Coastal Mangroves Face Threats to Survival

Nov. 12, 2020
Some of the world’s most valuable ecosystems are facing a "triple threat" to their long-term durability and survival, new research shows.

New research shows that coastal mangrove forests, some of the most valuable ecosystems in the world, are facing three distinct threats to their long-term survival and resilience. Mangrove forests provide both significant biodiversity and coastal protection from wave and storm erosion, but sea-level rise, lack of mud, and reduced habitats are putting the forests under increased pressure.

The study was conducted by an international team of experts from the University of Delaware, Utrecht University, and the University of Exeter, and the researchers identified a number of human activities and infrastructure that stress the coastal forests. River dams cause negative effects that decrease the supply of mud that raise mangrove soils, while buildings and seawalls often occupy spaces that mangroves require to survive. 

The researchers used computer simulations to show how coastal forests retreat under sea-level rise. The simulations include the interactions of tides, mud transport, and multiple mangrove species. In coastal areas where mud and sediment in the water are declining, the results were especially stark. 

Coastal mangrove forests provide valuable protection against storms and erosion to coastal communities. Mangroves can withstand tidal flooding and capture mud in order to raise their soils. However, since the trees cannot survive if they are under water too long, the combination of sea-level rise and a decreasing supply of mud from rivers is a serious threat. 

Dr. Barend van Maane, 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.”

The modeling was also able to show that mangroves with dense roots can trap mud more effectively. Although this is likely unsurprising for those familiar with the erosion-mitigating and soil-stabilizing properties of plant roots, it has the negative side effect of stopping mud from reaching forested areas further inland. 

Danghan Xie, PhD researcher at Utrecht University and lead author of the study, said, “This makes the more landward-located trees flood for longer periods of time, an effect that is intensified by sea-level rise.

“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.”

The narrower a mangrove zone, the less effective the forest is at protecting the coast from storms and, in the worst cases, can lose its protective abilities altogether. 

Co-author Dr. Christian Schwarz, an environmental scientist at the University of Delaware, added, “The loss of mangrove species will have dramatic ecological and economic implications, but fortunately there are ways to help to safeguard these ecosystems.

“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.”

The study, "Mangrove diversity loss under sea-level rise triggered by bio-morphodynamic feedbacks and anthropogenic pressures," was published in Environmental Research Letters.

Source: The University of Exeter