Loss of Mangroves

Primary reference(s)

Goldberg, L., D. Lagomasino, N. Thomas and T. Fatoyinbo, 2020. Global declines in human‐driven mangrove loss. Global Change Biology, 26:5844-5855.

Ellison, A.M. and E.J. Farnsworth, 1996. Anthropogenic disturbance of Caribbean mangrove ecosystems: past impacts, present trends, and future predictions. Biotropica, 28:549-565.

Polidoro, B.A., K.E. Carpenter, L. Collins, N.C. Duke et al., 2010. The loss of species: mangrove extinction risk and geographic areas of global concern. PLoS ONE 5(4): e10095.

Additional scientific description

Mangroves are distinctive tropical plant communities that occupy the intertidal zone between sea and land, or in areas that are subject to the indirect influence of tides (Tomlinson, 2016). Fewer than 10 mangrove species are found in the New World tropics, while 36 species have been reported for the Indo-West-Pacific region. These species communities together make up a forest, which can be categorised as fringe, riverine, overwash, basin, or dwarf (Lugo and Snedaker, 1974). The ecosystem services that healthy mangrove stands provide include climate regulation, water purification, coastal protection, timber and fuel supply, fisheries generation, and eco-tourism support (Worthington et al., 2020).

Metrics and numeric limits

The Global Mangrove Watch (GMW), an initiative of the Global Mangrove Alliance, is an online platform that provides the remote sensing data and tools for monitoring mangroves. The GMW is made up by Japan Aerospace Exploration Agency Kyoto & Carbon Initiative by Aberystwyth University, solo Earth Observation, The Nature Conservancy, and Wetlands International. The GMW maps also constitute the official mangrove datasets used by the United Nations Environment Programme (UNEP) for reporting on Sustainable Development Goal 6.6.1 (change in the extent of water-related ecosystems over time) (GMW, no date).

The most recent global extent maps are for 2016, and historical assessments span as far back as 1996. Between 1996 and 2016, global mangrove extent decreased by 6057 km2. Total loss during these 20 years amounted to 15,261 km2.

The availability of GMW has resulted in the Mangrove Restoration Potential Map (MRP Map), a unique interactive tool developed to explore potential mangrove restoration areas worldwide and model the potential benefits associated with such restoration. The mapping tool was developed by The Nature Conservancy and the International Union for Conservation of Nature (IUCN), in collaboration with the University of Cambridge (IUCN, University of Cambridge and The Nature Conservancy, no date).

The MRP Map includes assessments of global maximum mangrove extent, mangrove deforestation, and potential restorable area. From 1996 to 2016, 1389 km2 of mangroves have been degraded, and a total of 8120 km2 of area have potential for mangrove restoration.

Data suggest an average loss rate of 0.13% annually from 1996 to 2016, higher than the average for tropical and subtropical forest losses (Goldberg et al., 2020). At national levels, losses are recorded in 97% of the countries and territories with mangroves (105 out of 108), while degradation is recorded in 76% (82 out of 108).

Examples of drivers, outcomes and risk management

Mangrove loss is primarily due to the deforestation of existing mangrove areas (Hamilton and Casey, 2016). An estimated 62% of global mangrove loss is due to land-use conversion, namely into aquaculture and agriculture (including rice, oil palm and coconut plantations) and urbanisation (Friess et al., 2019; Goldberg et al., 2020). The importance of these specific mangrove deforestation drivers varies regionally, with southeast Asia a particular hotspot for mangrove loss (UNEP-WCMC, 2014).

Mangrove extent is further threatened by increased rates of sea-level rise (Lovelock et al., 2015). Mangroves are vulnerable to sea-level rise when they are unable to build surface elevations commensurate with the rate of sea-level rise, which results in their submergence and subsequent loss (Krauss et al., 2014). This is compounded by their inability to migrate inland to higher elevations when suitable migration space is lacking due to coastal development or natural barriers, resulting in coastal squeeze (Alongi, 2015; Schuerch et al., 2018).

Mangroves are also sensitive to other climatic events, such as storms (Krauss and Osland, 2019), high water events, precipitation and drought, and climate fluctuations such as the El Niño-Southern Oscillation (ENSO) (Field, 1995; Sippo et al., 2018). Mangroves are most vulnerable when they experience combinations of these climatic events; for example, mangroves along a 1000-km length in northern Australia recently experienced substantial dieback after an ENSO event led to a temporary sea-level drop and a marked reduction in precipitation (Duke et al., 2017).

Outcomes involve reduced capacity to provide ecosystem services to local coastal communities and global populations (Duke et al., 2007; Estoque et al., 2018). These include reduced ability to mitigate climate change as a sink of atmospheric carbon; reduced disaster risk reduction and lower protection of coastal communities from hazards such as sea-level rise and storm surges; reduced adaptive capacity of coastal populations through reduced access to sources of food, fibres, timber, chemicals, and medicines. The loss of these resources often translates into loss of revenue, as local fisheries, tourism, and related industries are sustained by the forests. In addition, as mangroves store a large amount of carbon in their soils, mangrove deforestation can release 25–100% of the total cleared biomass as carbon dioxide (CO2) emissions and that as much as 45% of soil carbon is lost within three years (Pendleton et al., 2012; Lovelock et al., 2017). Emissions from wetlands are explicitly considered in national greenhouse gas emissions reporting through the Intergovernmental Panel on Climate Change (Hiraishi et al., 2014). Mangrove loss may also have unintended impacts on other connected coastal ecosystems (UNEP-WCMC, 2006).

Mangrove loss can be reduced or managed by efforts to stabilise existing mangrove extent and restoring (replanting) mangroves that have been lost. Mangrove coverage can be stabilised by increased protected area management, enforcement of national laws that prohibit mangrove loss, integration of local knowledge and community participation, and promotion of flexible and robust governance in dealing with uncertainty, complexity, and dynamics of mangroves and ecosystems (e.g., adaptive management). Investigating, monitoring, and mitigating drivers of mangrove loss on regional and national levels are critical to maintaining existing mangrove forests.

Mangrove coverage can be increased by rehabilitation of previously deforested mangrove areas, such as the rehabilitation of abandoned aquaculture ponds. Potentially 800,000 hectares globally may be biophysically suitable for rehabilitation (Worthington and Spalding, 2018), although several socioeconomic and governance challenges exist (Wodehouse and Rayment, 2019) that may reduce the area that is ultimately feasible to rehabilitate. The success of these interventions requires an expansion of research on the basic ecology and hydrology of mangroves and social sciences of human-mangrove interactions. Understanding local enabling conditions (social equity, environmental sustainability, and economic viability) are key to successful mangrove management and its associated blue economy (Cisneros-Montemayor et al., 2021).