Effects of Ecological Restoration Using Non-Native Mangrove Kandelia obovata to Replace Invasive Spartina alterniflora on Intertidal Macrobenthos Community in Maoyan Island (Zhejiang, China)

Assessment of macrobenthos in evaluating the restoration effects of artificial mangrove planting on tidal flats in Zhejiang, China

Artificially planted mangroves are widely used for tidal flat restoration, but their effectiveness in restoring these ecosystems remains unclear. Macrobenthos, as key indicators of tidal flat ecosystem health, can reflect changes in environmental conditions and quality resulting from the introduction of artificial mangroves. This study aimed to investigate the ecological restoration effects of artificially planted mangroves in northern China by surveying macrobenthic communities across four regions in June 2023. We compared the macrobenthic community structures in mangrove zones with those in Spartina alterniflora or unvegetated mudflat control zones by estimating habitat quality indices and community structure analyses. Additionally, we explored the relationship between community structures and environmental factors to identify key drivers of community structure changes. Significant differences in species abundance and biomass were observed across the four regions, with mudflat zones exhibiting the highest values, followed by mangrove and Spartina alterniflora zones. Beta diversity was higher in mangrove zones compared to control zones, driven primarily by turnover components. The AMBI index indicated excellent ecological conditions in 18-year-old mangrove zone on Maoyan Island, while poorer conditions were observed in the mangrove zone of Zhoushan City. Environmental factors, such as sand content, organic carbon content, and elevation, significantly influenced macrobenthic community distribution. Overall, our findings suggest that the 18-year-old mangrove zone on Maoyan Island is ecologically stable and demonstrates significant restoration effects. Using macrobenthos to assess the restoration efficacy of mangroves proves to be both effective and feasible. Furthermore, while the mangroves have successfully established and grown, their long-term survival depends on the active involvement of local stakeholders, highlighting the importance of long-term monitoring and adaptive management strategies.

Functional traits of macrobenthos substantially indicated habitat change from the invasive saltmarsh to introduced mangrove

Mangrove afforestation is usually thought to be beneficial to mitigate the degradation and loss of mangroves. In Southern China, planting mangroves with the introduced Sonneratia apetala is also supportive to remove the invasive Spartina alterniflora. However, the influence of mangrove afforestation dominated by introduced species on macrobenthos, a vital joint of energy flow and nutrient cycling in mangroves, remains unclear. We explored the linkage between the functional traits of macrobenthos and the physicochemical properties of sediments in a coastal continuum including the mudflat (MF), exotic Spartina alterniflora saltmarsh (SL), natural Avicennia marina forest (AM), and introduced S. apetala afforestation (SA) via a seasonal field survey. After removing the S. alterniflora invaded into mudflat via S. apetala afforestation, the sediment C/N ratio decreased compared to that of natural forest, while the concentrations of microphytobenthic chlorophyll-a increased. The macrobenthic inhabiting mode shifted from epifaunal to infaunal as well. The biomass and density of microbenthic community decreased along MF, SL, AM, and SA. SL had greater C/N ratio and smaller functional richness (FR) than MF. AM was characterized by similar functional diversities, and pH value and salinity of sediment to those of MF, and greater microphytobenthic chlorophyll-a was found in AM. Compared to AM, the introduced S. apetala substantially engineered the habitat due to its flourishing above-ground pneumatophore system which caused faster deposition process, subsequently changed the resource utilization strategies of macrobenthos considerably. Overall, the use of Sonneratia afforestation on Spartina removal could not replace the contribution of natural Avicennia forest with respect to the functional traits of macrobenthos. Careful consideration on ecosystem functionalities would be indispensable for conducting saltmarsh eradication and mangrove afforestation in the future.

The diversity, drivers, consequences and management of plant invasions in the mangrove ecosystems

Mangrove ecosystems, which occupy intertidal environments across tropical and subtropical regions, provide crucial ecosystem services, such as protecting the coastal areas by reducing the impact of cyclones, storms, and tidal waves. Anthropogenic activities such as human settlements, deforestation, pollution, and climate change have increased the risk of biological invasions in mangrove habitats. Plant species can be introduced to mangrove habitats via anthropogenic means, such as trade and transportation, urbanisation, and agriculture, as well as through natural processes like wind, floods, cyclones, and animal-assisted seed dispersal. Additionally, some native plants can become invasive due to the changes in the mangrove ecosystem. Invasive species can significantly affect coastal ecosystems by out-competing native flora for resources, thereby altering fundamental properties, functions, and ecosystem services of the mangrove forests. The successful establishment of invasive species depends on a complex interplay of factors involving the biological attributes of the invading species and the ecological dynamics of the invaded habitat. This review focuses on exploring the mechanisms of invasion, strategies used by invasive plants, the effects of invasive plants on mangrove habitats and their possible management strategies. Based on the literature, managing invasive species is possible by biological, chemical, or physical methods. Some non-native mangrove species introduced through restoration activities can often become more intrusive than native species. Therefore, restoration activities should prioritise avoiding the use of non-native plant species.

Invasion of Spartina alterniflora in the coastal zone of mainland China: Control achievements from 2015 to 2020 towards the Sustainable Development Goals

The Sustainable Development Goals (SDGs) and the Convention on Biological Diversity's 15th Conference of the Parties (CBD COP15) both emphasized the urgency of protecting biological diversity. Spartina alterniflora (S. alterniflora), as an invasive species in China, has posed severe biodiversity challenges, demanding nationwide control and management. This study aims to assess the effectiveness of S. alterniflora management during China's SDGs implementation from 2015 to 2020. Landsat images acquired in 2015 (the beginning year of SDGs), 2018, and 2020 (the end year of SDGs' targets 6.6, 14.2, 14.5, and 15.8 related to alien invasion) were applied to quantify the spatiotemporal dynamics of S. alterniflora extent. The results revealed a consistent shrinkage of S. alterniflora, with a net areal reduction of 2610 ha from 2015 to 2020, implying the effectiveness of control measures on S. alterniflora invasion. Provinces including Zhejiang, Jiangsu, and Shanghai have succeeded in controlling S. alterniflora, evidenced by the sharp reduction in S. alterniflora area by 4908 ha, 2176 ha, and 1034 ha, respectively, from 2015 to 2020. However, better management of S. alterniflora is needed in regions with more severe S. alterniflora invasion, e.g., Shandong, Fujian, and Guangdong provinces. Our results suggest that relevant policies, regulations, and ecological restoration projects implemented by national or local governments in China received satisfactory results in S. alterniflora control. Nevertheless, S. alterniflora potential utilities and its governance effectiveness should be objectively evaluated and weighed to obtain the greatest ecological benefits and promote sustainable coastal ecosystems. The results of this study are expected to provide important baseline information benefitting the formulation of coastal protection and restoration strategies in China.