Restoration of native mangrove wetlands can reverse diet shifts of benthic macrofauna caused by invasive cordgrass

Abstract

Ecological replacement using native mangrove species combined with physical treatments has become an effective method in controlling the spread of invasive cordgrass Spartina alterniflora. To re-establish ecosystem functions, trophic interactions between macrofauna and their potential food resources must be considered during the restoration process. Here, we examined the changes in the diets of macrofauna in three restored mangrove ecosystems with different invasion histories following the removal of S. alterniflora in southern China. Carbon and nitrogen stable isotope ratios for dominant macrofauna, as well as their potential food sources, were analysed. The relative contributions of the different carbon sources to the diets of benthic macrofauna and isotopic niche width, including convex hull areas and standard ellipse areas of the macrofaunal community, were then calculated and compared among sampling sites in each region. Our results indicated that Spartina-derived detritus contributed to >80% of the organic carbon sources of dominant macrofauna in the S. alterniflora communities in all three regions. Spartina alterniflora communities had lower convex hull areas and standard ellipse areas than natural mangrove forests, indicating significant resource competition among different consumer populations. Replacing S. alterniflora with the native mangrove species Kandelia obovata could reverse the diets of these macrofaunal species, resulting in a shift from homogeneous Spartina-dominated diets to more heterogeneous algae-based diets. It could take several decades to restore food web interactions to a pre-impacted state. Even the diets of macrofauna in mature mangrove (>40 years) remained affected by the Spartina-derived organic matter. Synthesis and applications. Our study reveals that the ecological replacement of invasive Spartina alterniflora using native mangrove species could restore food web function gradually. However, this removal and replacement approach is a long process and requires significant manpower and resources. Furthermore, the native ecosystem will continue to be influenced by the cordgrass as long as large areas of wetland nearby are occupied by S. alterniflora. Consequently, preventing the colonization of S. alterniflora should be a priority for coastal ecosystem management in southern China.