On World Mangrove Day (26 July 2016) we’re highlighting one of the most ecologically and economically important ecosystems in the world, the mangrove forest.
What is a mangrove?
Mangroves are uniquely-adapted, saltwater-tolerant trees and shrubs that grow between the land and sea in the tropics.
They are home to an enormous diversity of animals, and play a crucial role in filtering sediments from the land and providing “nursery” habitat for many marine species. Because of this, healthy, productive mangroves are key in supporting the world’s coral reefs and maintaining populations of fish, shellfish and crustacean species.
But mangrove forests are important in many more ways. Mangroves can capture (or sequester) among the largest carbon stores per unit area of any tropical ecosystem, making them hugely valuable in the battle against global climate change, and they can provide essential protection for vulnerable coastal communities from the devastating impacts of frequent tropical typhoons and tsunamis.
Decline and deforestation
However, in recent decades, mangroves have suffered extensive declines globally, predominantly due to the development of fishpond aquaculture in Asia. This means cutting and clearing mangrove areas for the creation of large (several hectares) fishponds for rearing fish, crabs and shrimp.
In the Philippines, more than 50% of the country’s former 500,000 hectares of mangroves have been converted to fishponds over the last century. As a result of global mangrove deforestation, many millions of tonnes of carbon are released annually and coastal greenbelt defenses have disappeared or their effectiveness has been severely degraded.
Governments and NGOs now place much focus on mangrove replanting and rehabilitation programs, although these often centre on planting in easily-accessible, low-conflict low-intertidal areas where mangroves are less well-equipped to cope with high salinities and wave action. This can result in low mangrove survival rates and threaten other important coastal ecosystems.
Seafront areas available for mangrove planting are also often very narrow, so where mangroves do establish here they may be less effective at protecting local communities from typhoons.
ZSL’s Philippines hub and other conservation NGOs instead advocate and prioritize mangrove rehabilitation in abandoned fishpond areas (abandoned for financial or low-productivity reasons). Because fishponds were originally constructed in former mangrove areas, their conditions are more suitable for new mangrove establishment. As well as this, they are often very large and so could provide much greater carbon storage and coastal protection potential than seafront areas.
And fishpond abandonment is high: thousands of hectares are estimated to be abandoned in the Philippines alone. However, tenure issues surrounding actual or perceived ownership of fishponds in the country are complex and can be dangerous, and monitoring and action on abandonment is therefore minimal. And the former mangrove lands remain fallow.
So how beneficial is mangrove rehabilitation for carbon storage and coastal protection in abandoned ponds? And is a strategy for abandoned fishpond rehabilitation actually feasible due to tenure issues at a regional level?
Studying abandonded ponds
To attempt to answer these questions, we researched mangrove-rehabilitated abandoned fishponds and seafront areas in Panay Island, Philippines. We used field- and satellite-based data to index carbon stocks and coastal protection potential of these currently rehabilitated areas.
In a specific municipality case study, we then located all abandoned fishpond areas and used our data to estimate potential carbon stocks and coastal projection potential gains if all of these were reverted to mangrove forest.
Lastly, we used a recent ZSL Philippines study identifying legal tenure (ownership) status of all fishponds in the municipality to explore the feasibility of reverting them.
What did we find?
- Carbon storage and greenbelt protection was greater in abandoned ponds compared to other seafront mangrove sites, despite slower mangrove growth.
- Almost 10% of all the municipality’s fishponds were abandoned (an area greater than 370 hectares). We estimated that between 5,800-8,000 tonnes of carbon could be stored in mangrove trees and new soils over just six and a half years if all of these ponds were reverted tomorrow (the equivalent CO2 emissions of 17,000-24,000 return flights from London to New York!).
- 13% of the municipality’s coastline could return effective coastal protection from regular waves after just eight years under targeted abandoned fishpond reversion. And more than 96% of the sea-facing abandoned ponds along this coastline are currently in breach of lease terms, and thus under current Philippines law should be being formally reverted back to mangroves.
To sum up, we found that rehabilitating abandoned fishponds to former mangrove forest could bring substantially greater climate change mitigation and adaptation potential and for the protection of vulnerable coastal communities.
In the Philippines, new mangrove forests in unused fishponds could help secure the restoration and protection of coastal biodiversity, because the diversity of mangrove species that can grow further inland in abandoned fishponds is substantially greater than the few species that can persist in narrow, high salinity seafront areas.
And it is not just the people and plant communities that could benefit from such a strategy. The large area of abandoned fishpond coverage means their reversion would increase intertidal habitat availability, providing greater support for terrestrial and marine animal communities.
To date, ZSL-led projects in the Philippines have rehabilitated and protected over 100 hectares of abandoned fishpond mangroves - around the size of 140 football pitches. We are working closely with local government to increase this area and advocate national government-level involvement in fishpond reversion.
By Clare Duncan, PhD candidate at ZSL's Institute of Zoology and University College London, and Dr Jurgenne Primavera, ZSL Chief Mangrove Scientific Advisor.
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