It is surprisingly difficult to give a single, definitive response to this seemingly simple question, so in 2008, Arturo Ruiz-Luna and a group of researchers set out to answer the question once and for all, by taking a close look at the available data and its reliability.
The first challenge is to define precisely what counts as “an area of mangroves”. Since the edges of any one type of vegetation tend to merge gradually into the next, the boundaries between types are often not clear or discrete. The second challenge is to find comparable maps or records of the extent of mangroves in former times. In many cases, earlier maps were based on simple aerial photography as opposed to the more sophisticated satellite imagery available today. Equally, studies relying on different technology may produce quite different results.
Ruiz-Luna and his colleagues concluded that there was a high degree of uncertainty over the true extent of mangrove cover in Mexico and over the rates of mangrove depletion. The unreliability stemmed from the very different estimates of mangrove cover given in earlier studies.
They found that the earliest estimate of the extent of mangroves in Mexico, made in 1973, was 700,000 ha (1.7 million acres). From 1980-1991, a figure of 660,000 ha was widely quoted. This is thought to have been derived from the previous figure using a linear regression to include estimated deforestation rates.
On the other hand, a study in 1992, based on 1:3,800,000 scale maps, arrived at an areal extent of 932,800 ha. A 1993 government estimate, based on satellite imagery and supporting ground survey, came up with a figure of 721,554 ha. A 1994 estimate, also using satellite data, arrived at a similar figure.
Since 2000, estimates vary from a low of 440,000 ha (based on deforestation rates and linear regression) to a high of 955,866 (almost all of it mangrove, with a small area of secondary mangrove succession).
According to Ruiz-Luna and his co-authors, it is therefore likely that the true value of the extent of mangroves lies somewhere between 600,000 and 900,000 ha, with the authors plumping for 800,000 ha as a reasonable estimate.
Why are mangroves lost?
Mangrove habitats are lost due to damage (deforestation) from logging and land use changes. Examples of land use changes adversely affecting the extent of mangroves are:
- the conversion of mangroves to harbors, as in Manzanillo (Colima)
- mangroves being converted for hotels and tourist use (Cancún)
- mangrove swamps being reassigned for aquaculture (San Blas)
The loss of mangroves can also result from changes to hydrological systems on the landward side of the mangroves. For example, when an artificial channel was opened in the 1970s in Cuautla (Nayarit) to connect the Marismas Nacionales with the sea, the channel was originally about 50m wide. It is now about 600m wide in the middle, and more than 1000m wide at its mouth, due to damage caused by the river flow, and from the greater exposure of mangroves to higher salinity water, as well as increased mangrove mortality from storms and hurricanes.
While there are some small-scale projects to replant mangroves in some tourist area, this is unlikely to be a good substitute for the original mature mangrove ecosystem. This is why Geo-Mexico is happy to help publicize public protests against mangrove destruction, such as the one pictured above which took place at the Cancún climate summit in 2010.
Arturo Ruiz-Luna, Joanna Acosta-Velázquez, César A. Berlanga-Robles. 2008. “On the reliability of the data of the extent of mangroves: A case study in Mexico.” Ocean and Coastal Management 51 (2008) 342-351.
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