Density-dependent role of an invasive marsh grass, <i>Phragmites australis</i>, on ecosystem service provision

<div><p>Invasive species can positively, neutrally, or negatively affect the provision of ecosystem services. The direction and magnitude of this effect can be a function of the invaders’ density and the service(s) of interest. We assessed the density-dependent effect of an invasive marsh grass, <i>Phragmites australis</i>, on three ecosystem services (plant diversity and community structure, shoreline stabilization, and carbon storage) in two oligohaline marshes within the North Carolina Coastal Reserve and National Estuarine Research Reserve System (NCNERR), USA. Plant species richness was equivalent among low, medium and high <i>Phragmites</i> density plots, and overall plant community composition did not vary significantly by <i>Phragmites</i> density. Shoreline change was most negative (landward retreat) where <i>Phragmites</i> density was highest (-0.40 ± 0.19 m yr^-1 vs. -0.31 ± 0.10 for low density <i>Phragmites</i>) in the high energy marsh of Kitty Hawk Woods Reserve and most positive (soundward advance) where <i>Phragmites</i> density was highest (0.19 ± 0.05 m yr^-1 vs. 0.12 ± 0.07 for low density <i>Phragmites</i>) in the lower energy marsh of Currituck Banks Reserve, although there was no significant effect of <i>Phragmites</i> density on shoreline change. In Currituck Banks, mean soil carbon content was approximately equivalent in cores extracted from low and high <i>Phragmites</i> density plots (23.23 ± 2.0 kg C m^-3 vs. 22.81 ± 3.8). In Kitty Hawk Woods, mean soil carbon content was greater in low <i>Phragmites</i> density plots (36.63 ± 10.22 kg C m^-3) than those with medium (13.99 ± 1.23 kg C m^-3) or high density (21.61 ± 4.53 kg C m^-3), but differences were not significant. These findings suggest an overall neutral density-dependent effect of <i>Phragmites</i> on three ecosystem services within two oligohaline marshes in different environmental settings within a protected reserve system. Moreover, the conceptual framework of this study can broadly inform an ecosystem services-based approach to invasive species management.</p>