Mangroves growing on the edge of a channel, showing distinctive prop roots of the black mangroves, Rhizophora mucronata (Kiunga, Kenya).
© David Obura

Mangrove seeds develop on the tree into seedlings, the spear shape helps them to float upright and then root in soft muds when deposited in a new location for growth.
© Rudy van der Elst


Mangrove forests are among the most common of coastal habitats in the WIO, typically in river estuaries, with smaller stands in enclosed lagoons and on sheltered open sea coasts. There are ten species of mangrove trees in the WIO, most of which are distributed widely, and a variety of shrubs and palms that also grow in mangrove forests.  Mangroves are characterized by their resistance to salt water, enabling them to grow in brackish and full-salinity seawater, and by their ability to root in mud or sand.
The WIO has an estimated 7,900 km² of mangroves, 5% of the world’s total. WIO mangroves represent a subset of the species found in the West Pacific region, isolated by the expanse of the Indian Ocean and the arid coastlines of the Middle East. They may thus represent a distinct subregion of the Indo-West Pacific mangrove fauna and flora.
The best-developed mangrove forests occur around river mouths where they are important in trapping river sediments that would otherwise be washed out to sea. Madagascar and Mozambique have the largest mangrove areas in the region with 2990 and 2910 km² respectively, along their coastlines in the Mozambique channel. The largest contiguous stand of mangroves in the WIO is the Rufiji delta (Tanzania, 480 km²), followed by Mahajamba (Madagascar, 420 km²), then the Zambezi and Ruvuma rivers. Coastal island chains, such as the Lamu Archipelago in northern Kenya and Quirimbas in northern Mozambique have extensive mangrove formations in river estuaries and sheltered shorelines. South Africa supports a small extent of mangrove, among the southern-most mangroves in the world, at 32°S.
In the island states, the high islands have no major estuaries, support little mangrove growth except for Mayotte and the south coast of Moheli, Comoros. By contrast, the coralline islands with major lagoons support significant mangroves, in particular Aldabra (20 km²), Cosmoledo and Europa (Scattered Islands, 7 km²).

Significance - Mangroves are among the most productive habitats on earth and have tremendous social and ecological value. Wood production and growth rates are very high. Mangroves support complex and rich food webs, due to high primary production, influx of nutrients from rivers and the complex structures provided by the mangrove roots. They provide sheltered nursery grounds for many fish and invertebrate species that spend their adult lives in other coastal and pelagic ecosystems.
Human use of mangroves is extensive, with harvest of molluscs, crustaceans, and fish as well as of wood for fuel, charcoal, timber, and wood chips. Mangrove wood is extremely hard, insect-resistant and with a high energy content as fuel. More indirect ecosystem goods and services include the role of mangroves as nurseries for economically important fisheries, especially for shrimp, and others such as filtering and trapping of pollutants, stabilization of coastal land by trapping sediment and protection against storm damage.
Mangrove forests are perhaps the most efficient ecosystem for carbon sequestration, with estimates of carbon storage of 500 – 1300 tonnes/ha (or 50-130 g/m²), of which 70% is trapped in the sediment. This is from 1-3 times the amount of carbon stored in primary tropical forests.
Taken altogether, the annual economic value of mangroves, estimated by the cost of the products and services they provide, has been estimated at between $2,000 and $9,000 per ha per year.

Mangrove roots underwater at high tide – at low tide these are exposed to the air to breath oxygen, enabling mangroves to live in oxygen-poor silt and sand in estuaries.
© Keith Ellenbogen

Mangroves are among the few marine ecosystems that can successfully be replanted and rehabilitated by manual planting of seedlings.
© Rudy van der Elst

Threats - Because mangrove ecosystems have tremendous value for coastal communities and associated species, they are being destroyed at alarming rates. Over the last 50 years, about one-third of the world’s mangrove forests have been lost. Within this region, mangrove decline is estimated to have been about 8%, from 1980 to 2005, as against global projections of 25% decline by 2025.
The harvesting of mangrove trees has persisted for centuries in East Africa, with export of poles to Oman and the Middle East being a staple of trade throughout this time, and continuing today. Uncontrolled cutting of mangroves has cleared large areas of previously productive forest. Mangrove forests are also the first to be cleared for the construction of salt pans from which most of the region’s sea salt is produced. Additional pressure from tourism developers, coastal construction, farmers and the ever-growing need for fuel wood, further encourages large swathes of primary mangrove forest to be cut indiscriminately with little or no replanting. Climate change, in particular sea level rise, is a growing threat to mangroves, as their vertical zonation is pronounced, and many mangrove systems may have little space to recede inland as sea levels rise, due to topographic factors and blockage by developed and altered ecosystems on their landward boundary.


Mangrove species found in the WIO

  • Brugeria gymnorrhiza – variable salinity, often mixed with Rhizophora and Ceriops zones; coppices well and poles used in housing.
  • Rhizophora mucronata – found in the upper eulittoral forming extensive pure strands in estuarine conditions. Good source of tannins for leather and poles used for houses.
  • Ceriops tagal – littoral fringe on landward side of forests. Strong poles resistant to termite attack.
  • Avicennia marina – found on compact substrates, sand flats and new sediment. Tolerates high salinity and varied flooding regimes thus widespeard throughout the mangrove forest, found either on the landward margin as forest stands, or seaward side mixed in with Sonneratia, Ceriops and Xylocarpus. High tolerance therefore the most widely distributed mangrove species in the WIO. Copices well timber used for construction, trunks usedfor boats and flowers used for honey.
  • Lumnitzera racemosa – a landward mangrove in the littoral fringe where there is an influence of fresh water. Good fuel wood and coppices well.
  • Heritera littoralis – found in the littoral fringe or riverine, especially estuaries in sandy loan and inland areas. Wood very tough used in ships, boat masts.
  • Sonneratia alba – found in the upper eulittoral. Arial roots used by fishers as floats. Tree coppices well.
  • Pemphis acidula – found in the supralittoral fringe, common landward reaches of mangroves on sand beaches and more inland.
  • Xylocarpus granatum, X. moluccensis – usually landward margin of mangrove forests in the littoral fringe where there is an influence of fresh water. Fruits are known for their medicinal properties. Regarded as a mangrove associate due to the absence of specializations for coping with saline conditions.

Management, mitigation and restoration - In the WIO mangroves are more commonly managed as forest reserves than for biodiversity, falling under forestry regulations, and being viewed more from a utilization perspective than conservation. Their role as ecosystem service providers to a broad diversity of other systems (e.g. coral reefs, fisheries, prawns, land protection) generally goes unrecognized and therefore is not valued in competition with development alternatives. This is, however, changing in some countries, where community dependence on mangroves and local tourism are combining in the form of visitor-oriented boardwalks to enable tourists to experience the mangroves. Madagascar and Mozambique, with the largest area of mangroves in the region, also have the lowest coverage of existing protected areas in mangrove systems.
In restoration and reforestation of mangroves, Kenya is among the leading countries in both research and in community efforts at restoration, with broad replication of the latter throughout the region. The role of mangroves in sequestering carbon dioxide is a new issue for the region to deal with, and may potentially place high value on avoided deforestation of mangroves systems and thus their conservation, as well as on reforestation.
The World Heritage Convention, Ramsar Convention and Man and the Biosphere (MAB) programme are three international instruments relevant to protection of mangrove systems, and all of them have some coverage in the WIO.

Key References - Alongi (2002); Andriamalal (2007); Ellison and Stoddart 1991; Erftemeijer and Hamerlynck (2005); Ewel et al. (1998); FAO and UNEP 1981; IUCN (2004); Kennish MJ (2002); Lebigre (1990); McLeod and Salm (2006); Murray et al. (2011); Primavera (1997); Tomlinson 1986; UNEP (1994); Wells et al. (2006); Wilkie and S Fortuna (2003). --> References