Mangrove coasts face 'triple threat', according to new research

A new study has found that mangrove forests face a ‘triple threat’  to their long-term durability and survival – sea-level rise, lack of mud and squeezed habitats.

Considered to be some of the world’s most valuable ecosystems due to the coastal protection they provide and the biodiversity they host, this research highlights that mangrove coasts urgently need better protection.

mangroves under threat

The research identifies how these coastal forests get pushed against their shores and the causes their diversity loss. It also highlights the negative impacts of river dams, which decrease the supply of mud that could otherwise raise mangrove soils. Additionally it notes how buildings and seawalls largely occupy the space that mangroves require for survival.

Dr Barend van Maanen, senior lecturer at the University of Exeter and supervisor of the project, said: “Both mangrove coverage loss and diversity loss go hand in hand when that landward retreat is limited by expanding cities, agriculture or flood protection works.”

Mangroves withstand flooding by tides and capture mud to raise their soils. However, mangrove trees cannot survive if they are under water for too long, so the combination of sea-level rise and decreasing mud supply from rivers poses a serious threat to these forests.

New computer simulations show how coastal mangrove forests retreat towards land under sea-level rise, especially in coastal areas where mud in the water is declining. These simulations include interactions among tides, mud transport and multiple mangrove species.

The simulations also show that mangrove trees with dense roots trap mud more effectively and can stop it from reaching forest areas further inland.

“This makes the more landward-located trees flood for longer periods of time, an effect that is intensified by sea-level rise,” said Danghan Xie, PhD researcher at Utrecht University and lead author of the study.”Increasing landward flooding then seriously reduces biodiversity. Human land use prevents the mangroves ‘escaping’ flooding by migrating inland, narrowing the mangrove zone and further endangering biodiversity.”

Coastal mangrove forests are valuable, highly biodiverse ecosystems that protect coastal communities against storms. A narrow mangrove zone is much less effective in protecting the coast against storms, or in the worst case loses its protective properties altogether.

“The loss of mangrove species will have dramatic ecological and economic implications, but fortunately there are ways to help safeguarding these ecosystems,” added co-author Dr Christian Schwarz, environmental scientist at the University of Delaware. “It is essential to secure or restore mud delivery to coasts to counter negative effects of sea-level rise. For coasts where mud supply remains limited, removal of barriers that obstruct inland migration is of utmost importance to avoid loss of mangrove forests and biodiversity.”

For more from our Ocean Newsroom, click here.

Photography by Barend van Maanen.

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