Drivers of global mangrove loss and gain in social-ecological systems

Mangrove forests are vital coastal ecosystems providing numerous essential services. They act as significant carbon sinks, mitigating climate change by storing substantial amounts of carbon (8.5 gigatons). They also offer crucial coastal protection, reducing the impact of storms and providing flood defense valued at US$65 billion annually. Furthermore, mangroves support thriving fisheries, sustaining the livelihoods of 4.1 million small-scale fishers. Despite their importance, mangrove forests have suffered considerable decline, with estimates suggesting a 35% loss by the end of the 1990s and further reductions in subsequent years. This loss is primarily attributed to conversion for aquaculture and agriculture. This research addresses the critical need to understand the complex interplay of socioeconomic and biophysical factors influencing mangrove loss and gain. The study aims to identify interventions that can effectively reverse mangrove decline by analyzing the drivers that decrease loss rates and increase gain rates. Previous research has highlighted socioeconomic factors such as effective protected area management, strong governance, community dependence on resources, control of corruption, and higher levels of democracy as crucial for mangrove conservation. However, the impact of national governance and conservation policies at a landscape scale, along with their interaction with local economic pressures and biophysical drivers, remains poorly understood. This gap in knowledge limits the ability of resource-constrained governments and conservation practitioners to implement effective interventions. Therefore, this study seeks to bridge this gap by investigating the relationships between socioeconomic and biophysical variables and mangrove cover change across various coastal geomorphic units worldwide.

Existing literature identifies various socioeconomic factors that influence mangrove conservation success. Effective management of protected areas, strong governance structures, and community reliance on mangrove resources are consistently highlighted as positive influences. Studies also emphasize the role of controlling corruption and promoting democratic governance in mediating human pressures on mangrove ecosystems. Previous research has also explored the impact of high population density on mangrove loss, demonstrating that effective regulations can mitigate this pressure, even offering greater protection beyond protected areas. However, the increasing influence of biophysical drivers, such as shoreline erosion and extreme weather events, poses significant challenges. Identifying these drivers is vital for efficient resource allocation and the development of targeted conservation policies. For terrestrial forests, initiatives like community forestry, recognition of indigenous land tenure rights, and climate policy programs have demonstrated success in reducing deforestation. While community forestry is being promoted in mangrove ecosystems in some regions (Myanmar and Kenya), its global-scale impact remains unknown. This study aims to address this knowledge gap and inform the post-2020 global biodiversity framework and national climate commitments under the Paris Agreement.

This study employed a hierarchical modeling approach to analyze the relationship between mangrove cover change and socioeconomic and biophysical variables. High-resolution, global time-series data on mangrove cover from 1996 to 2016 were used, focusing on two decades (1996–2007 and 2007–2016) to capture recent trends in mangrove loss and gain. The data were processed to calculate four measures of mangrove cover change: percent net loss, percent net gain, percent gross loss, and percent gross gain. These metrics allow for a nuanced understanding of the interplay between losses and gains. Landscape mangrove geomorphic units were delineated based on maximal mangrove cover extent and classified into typologies based on geomorphic settings (delta, estuary, lagoon, and open coast). National-scale variables included indicators of conservation policies (community forestry support, restoration effort, NDC commitments, proportion of Indigenous people’s land, and Ramsar wetland area), governance (level of democracy and economic complexity), and landscape-scale variables included economic growth (night-time lights growth), access to markets (travel time to the nearest city), and several biophysical variables known to influence mangrove extent (mangrove fragmentation, sediment availability, tidal amplitude, antecedent sea-level rise, drought severity, tropical storm frequency, and extreme low temperatures). The researchers used multi-level linear modeling and spatial autoregressive (SAR) models to account for national and landscape-level variability and spatial autocorrelation. Model fit was validated through residual plots, and likelihood ratio tests were conducted to assess model fit and the significance of random effects. Finally, hotspot analysis was used to identify areas with unusually high or low mangrove loss or gain, examining those cases using satellite imagery and expert knowledge to further determine the underlying drivers.

Globally, the rate of net mangrove loss decreased from 2.74% in 1996–2007 to 1.58% in 2007–2016. While overall mangrove extent continued to decline, several countries experienced net gains or no change due to low gross losses and high gross gains. In the first decade (1996–2007), landscape-scale net losses were negatively associated with proximity to cities, mangrove clumpiness (fragmentation), tidal amplitude, drought severity, and minimum temperature, while positively associated with antecedent sea-level rise. At the national scale, net losses were positively associated with mangrove restoration effort. In the second decade (2007–2016), net losses remained high in lagoons and showed similar negative correlations with proximity to cities, clumpiness, and tidal amplitude at the landscape scale. The relationship between economic growth (night-time lights growth) and mangrove loss showed a reversal. While mangrove gross loss increased significantly with economic growth in the first decade, this relationship disappeared in the second. Conversely, greater net gains were observed in areas with higher economic growth in the second decade, suggesting a potential shift where economic development is no longer necessarily detrimental to mangroves. A strong positive association was found between community forestry effort and mangrove gains, particularly in the second decade for both net and gross gain. This highlights the potential of community-based management for mangrove conservation. Mangrove restoration effort and NDC commitment did not show a consistent positive impact on mangrove conservation, possibly due to time lags between policy implementation and on-ground responses. Biophysical drivers played a significant role, with higher mangrove fragmentation strongly associated with greater loss but also with greater regeneration. Reduced sediment availability was linked to higher loss, while smaller tidal amplitudes were associated with greater vulnerability. While drought and extreme low temperatures caused mangrove dieback in the first decade, mangrove gains were also higher in areas with lower minimum temperatures, indicating potential regeneration after disturbance. Lagoonal mangroves showed persistent high net losses, highlighting a potential vulnerability. Hotspot analysis revealed that high access to markets was a major driver of mangrove deforestation, often in protected areas, due to conversion to aquaculture and agriculture. Remote areas tended to be better conserved.

This study's findings highlight the complex and dynamic interplay of socioeconomic and biophysical drivers influencing global mangrove change. The reversal in the relationship between economic growth and mangrove loss suggests a potential turning point where economic development, if managed sustainably, can contribute to mangrove expansion. This could be attributed to factors such as increased wealth and education leading to reduced pressure on mangrove ecosystems for alternative land uses. The strong positive effect of community forestry on mangrove gains emphasizes the critical role of local communities in conservation efforts. Successful community-based initiatives often involve well-defined governance, secure user rights, and support from external organizations. The lack of consistent positive association between national-level conservation policies (NDCs and restoration efforts) and mangrove conservation likely stems from time lags between policy implementation and on-ground effects. Therefore, it is crucial to assess the effectiveness of these policies over longer time scales. Biophysical factors remain important drivers of mangrove change, emphasizing the need to consider these aspects when planning conservation interventions. The persistent high losses in lagoonal mangroves highlight an area requiring further investigation and targeted conservation efforts. This research underlines the necessity for integrated approaches to mangrove conservation that consider both socioeconomic and biophysical contexts. Future research could focus on further clarifying the mechanisms through which economic growth supports mangrove expansion and on developing more robust methods for assessing the effectiveness of national-level conservation policies.

This study provides a comprehensive global assessment of mangrove loss and gain, revealing a complex interplay of socioeconomic and biophysical factors. The findings demonstrate that the relationship between economic growth and mangrove change has reversed, highlighting the potential for sustainable development to support mangrove expansion. The critical role of community forestry in promoting mangrove gains underscores the importance of participatory management approaches. While national-level policies show mixed results, the study emphasizes the need for long-term monitoring and evaluation. Future research should focus on understanding the specific mechanisms driving these trends and developing effective strategies for mangrove conservation within diverse social-ecological systems. This work contributes significantly to our understanding of mangrove dynamics and provides valuable insights for achieving sustainable development goals and ecosystem restoration.

This study relies on globally available datasets, which may introduce uncertainties in the estimation of some indicators. The availability of governance indices was limited for some countries, potentially influencing the results. To avoid bias from small geomorphic units, units smaller than 100 ha were removed, potentially excluding some hotspots of mangrove change. The correlative nature of the models does not establish direct causation, requiring expert interpretation to understand the underlying mechanisms driving changes. Finally, the temporal scope of the study may not capture the full long-term effects of certain policies and interventions.