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In 2007, a group of eminent mangrove scientists warned that if mangroves continue to disappear at the current rate of 1 to 2 percent per year, within 100 years they will be gone. Not extinct as individual species — there are enough protected stands to guarantee their biological survival — but finished as providers of ecological services to the planet. As mangrove habitats become smaller and more fragmented, a tipping point is reached. They can no longer support the diversity of organisms that depend on them, or play their ecological roles.
How do mangroves serve the planet? Let us count the ways — for it turns out that mangroves are ecological Swiss Army knives, with a blade for every purpose. Some services they offer by virtue of being plants. They consume carbon dioxide, release oxygen, and create carbohydrates during photosynthesis. They form soil, store and sequester carbon, and cycle water and nutrients through the ecosystem. These processes are fundamental to life on earth.
Other services arise from mangroves’ location as trees of the tide. They buffer tropical coastlines from storm winds and waves. They capture and stabilize fine sediments. They act as biofilters, controlling nutrient runoff from the land and maintaining the quality of coastal waters on which other ecosystems such as coral reefs depend. They are also key suppliers of organic carbon to the oceans, dripfeeding a source of primary productivity to marine food webs. They provide nursery habitats and havens for marine organisms, and nesting and roosting space for birds. They are a source of pollen and nectar for bees and a source of fodder for browsing herbivores. Root to tip, they support great biodiversity.
The total value of mangrove ecosystem services has been estimated to be $10,000 per hectare per year ($4,000 per acre). In other words, when a hectare of mangroves is cut down, the environment loses $10,000 worth of annual support. Thus a 100-hectare (250-acre) shrimp farm constructed by clearing mangroves incurs an annual environmental deficit of $1 million — a cost that, if it were included in the price of the product, would take farmed shrimp off the fast-food menu.
It is precisely this point that incenses ecological economists: ecosystem services are not included in the market economy. They are referred to as externalities. The global free market thrives, say these economists, because externalities don’t appear on the balance sheet; they are a debt to nature that is never paid. Or has not been until now.
Climate change, the bellwether issue of our time, has prompted a reevaluation. Since the Industrial Revolution — the commencement of the era of carbon profligacy — developed nations have racked up a huge debt with one particular ecosystem service: carbon-dioxide storage in atmosphere and ocean. Now that bill is being collected. Warming, acidifying, rising oceans on the one hand; a warming, storming terrestrial climate on the other. It’s like the landlord paying a visit, demanding 200 years of unpaid rent.
Carbon trading — the attempt to use market mechanisms to rescue the planet from CO2 overload — requires precise accounting of carbon sources and sinks, and has brought the tools of economics to bear on the issue. A spin-off from this activity has been an effort to put a monetary value on other ecosystem services, including those provided by forests. In 1997, a team led by American ecological economist Robert Costanza estimated the total value of global ecosystem services, along with the natural capital stocks that produce them, to be between $16 trillion and $54 trillion, roughly one to three times more than the global gross national product at that time. The global economy was being subsidized at a ratio of between one-to-one and three-to-one by the environment.
In Costanza’s calculation, mangroves had among the highest per-hectare value of any ecosystem — higher than coral reefs, continental shelves, and the open sea. Not bad at all for mosquito-infested wastelands.
Among the most studied services mangroves provide — because of their commercial importance — are those to fisheries. There is the nursery role, of course. One study found that 90 percent of commercial fish species in south Florida use mangrove estuaries as habitat during some stage of their life cycle. But other services, such as nutrient provision, water filtration, and uptake of agricultural chemicals, are also significant. A study of fish landings at 13 sites in the Gulf of California recently found that yields were directly proportional to the length of coastline inhabited by mangroves. The researchers determined that the presence of mangroves was worth $37,500 per hectare ($15,000 per acre) per year for its fish-related services. This figure is 600 times the value the Mexican government places on mangrove land, which is acquired cheaply for tourism development and conversion to shrimp farms.
In 1994, a group of scientists estimated that for intensive shrimp farming to be environmentally sustainable, each farm would need an area of mangroves 35 to 190 times its size to provide fish meal, clean water, and brood stock, and to assimilate its wastes. For each calorie of edible shrimp protein produced by a shrimp farm, they estimated that approximately 295 calories of ecosystem work was required. Suppose each calorie of shrimp had a market price of one cent, and that the ecosystem services were valued at the same rate. In that case, the environment would be discounting the price of shrimp to the tune of $2.95 per calorie, or roughly $1,500 per pound of shrimp. Pink gold, indeed.
But this discount doesn’t include the environmental liabilities of pond aquaculture that don’t relate to mangroves, such as disruption of natural tidal flows, creek blockage, salinization of ground water, acidification of soil, release of toxic wastes and excess nutrients into coastal waters, overexploitation of wild larvae, and destructive bycatch of other marine life. None of these damages are accounted for in the market price of the farmed product; they are externalities — costs paid by the environment.
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