J. N. Sanchirico & P. Mumby
Habitats and the ecosystem services they provide are part of the world’s portfolio of natural capital assets. Like many components of this portfolio, it is difficult to assess the full economic value of these services, which tends to over-emphasize the value of extractive activities such as coastal development. Building on recent ecological studies of species–habitat linkages, we use a bioeconomic model to value multiple types of habitats as natural capital, using mangroves, sea grass, and coral reefs as our model system. We show how key ecological variables and processes, including obligate and facultative behaviors map into habitat values and how the valuation of these ecological processes can inform decisions regarding coastal development (habitat clearing). Our stylized modeling framework also provides a clear and concise road map for researchers interested in understanding how to make the link between ecosystem function, ecosystem service, and conservation policy decisions. Our findings also highlight the importance of additional ecological research into how species utilize habitats and that this research is not just important for ecological science, but it can and will influence ecosystem service values that, in turn, will impact coastal land-use decisions. While refining valuation methods is not necessarily going to lead to more rational coastal land use decisions, it will improve our understanding on the ecological–economic mechanisms that contribute to the value of our natural capital assets.
Main Results and Conclusions:
- Overview: “The contributions of the paper are: (1) to illuminate how key ecological variables and processes, including different species–habitat associations map into ecosystem service values and (2) to show how the valuation of these ecological processes maps into the opportunity cost of clearing the mangroves that can be used to inform decisions regarding coastal development (habitat clearing)”(68).
- Obligate (necessary) and facultative (unnecessary but supplementary) use of mangrove ecosystems by reef-fish is used to help determine the economic value of the ecosystem studied (68).
- An overview of the bioeconomic model is as follows: “The key findings in Mumby et al. (2004) that guide the development of our bioeconomic model are: (1) the availability of different juvenile habitats (sea grass beds and mangroves) partly determine the biomass of adult fish on coral reefs because survival rates of juveniles vary among habitats; and (2) fish species–mangrove functional relationships in coral-reef systems tend to be facultative rather than obligate… We describe the biological and economic model and then the calculation of mangrove habitat value, based on mangroves serving as an input into the “production” of the fish population being harvested”(68).
- The results of the article explain the ways by which the value of mangrove habitat was derived by examining the total value of the fishery, the average value of the fishery, and the opportunity cost of clearly mangroves.
- Total fishery value is explained: “Figure 2c and d shows that for a given effort level, the total value of the fishery increases as the mangrove availability increases but at a decreasing rate. We also find that a complex interaction emerges among the level of fishing effort, mangrove coverage, and total fishery value, where the effort level that results in the highest total value depends critically on the level of the mangrove coverage. We also find that the greater the amount of fishing effort level, the greater is the minimum threshold of mangrove coverage needed to ensure that the value of the fishery is positive” (72-73).
- Average fishery value is explained: “Because many studies of habitat valuation use average values (Costanza et al. 1997), we also show how the average value of the fishery with respect to fishing effort (V (ET,L)/ET=Leq), and the average value with respect to mangroves (V(ET,L)/M) varies with the coverage of mangroves (Fig. 3). Intuitively, the license price (Leq) is the amount that a vessel owner would be willing to pay (e.g., per hour) to fish on the reef. In the facultative case (panel A) and the obligate setting (panel B), the shape of the license price increases at a decreasing rate in the level of mangroves, which is driven by the properties of W(M). We also find that there is endogenous minimum threshold of mangroves needed at a given fishing effort level for the fishery to be become profitable” (73).
- Facultative and obligate association were important factors in the assessment of the opportunity cost of clearing mangroves:
- “For a facultative association, we find that the opportunity cost increases with the amount of mangroves cleared and that it increases with increased fishing effort levels (see Fig. S2 in supplementary material). The coastal land use implication is that with higher fishing effort levels, it pays to clear less habitat, everything else being equal. Qualitatively similar results hold for the obligate association. It is also easy to show that the opportunity costs decrease with improved conditions in the sea grass beds (dV/dMdθ<0). This is because with greater numbers of juveniles surviving in the sea grass beds (θ ↑), the number of recruits to the reef is less dependent on the predation refuge in the mangroves” (74).
- “We find that the opportunity cost is higher for obligate than for facultative associations (Fig. 4a). Therefore, even though the total and average values from fishing were lower for the obligate relationship, we find that the marginal value is higher due to the greater (marginal) effect that mangroves have on the value of the fishery in the obligate rather than facultative setting. An obligate habitat association, therefore, results in less conversion of the coastal environment than when the association is facultative” (75).
- Due to the fact that, sometimes, facultative association can show maximum returns by clearing an entire mangrove habitat, “Our results illustrate the importance of utilizing the incremental (or marginal) value for policy decisions (Bockstael et al. 2000), as opposed to total or average values that are commonly used in habitat valuation studies (Costanza 1997).
Bockstael NE, Freeman AM et al. (2000) On measuring economic values for nature. Environ Sci Technol 34:1384–1389 doi:10.1021/es990673l.
Costanza R, d’Arge R et al. (1997) The value of the world’s ecosystem services and natural capital. Nature 387(6630):253–260 doi:10.1038/387253a0.
Mumby PJ, Edwards AJ et al. (2004) Mangroves enhance the biomass of coral reef fish communities in the Caribbean. Nature 427 (6974):533–536 doi:10.1038/nature02286.