donderdag 15 mei 2014

Matteo's mangrove paper

Go here for the very first peer-reviewed article of our former MSc student, Matteo Zavalloni. The article quantifies the trade-off between two different alternative uses of a mangrove ecosystem and finds that it is crucial to take into account spatial links in this process.

The general idea behind the paper is that a mangrove ecosystem can serve many different purposes, but as space is limited you cannot have them all: in other words, we have a typical economic problem of satisfying wants under limited resources. In his paper Matteo (and I, and Paul van Zwieten) focused on the trade-off between two uses: cultivating shrimp in aquaculture ponds; or providing nursery habitat for juvenile wild shrimp. There is a lot more to mangrove ecosystems than just those two functions: tourists also like to come to mangrove forests (I loved the mangrove forest in Ca Mau, for example), and mangroves also form important coastal protection. In this paper, however, we wanted to explore some methodological issues in a spatially explicit manner, and these two uses were the most appropriate for this analysis.

So Matteo developed a model that maximizes the mangrove forest's nursery function, under the restriction that aquaculture production should not drop below some given level. If you run the model for many different levels of aquaculture production, you typically get a picture like this:
(Be aware that this is not the original picture in the paper but a stylized version of it.)

In economics lingo we call this a production possibilities frontier (PPF). It shows all combinations of two goals (in this case aquaculture production and nursery habitat) that are maximally attainable. Combinations above the curve are impossible to attain; combinations below the curve are feasible, but not very efficient.

Why are there two curves? The green curve indicates the PPF you get if you use all available information on differences in habitat quality, and you take into account that in order to function as a nursery, a mangrove forest must in some way be connected to the water course. The blue curve indicates the PPF if you would ignore (or simply not know) that last piece of information. You see that if you ignore the connectivity you arrive at solutions that provide much less benefits in terms of nursery habitat than what would in theory be possible. The reason is that you locate your shrimp farms at the wrong places:

Maximum provision of the nursery ecosystem service at 24% of the possible aquaculture benefits, with and without taking into account the connectivity of nursery habitat
Note that the left picture has all aquaculture clustered together in the west corner of the study area, whereas the right picture has aquaculture located along the water course, blocking the mangrove forest from access to the river. The reason is that farms near the river have lower transport costs, but the consequences for the nursery function are not taken into account.

So how does this help us? First, we wanted to demonstrate that conservation, no matter how noble, has costs, and that those costs should be considered in policy decisions. Second, it illustrates that functions can be combined if you use your information wisely. The approach we developed is one small step towards methods to find the best compromise between different interests in coastal zones.

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