A new study published in the Wiley Journal of Global Ecology and Biogeography has determined that both human pressures, when coupled with erosion, drought, and sea-level changes (i.e. certain effects of climate change), are the main culprits behind the diminishing mangrove forests in the Western Indian Ocean region.
Mangroves, unique trees that thrive in saltwater, are among the most productive ecosystems on earth. Not only do they protect shorelines from eroding, shielding us from floods, cyclones and hurricanes, and storms, but they also provide nursery areas for fish, crustaceans, mollusks, and more, while supporting many threatened and endangered species. Globally, mangroves provide goods and services estimated to be worth billions per year, including 65 billion USD in shoreline protection alone.
But for many communities, including those in the Western Indian Ocean region, they are worth much more. Coastal residents eat or sell the fish who live around the mangroves, while others use their wood as building material and fuel to use themselves or sell—underscoring the importance of mangroves to both economic and food security. Mangroves also have important cultural and recreational value for adjacent communities. Yet the region has lost 20-33% of its mangroves in the past 25 years alone, a trend that—if left unchecked—is only likely to accelerate with rapid increases in population and coastal development.
These and other causes of mangrove degradation, including human pressures (logging, land-use conversion, etc.) and the effects of climate change (i.e. sea-level rise, cyclones including related wind and flood impacts, erosion, ocean climate, etc.), are well-known. “However, not all mangroves suffer equally from these drivers of deforestation,” noted Dr. Joseph Maina of Macquarie University (Australia), lead author of the study. “To create tailored responses to address mangrove degradation in different regions and increase their resilience to climate change, it is necessary to understand how mangroves are affected locally by these factors.”
This study, supported by the Nairobi Convention’s WIOSAP project, the Western Indian Ocean Marine Science Association (WIOMSA), and Macquarie University, examines the level of exposure to threats to mangroves in Kenya, Madagascar, Mozambique, and Tanzania, applying the recently developed Western Indian Ocean Climate Change Vulnerability Assessment Toolkit.. It finds that human pressure, erosion, drought, and sea-level changes are the main drivers of changes in mangrove cover in these four countries.
Specifically, mangroves in Mombasa and Kilifi, Kenya were severely threatened due to erosion, human pressure, and sea level. However, mangroves in Lamu were only moderately exposed, while Kipini, southern Kenya, and Mida were low to moderately exposed.
In Tanzania, Pangani in the north, Bagamoyo in the centre, Lindi and Kilwa in the south, and Unguja were severely exposed because of high erosion, sea-level anomalies, and human pressures. Kilwa, for its part, was exposed as a result of land development intensity and drought conditions, while Tanga, Mafia, Mtwara and Rufiji were the least exposed mangrove areas in Tanzania.
In Mozambique, though the exposure was low or moderate in areas (namely, Inhambane, Zambezi, and Vilankulos), Angoche and Nacala were highly exposed due to sea-level anomaly, drought, and erosion. Meanwhile, Maputo Bay and Nhangau (the outskirts of Beira town) were also severely exposed to human pressure, land development intensity, and erosion. Indeed, Beira town was recently hit by tree extreme climatic events (Cyclone Idai in 2019; Tropical storm Chalane in 2021, and Cyclone Eloise in 2021), which may corroborate the predictions of this study – especially high exposure to erosion in central Mozambique, where Beira is located.
In Madagascar, however, all mangrove regions ranked as severely exposed, although the reasons for this exposure varied across the country. For example, Northwest, southwest, and western Madagascar were all highly exposed to erosion, while the southwest was also severely exposed to sea-level anomalies. While mangroves in Madagascar were all exposed to drought conditions, they were overall less impacted by human pressure than other countries in the region.
These variations demonstrate why it is so important to understand the main drivers behind the degradation of a certain mangrove area, so that tailored, adaptive management approaches can be applied and often limited resources appropriately directed. Depending on the driver of change, these adaptive management approaches could include establishing buffer zones, awareness-raising, enforcement of anti-logging rules, and mangrove rehabilitation and restoration. UNEP – Nairobi Convention and WIOMSA’s recent Guidelines on Mangrove Ecosystem Restoration for the Western Indian Ocean region could be helpful in this regard, as they analyze risk factors and challenges facing restoration projects and provide possible solutions. By understanding the specific drivers of change and then adapting responses accordingly, governments, donors and other managers of restoration projects can ensure that their investments are used wisely—thus reducing the amount of wasted effort and resources and increasing the benefits provided by this critical habitat.