Mangroves in coastal Thailand are the main protection against deadly flooding from tsunamis, so it might seem wise to protect them at all costs. However, ripping out a few mangroves and replacing them with shrimp farms, an important local industry, doesn't necessarily have to reduce the plants' power to blunt tsunamis. And in that observation lies a fresh, quantitative approach to how policy makers can protect the environment and allow growth and development that improve local residents' lives.
So says a University of Florida zoologist and co-author of a paper on the topic set to appear this week in the journal Science.
Brian Silliman, a UF assistant professor of zoology, said governments and managers worldwide are leaning toward a system known as 'ecosystem-based management' to achieve environmental protection goals. Contrasting traditional techniques that focus on single species, ecosystem-based management seeks to conserve not only species but also habitats and the services they provide to humans by conserving entire ecosystems.
Under ecosystem-based management, policy makers in Thailand would not only take into account its mangroves, but also the needs of shrimp farmers, threats from ocean pollution, potential damage from storms, and other factors, Silliman said.
Shrimp farming in Thailand has led to destruction of thousands of acres of mangroves and is a major environmental issue in Asia. In many areas, mangroves no longer exist, leaving shoreline towns and cities unprotected from tsunamis. To increase mangroves' protective services, governments must restore or conserve them.
For Silliman and his colleagues, the question was, can some mangroves be converted to shrimp farms without losing too much storm buffering?
Silliman said that managers might make a common assumption: The amount of benefits from a natural amenity - whether sea grass, forests or mangroves - are linked directly to its size. So, more mangroves would mean proportionally more storm surge protection, more habitat for juvenile fish and more pollution-filtering capacity.
That assumption inevitably leads to either-or conclusions about environmental protection and development, Silliman said. If more mangroves always means more environmental benefits, then none should be destroyed.
The main point of the Science paper is that assumption and its inevitable conclusion are not always right and should be questioned.
Ecologists have long understood that species reach thresholds at which their environmental benefits to humans are greatest, and saturation points at which those benefits trail off, Silliman said. In other words, the relationship between the amount of services provided by an ecosystem and the area of that ecosystem is not a straight line.
When the authors reviewed studies of the mangroves and how they buffer tsunamis, it quickly became clear that the mangroves don't offer much protection until they cover a certain critical area, Silliman said. And their protection doesn't get much better after this area reaches a certain, 'saturation' size compared to the size of the vulnerable coast they shelter.
Merging this ecological knowledge and economic valuation theory to create a technique useful in many other scenarios, the researchers assigned dollar values to the mangroves' protective powers, and then compared those values to the dollars earned from shrimp farming. Their conclusion: Small losses of between 10 and 20 percent of mangroves, where massive expanses of mangroves already exist, are outweighed by gains by shrimp farmers.
In other words, as long as farmers don't destroy too many of the plants, they can uproot some mangroves, build shrimp ponds and make money - and the remaining mangroves will still protect the shoreline from tsunami storm surges.
'It makes intuitive sense when you say, 'both sides can benefit,' but the important quantitative question is, 'How much land can mangroves give up and not lose that most important service of protection against tsunamis,'' Silliman said. 'We provide a technique to answer that question based on combining some basic principles of ecology and economics.'
That said, Silliman noted that the argument assumes there are plenty of mangroves. If most have already been destroyed as is the case in most areas, then restoration, not shrimp farming, is essential, he said. The research was funded by the National Centre of Ecological Analysis and Synthesis, and the Packard Foundation, which encourages interdisciplinary research. The authors relied on published studies as part the centre's goal of tying together seemingly unrelated research to find new approaches to solving problems.