Threats to Mangroves

There are approximately 70 species of mangroves around the world (Polidoro et al. 2010). When activities such as logging, shrimp farming, coastal agriculture, hotel development, and other activities are valued over the ecosystem services the intact mangroves provide, genetic diversity is among the first—but least considered—casualty. The trees and associated species (e.g., birds, snakes, crabs) are visibly lost, but so too are the specific genotypes and phenotypes that have evolved in microhabitats around the world to withstand insects, tidal fluctuations, precipitation patterns and salinity regimes. Mangroves are not species-rich to begin with, especially in comparison with other tropical forests (Alongi 2002). And in the areas where replanting is attempted, it is often done with seeds from elsewhere, and often done with one species, rather than the mix of species that originally existed. As Polidoro et al. (2010) have stated:

“Although regeneration of degraded mangrove areas is thought to be a viable option in some areas (Saenger 2002, Walters et al. 2008), successful regeneration is generally only achieved by the planting of monocultures of fast-growing species, such as Rhizophora or Avicenna species. Many rare and slow growing species are not replaced (Alongi 2002), and many species cannot be easily replanted with success. In sum, mangrove areas may be able to be rehabilitated in some regions, but species and ecosystems cannot be effectively restored” (p. 8).

Thus, once these species and subspecies are lost, they cannot be recovered, no matter how many well-intentioned restoration efforts are made. The solution to ensuring the mangroves are there to provide the ecosystem services we depend on is not to remove them in the first place.

Climate change is causing two important impacts along the world’s coastlines. Sea levels are rising and the chemistry of the oceans is shifting (IPCC 2007 and NRC 2011). The rates at which these impacts are occurring is likely to exceed the ability of mangrove forests and the species that live within them to adapt (Gilman et al. 2008).

In areas with upland coastal development, mangroves cannot retreat landward. Mangroves around the world are adapted to specific tidal regimes. If they spend increasing amounts of time inundated, at some point they will not be able to rid themselves of the ocean salt quickly enough, and will whither and die. They will also not receive the nutrients and sediment from freshwater flowing seaward that they require to survive. Compounding this, as the ocean becomes more acidic, mangrove animals with shells and other hard structures (e.g., oysters, snails, crabs, sea stars) will have an increasingly difficult time taking up calcium carbonate from seawater, leaving them with developmental deformities and thin, less protective shells as adults (Doney et al. 2012). And coral reefs—already stressed by increasing temperatures and bleaching—will likewise be weakened by increasing acidity (Hoegh-Guldberg et al. 2007). Many species—including many commercial fishery species—rely on both mangrove and reef habitats during different parts of their life cycle. Climate change directly threatens these habitats, which are responsible for providing food to billions of people around the world. Mangroves provide the critical nursery and adult habitats for 30-80% of commercial fisheries around the world (Rönnbäck 1999). Finally, mangroves are among the most important carbon sinks on the planet (Donato et al. 2011). Losing them will cause even greater carbon releases (Donato et al. 2011), creating a positive feedback loop that will further exacerbate sea level rise and increased ocean acidity.

Most destructive uses of mangrove forests require their removal. The motivations behind deforestation include direct use of the mangrove wood and leaf products, use of the wetland habitat, or complete fill and conversion for coastal developments.

Deforestation for fuel & timber accounts for the ongoing loss of approximately 26 percent of existing mangroves (Valiela et al. 2001). Mangrove reforestation has had very low success, although new hydrology-based methods may be more promising (Lewis & Gilmore 2007). Even so, we cannot rely on reforestation to prevent mangrove loss. These fragile and rare ecosystems are being lost at such a tremendous rate that mangrove experts predict that without changes to current practices, mangroves will be functionally extinct in less than a century (Duke et al. 2007). A world without mangroves means a world without most fisheries, without bioshields from storms, and without many bird and other species. The loss of mangroves as a unique habitat would directly jeopardize more than a billion of the world’s human population.