Mangrove Forest: One of the World’s Threatened Major Tropical Environments

Study Number:

18

Author:

I. Valiela, J. L. Bowen & J. K. York

Abstract:

“We collated and revised published information to review the status of mangrove swamps worldwide. To assess the status of this major coastal environment, we compiled and examined available data to quantify the extent of mangrove forest areas in different parts of the world, the losses of mangrove forest area (deforestation) recorded during recent decades, and the relative contributions by various human activities to these losses. We first assessed current mangrove forest area in tropical countries of the world. It is difficult to judge the quality of these data in the published literature, because in many cases the methods used to obtain them were insufficiently described and the associated uncertainty was not indicated. Much information based on satellite imagery is summarized in the World Mangrove Atlas  (Spalding et al. 1997). We compared and supplemented data from the atlas with data from local or regional publications, where available, to update estimates of the area covered by mangrove forests for as many countries as possible. We then estimated long-term changes in mangrove habitat areas by compiling information on time courses of areas of mangrove forests. These data too were compiled for as many countries as published sources allowed. Few papers reported multiyear data; most of our compilation consists of data for a given country, published in different years (Table 1). To examine linkages between human activities and losses of mangrove habitats, we compared overall economic activity by compiling data on per capita gross national product (GNP; data from the Central Intelligence Agency’s World Factbook are available at Web site http://www.cia.gov). We also compared loss of mangrove habitat with absolute coastal population density (calculated as the length of coastline per country and estimating that 37% of the population lives within 100 km of the shore (coastal development) [Cohen et al. 1997]). Then, to identify the relative contribution of specific human activities responsible for losses of mangroves, we compiled published data on areas of mangrove that were lost or converted to other land covers through diverse human uses” (807-808).

Main Results and Conclusions:

• An estimate for total global mangrove coverage is as follows:
• “We estimate, from our compilation of the most recent data for all countries where mangroves have been reported, that there are roughly 1.7 x 105 km2 of mangrove habitats along the shorelines of the world (Table 2)” (808).
• “By far the largest proportions of mangroves occur in Asia and the Americas (Table 2)” (809).

• Rates of global mangrove loss are provided: “For all continents, present-day mangrove forest area is substantially smaller than the original area (deforestation), with a world average loss of 35% (Table 2). On a worldwide scale, just under 3 x 103 km2 of mangroves have been lost each year since the early 1980s, which translates into an overall aerial loss rate of 2.1% per year”(810).
• Humans are the main cause of mangrove loss:
• “In countries with reductions in area of mangrove forests (deforestation), the percentage loss of mangrove forests increased as per capita GNP increased (Figure 2a)” (811).
• “Shrimp culture (aquaculture) is, by a considerable margin, the greatest cause of mangrove loss; including losses attributable to fish culture, maricultural (aquaculture) endeavors are responsible for more than half (52%) of the losses of mangroves”(812).
• “The sum of losses from all the various human activities, across all continents, reached 36 x 103 km2 (Table 3). This sum was obtained from data from countries that hold 66% of the area of mangrove forests”(812).

• Maricultures (aquaculture) have a significant impact on mangrove habitat loss: “It is apparent that maricultural practices are responsible for the bulk of the increasing losses of mangrove swamps worldwide…Most of the damage is attributable to the direct loss of habitat from conversion of “cheap” mangrove land to valuable shrimp, prawn, and fish ponds (Figure 3)…Because the conversion of food to shrimp is incomplete, considerable amounts of organic matter and nutrients pass through the ponds, and the effluent may cause or exacerbate the eutrophication of downstream estuaries and mangrove forests (Wolanski et al. 2000)” (812).
• In conclusion, mangrove habitat loss along with tropical habitat loss in general, has and is experiencing major anthropogenic influences: “Losses of terrestrial tropical forests have justifiably received much attention recently. By the end of the 20th century, human activities converted up to 30% of the original pristine acreage of terrestrial tropical forests to other land covers (Houghton 1995). Estimates by the Food and Agriculture Organization of the United Nations (FAO 1997) suggest that 1.54 x 107 ha y-1 of terrestrial tropical forest were lost during the 1980s, and 1.37 x 107 ha y-1 during 1990–1995.These losses translate into an annual loss of 0.8% of the area of terrestrial tropical forests. Losses of coral reefs have also received considerable press and scientific attention. Best estimates are that about 10% of the world’s coral reefs have been lost and perhaps up to 30% will be degraded in 10–20 years (Wilkinson 1992). For comparison, the world’s area of mangrove forests has been reduced by about 35% on a worldwide scale since the 1980s, and 2.1% of the existing worldwide mangrove area is lost each year (deforestation). The rate is as high as 3.6% in the Americas (Table 2). Such losses of mangrove forests are alarming rates of loss of a major coastal environment” (813).

Works Cited:

[FAO] Food and Agriculture Organization of the United Nations. 1997. State of the World’s Forests. London: Oxford Words and Publications.

Houghton RA. 1995. Land use change and the carbon cycle. Global Change Biology 1: 257–287.

Wilkinson CR. 1993. Coral Reefs of the world are facing widespread devastation: Can we prevent this through sustainable management practices? Pages 11–21 in the Proceedings of the Seventh International Coral Reef Symposium; 22–27 June 1992; Guam, Micronesia.

Wolanski E, Spagnol S, Thomas S, Moore K, Alongi DM, Trott L, Davidson A. 2000. Modelling and visualizing the fate of shrimp pond effluent in a mangrove-fringed tidal creek. Estuarine, Coastal and Shelf Science 50: 85–97.