Abstract
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, Phragmites australis, 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 Phragmites density plots, and overall plant community composition did not vary significantly by Phragmites density. Shoreline change was most negative (landward retreat) where Phragmites density was highest (-0.40 ± 0.19 m yr-1 vs. -0.31 ± 0.10 for low density Phragmites) in the high energy marsh of Kitty Hawk Woods Reserve and most positive (soundward advance) where Phragmites density was highest (0.19 ± 0.05 m yr-1 vs. 0.12 ± 0.07 for low density Phragmites) in the lower energy marsh of Currituck Banks Reserve, although there was no significant effect of Phragmites density on shoreline change. In Currituck Banks, mean soil carbon content was approximately equivalent in cores extracted from low and high Phragmites 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 Phragmites 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 Phragmites 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.
Highlights
Invasive species pose significant conservation challenges, and their spread is considered second only to habitat destruction as the greatest cause of species endangerment and global loss of biodiversity [1]
We assessed the density-dependent role of Phragmites on ecosystem service provision at Currituck Banks and Kitty Hawk Woods Reserves, which are two Reserves within the NCNERR located in Currituck Sound, North Carolina (Fig 1)
Marsh shoreline change rates in Kitty Hawk Woods Reserve were most negative within the High Phragmites Density treatment (-0.40 ± 0.19 m yr-1, historical rate: -0.19 ± 0.05 m yr-1), moderate within the Low Phragmites Density treatment (-0.31 ± 0.10 m yr-1, historical rate: -0.07 ± 0.05 m yr-1), and least negative within the Medium Phragmites Density treatment (-0.07 ± 0.05 m yr-1, historical rate: -0.01 ± 0.04 m yr-1), differences were not significant across treatments
Summary
Invasive species pose significant conservation challenges, and their spread is considered second only to habitat destruction as the greatest cause of species endangerment and global loss of biodiversity [1]. Neutral, and negative impacts of invasive species on ecosystem service provision have been reported [3,4,5], with impacts often strongly linked to invasive species density [6]. There has been considerable interest in the carbon storage services associated with marsh ecosystems, the capacity of marshes to store atmospheric carbon trapped through primary production and vertical marsh accretion, and their ability to protect shorelines through baffling erosive wave energy [9, 10]. A key factor underpinning the ability of marshes to deliver ecosystem services is the diversity and structure of the marsh plant communities, which are often positively correlated with primary production, nutrient cycling, and resiliency to disturbance [11, 12]. Alterations to community composition or diversity through, for instance invasive species outcompeting native plant species, could disrupt the delivery of ecosystem services
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