A recent study reveals alarming rate of mangrove habitat loss in south-east Asia. The region contains the greatest proportion (33.8 per cent) of the global mangrove forest straddling across 1,34,257 sqkm. Mangroves are salt-tolerant evergreen forests that are found along coastlines, lagoons, rivers or deltas in tropical and subtropical countries and areas around the world, providing protection against erosion, cyclones and wind.
Mangroves are unevenly distributed across the tropics. “The distribution of mangrove forest is important in understanding the impact of observed processes of change on the global quantity of mangrove forest,” says the study titled ‘Distribution and drivers of global mangrove forest change, 1996–2010’. The study by Thomas N et al. published in PLoS ONE indicates that a significant driver of mangrove loss is anthropogenic activity in south-east Asia, in areas that are subject to aquaculture or agriculture practices.
The study says that “mangrove forests contain very high levels of above as well as below ground biomass, which equates to carbon storage levels equivalent to those measured in dense Amazonian rainforests. This makes mangroves amongst the most carbon rich ecosystems in the tropics. The high carbon content of mangroves, coupled with their financial value in terms of the ecosystems services that they support, makes them an important asset for carbon trading initiatives (e.g., REDD+)”.
Mangroves host a variety of biodiversity and offer many ecosystem services and support local livelihoods through the provision of fuel, food and construction materials. The study shows that mangrove losses can be estimated at 1 percent a year, a rate twice that of terrestrial rainforests.
This study investigates the distribution of observed drivers of change in mangrove forest extent, over the period 1996-2010 using time-series radar composite imagery. It uses both spatial and temporal information to achieve this novel understanding of changes in mangrove forest extent at the global scale. It presents evidence that presents evidence of human-induced land cover and land use change across the extent of the world’s mangroves, supporting previous claims that conversion of mangroves to commercial forms of food and resource production has been widespread.
The majority of loss and degradation, both locally and globally, occurred in Southeast Asia where aquaculture practices were widespread. This was exacerbated by the erosion of mangrove forest. This study was not only able to confirm prior observations, but was able to monitor changes in mangrove forest extent across their entire range, identifying the primary drivers of change and their spatial distribution.
Natural processes of erosion and deposition caused mangrove retreat and colonization or regrowth respectively. These processes of change cannot be readily managed as with human induced changes, yet highlight that mangrove forest extent is dynamic across its entire range and is capable of being influenced by external pressures. Upstream urbanization, mining and deforestation affect the influx of sediment into mangrove forests which provide the opportunity for new growth, but which can also be detrimental to some species. Influxes of sediment from urban development and mining can also deliver potentially harmful sediment to existing mangrove forests that can cause estuarine acidification and mangrove death.
Although loss and degradation of mangroves between 1996 and 2010 was substantial, this only partially revealed the extent of historic anthropogenic impact upon mangroves. The study demonstrates the need for a global mangrove monitoring system, as changes are occurring on a global scale and over short time periods.
It is estimated that over one third of mangrove forest had been lost at the close of the last millennium, yet the distribution of this loss and its drivers have not been spatially documented. Identifying such practices in their early stages is important for mangrove conservation. Whilst knowledge of prior disturbance is important for mangrove rehabilitation, identifying new and rapid degradation of pristine mangrove is of greater importance for mangrove preservation.
The study provides evidence that anthropogenic activities are detrimentally impacting mangrove forests and the ecosystem services that they provide. Emphasis should therefore be placed on quantifying these services in monetary terms, thereby forging an economic counterbalance in the face of heavy pressures from a lucrative aquaculture industry.
The full study is available here
Image courtesy: Fabian Lambeck, Wikimedia Commons (License: CC BY-SA 4.0)