Mangrove Ecosystem Restoration: a short insight

The mangrove tree, the cornerstone of the mangal ecosystem, bears its responsibility with poise and grace. The roots of this tree twist and overlap as they plunge into brackish water, creating habitats for sea-dwelling creatures to swim and shelter in through their gaps. It’s tall and thin trunk winds around, holding aloft bright green leaves, creating a green carpet along coastlines, protecting shores from the eroding effects of the sea.

Mangrove forests – or mangals – form along coastlines in brackish water environments; areas where fresh and saltwater meet. Here, they store carbon, maintain the coast, offer a buffer against hurricanes, typhoons and other storms, and care for much of the local wildlife. However, human activities in 21st century have resulted in the loss of 20-35% of Mangroves worldwide. Road construction and similar infrastructures in particular have been identified as having a negative effect on mangrove forests, chiefly by impeding the flow of currents. When water currents are impeded in a mangrove wetland, the water becomes filled with salt (hypersaline) and suspended solids (murkier). Hypersaline and murkier water makes it harder for mangroves to grow. Without roads in the way, mangrove vegetation lowers the speed of the water currents, trapping sediments that aid in filtering suspended solids like soil and algae from the water.

Around the world, there are efforts being made to restore the lost mangrove habitats. For example, small tunnels carrying water (culverts) have been constructed underneath roads in order to redirect water flow from healthy areas to those that have been damaged to “that should lead to restoring the hydrology of the zone”. Determining how well these restoration attempts are working is another matter: visual observation alone is not enough. In order to monitor the health of mangrove habitats researchers must evaluate what the mangrove trees are providing and, how well it’s being provided.

In a recent paper from the journal Regional Studies in Marine Science, researchers examined the fish populations of two areas of a mangrove forest, one in a conservation zone (healthier) and the other in an area undergoing restoration (damaged), in the Sian Ka’an ecological reserve in the Mexican Caribbean. The two habitats have been separated by a road across the reserve for almost 40 years. The road impedes the flow of water causing the area on the south end (the restoration zone) overtime to suffer significant damage to its ecosystem, with restoration only beginning in 2009. The aim of the research was to compare the structure of fish populations in each site in order to determine if the ecosystems maintain their function as nurseries. In a healthy mangrove forest the roots of the trees provide the optimal environment for juvenile fish to capture food with limited risk of getting hunted by their own predators. Data for this study came from six bimonthly fish collections in eight sites (four in each zone) over the course of a year (August 2017 to May 2018). These collections measured the number of different fish species, the types of fish species, and the life stage of the fish collected.

Findings revealed that in both the conserved zone and restoration zone marine fish species were most abundant. Marine fish made up nearly 60% of the total number of fish collected, many of which were juveniles. In the conserved zone juvenile fish made up 58% of collected fish. The abundance of marine juveniles in the conserved area implies that the healthy mangroves are continuing to function as nurseries for those species. However, in the restoration zone only 36% of fish were juvenile. The lack of juvenile fish species in the restoration zone indicates that the qualities that make the trees attractive as nurseries are present but not as abundant as in the healthier forest.

By providing a base of comparison this study has added means of measuring the recovery of mangrove tree forests. One more tool, for context-appropriate restoration projects. Yet, the work to support the restoration of whole ecosystems, and to ensure they are resilient for changing climate, biodiversity and human impacts is an ongoing and hopeful struggle. In some cases like the ones described in this article, we are collectively stepping in the right direction; the more information we collect and share the better equipped we will be to replenish not only the mangrove forests but, the many other ecosystems harmed by industrialisation that are in need of care and attention.

Written by Jack Gottesman, Oli Meese and Ian Cooke-Tapia

Illustration by Oli Meese