreased light availability, the filling in of open-water pools, and decreased interspersion that results in changes to habitat heterogeneity. These changes can affect the sensitive bird species that rely on wetlands for nesting, breeding, or foraging, leading to a reduction in abundance of marsh-reliant birds and an impoverished avian community characterized by generalist species. In hydrologically dynamic coastal marsh systems, inter-annual variation in water levels is common. High water years can increase interspersion with open-water habitat, whereas low water years can exacerbate the loss of open-water attributed to P. australis invasion, and may even facilitate its expansion. It is unclear, however, whether invasion by P. australisalters the way that birds respond to natural variability in water level. To assess whether the sensitivity of birds to changes in water level differs in coastal marsh invaded by P. australis compared with uninvaded cattail and meadow marsh habitats, we conducted a study in Long Point, on the North shore of Lake Erie. Long Point provides essential habitat for hundreds of bird species, including marsh-nesting species whose populations are in decline throughout the Great Lakes.
In 2014 and 2015 we surveyed birds using invaded and uninvaded marsh habitats. Water levels were significantly higher in 2015 than 2014 (paired T-test t19 = -7.942, p < 0.001). Bird use in uninvaded habitats differed significantly between 2014 and 2015, with overall lower abundances in the drier 2014 (F1,34 = 19.24, p < 0.001), as well as lower bird richness (F1,34 = 4.42, p = 0.043). However, the effect of year of sampling differed significantly among the invaded and two uninvaded marsh types for both abundance (F2,34 = 8.63, p <0.001) and richness (F2,34 = 5.35, p = 0.010). These significant interaction results may be because the increase in bird abundance and richness observed in cattail and meadow marsh in the wetter 2015 year were not evident in areas invaded by P. australis. Our study is the first we know of to find that P. australis invasion may modify the way that bird communities respond to natural fluctuations in water levels.