Effect of exotic cordgrass Spartina alterniflora on the eco-exergy based thermodynamic health of the macrobenthic faunal community in mangrove wetlands

The health of natural ecosystems, including mangrove wetlands, is being severely challenged by biological invasions worldwide. For a holistic evaluation of ecosystem health, researchers have commonly measured the ecoexergy of the system in question. Mangrove wetlands depend on their macrobenthic fauna and are threatened by the exotic cordgrass Spartina alternifiora, particularly in South China. However, the effect of S. alternifiora on the health status of macrobenthic faunal community in such wetlands is still unclear. In this study in Zhanjiang, China, eco-exergy was measured to compare the health status of macrobenthic faunal communities in (1) an Ayicennia marina monoculture vs. an S. alternifiora-invaded A. marina stand (an A. marina and S. alterniflora mixture) and in (2) an unvegetated shoal vs. an S. akemiliora-invaded shoal that had rapidly become an S. alternifiora monoculture. Macrobenthic faunal community eco-exergy and biomass but not specific eco-exergy were lower in S. alterniflora-invaded mangrove habitats than in non-invaded ones. In terms of eco-exergy, mollusks were the dominant group in all habitats, and were mainly responsible for the decrease in eco-exergy in the invaded habitats. Differences in the eco-exergy of the macrobenthic faunal community in S. alterniflora-invaded habitats were mainly associated with differences in sediment physicochemical properties and vegetation characteristics in general and with the greater plant density in invaded habitats in particular. The results indicate that the health of the macrobenthic faunal community in mangrove wetlands has been reduced by S. alterniflora invasion and the conservation of ecosystem health in mangrove wetlands will require eradication of S. alterniflora.