A spatial model of coastal ecosystem services

Author:

E. B. Barbier

Abstract:

Evidence suggests that the ecological functions underlying many ecosystem goods and services are spatially variable. For coastal systems, a simple model is developed incorporating a spatial production function that declines across an ecological landscape. The basic model demonstrates how spatial production of ecosystem services affects the location and extent of landscape conversion. An extension allows for the risk of ecological collapse, when the critical size of the remaining landscape that precipitates the collapse is not known. Both models are simulated using the example of spatial variation in ecosystem services across a mangrove habitat that might be converted to shrimp aquaculture.

Main Results and Conclusions:

• In simulation experiments, observations were made regarding ecosystem services provided by mangroves: “three benefits, which accrue mainly to local coastal communities in vicinity of the mangroves (Barbier, 2007; Barbier and Sathirathai, 2004; Sudtongkong and Webb, 2008). These are the role of mangroves as natural “barriers” to periodic damaging coastal storm events, their role as nursery and breeding habitats for offshore fisheries, and the exploitation of mangrove forests by coastal communities for a variety of wood and non-wood products” (74).
• The seaward fringe of mangrove ecosystems is very important: “Whereas it is difficult to determine whether the various resource products collected by local communities vary significantly across the mangrove landscape, the ecological evidence reviewed in this paper suggests strongly that the seaward fringe of the mangroves offer greater protection against storms and contain higher density of fish species that eventually populate near-shore fisheries” (75).
• Ecological collapse is more likely to occur when greater proportions of habitat are removed: “…Dobson et al. (2006) find evidence that the proportion of species remaining, species diversity and the trophic level of ecosystems tend to fall exponentially as the proportion of ecosystem habitat remaining declines. Similarly, for Thailand, it was found that pollution discharge from shrimp ponds in converted mangrove areas tend to decrease the growth rates and increase the mortality of the remaining mangroves (Vaiphasa et al., 2007). Both these phenomena suggest that the probability of ecological collapse will also decline exponentially as the proportion of habitat remaining becomes larger” (76).
• In terms of economic returns and conservation, Barbier’s findings suggest that “…the value of spatially declining production of ecosystem services equals the marginal economic returns foregone from converting the mangrove landscape to shrimp aquaculture…The result is that it is now optimal to preserve mangroves up to 869 m from the seaward boundary, and convert only the landward fringe of the coastal landscape to shrimp ponds”(76).
• In conclusion:
  • “This paper has shown how the spatial distribution of ecosystem services and the risk of ecological collapse can affect the allocation of an ecological landscape between preservation and coastal development options. The justification of this approach is from ecological evidence of spatial variation in the ecological production functions underlying key goods and services across landscapes… The resulting storm protection and support for near-shore fisheries provided by coastal ecosystems will also be greater for the seaward fringe compared to further inland. Although the critical landscape size that leads to the demise of the ecosystem is unknown, the risk of collapse is likely to increase with a fall in ecosystem area, which can be measured by the declining spatial distance across the natural landscape” (77).
  • Coastal development should be considered on a case-by-case basis and should not be generalized: “One interesting outcome is that, when conversion of the landscape is optimal, it always occurs in certain locations only. For example, if the seaward fringe of coastal ecosystems appears to be more valuable in terms of generating key ecosystem services, and if marginal development rents are constant across the landscape, then any conversion of the landscape should take place further inland”(77).
  • The more of an ecosystem there is in existence, the better chance it has against ecological collapse: “Incorporating the risk of ecological collapse is also extremely important to the landscape conservation decision. If the survival of the ecosystem is positively influenced by the scale of the landscape conserved, then there is an additional benefit of holding onto more landscape to ensure ecosystem survival”(77).

Works Cited:

Barbier, E. B. 2007. Valuing ecosystems as productive inputs. Economic Policy 22: 177-229.

Barbier, E. B., and S. Sathirathai, (Eds.). 2004. Shrimp Farming and Mangrove Loss in Thailand. Edward Elgar, London.

Dobson, A., Lodge, D., Alder, J., Cumming, G.S., Keymer, J., McGlade, J., Mooney, H., Rusak, J.A., Sala, O., Wolters, V., Wall, D., Winfree, R., and M. A. Xenopoulos. 2006. Habitat loss, trophic collapse, and the decline of ecosystem services. Ecology 87: 1915–1924.

Sudtongkong, C., and E. L. Webb. 2008. Outcomes of state- vs. community-based mangrove management in southern Thailand. Ecology and Society 13: 27.

Vaiphasa, C., de Boer, W.F., Skidmore, A.K., Panitchart, S., Vaiphasa, T., Bamrongrugsa, N., and P. Santitamnont. 2007. Impact of solid shrimp pond waste materials on mangrove growth and mortality: A case study from Pak Phanang, Thailand. Hydrobiologia 591: 47–57.