Economic and Ecological Effects of Sea Level Rise on Coastal Wetlands: A Case Study from Galveston Island, Texas

Coastal salt marsh wetland plants are expected to migrate upslope with the rise in sea level, but human development is expected to limit the potential migration. Feagin et al. (2010) explored the ecological and economic effects of projected Intergovernmental Panel on Climate Change<!–[if supportFields]> XE “Intergovernmental Panel on Climate Change (IPCC)” <![endif]–><!–[if supportFields]><![endif]–> (IPCC) 2007 report sea level changes at the plant community scale using the highest horizontal and vertical resolution data available. Their findings demonstrate that salt marshes do not always lose land with increasing rates of sea level rise. The lower bound of the IPCC 2007 potential rise actually increased the total marsh area, resulting in a net gain in ecosystem service values on public property. The upper rise scenario resulted in both public and private economic losses for this same area. Overall, Feagin et al. highlight the trade-offs between public and privately held value under the various IPCC 2007 climate change scenarios. As wetlands migrate inland into urbanized regions, their survival is likely to be dependent on the rate of return on property and housing investments.—Michelle Schulte
Feagin, R.A., Martinez, M.L. Mendoza-Gonzalez, G, Costanza, R., 2010. Salt marsh zonal migration and ecosystem service change in response to global sea level rise: A case study from an urban<!–[if supportFields]> XE “urban” <![endif]–><!–[if supportFields]><![endif]–> region. Ecology and Society 15, 14–32.

The authors chose Galveston Island, Texas, USA as the study site for projected sea level rise as the sea level has been well documented in the past. The coastal salt marshes at the study site exhibited the zonation patterns common to other Spartina alterniflora-dominated marshes in the U.S.. Five plant community zones have been previously defined as: open water, low marsh, salt flat, high marsh, and upland. The authors then created a map of the plant community zone based on elevation using the highest horizontal (1 m) and vertical (0.01 m) resolution Light Detection And Ranging (LIDAR) data available. Because the resolution of this model is quite fine, spatially and species-wise, Feagin et al. illustrate the effect of sea level rise within a discrete 6 x 6 km extent. Three IPCC<!–[if supportFields]> XE “Intergovernmental Panel on Climate Change (IPCC)” <![endif]–><!–[if supportFields]><![endif]–> scenarios were implemented in a time step fashion up to the year 2095 using a low rise (0.18 m increase in sea level), a mid rise (0.39 m), and a high maximum rise (0.59 m).
After running the model, the expected plant habitat loss/gain was calculated for all of the scenarios, both including and excluding potential barriers to plant migration. The authors’ goal was to best represent the different plant community zones in this salt marsh relative to one another, in terms of market and non-market based values. The ecosystem services being provided by each plant community at the study site were identified. Monetary values were associated with recreation, hunting and bird watching tourism values, carbon sequestration<!–[if supportFields]> XE “carbon sequestration” <![endif]–><!–[if supportFields]><![endif]–>, storm protection, fisheries support, and market-based property appraisal values. To estimate gains and losses, the authors calculated ecosystem service values considering the different areas covered by each ecosystem, given the modeled climate change.
The zonal migration of the plant community zones primarily depends on the relative sea level rise rate, the accretion rate as specific to zone and location, and the availability of land at a suitable base elevation. Also, the choice of whether to remove human-erected barriers greatly affects the availability of the land on which this migration could occur. There is no significant change, except at the high-marsh-to-upland interface, in the plant community zones for the low-rise scenario between 2005 and 2095. The rate of sea level rise equaled the rate of accretion. The anthropogenic barriers limited plant migration in the upland areas while protecting the developed land. In the mid-rise scenario, there was a net loss of Spartina alterniflora-dominated low marsh. There was, however, a net gain of salt flats and high marsh as these two plant communities found more locations at suitable elevations as they migrated upslope. Under the IPCC<!–[if supportFields]> XE “Intergovernmental Panel on Climate Change (IPCC)” <![endif]–><!–[if supportFields]><![endif]–> high scenario, the low marsh and salt flat zones surprisingly fared better than in the mid rise scenario because of the topographic relief. The slope appeared to be the primary factor in determining the plant community distribution in the study area.
In the different SLR<!–[if supportFields]> XE “sea-level rise (SLR)” <![endif]–><!–[if supportFields]><![endif]–> scenarios, it was predicted that the economic losses will generally outweigh the gains. The models indicated that there will only be economic gains in Spartina alterniflora-dominated low marshes during a low rise event. The uplands, with large property appraisal values, are likely to show large economic losses in all the projected scenarios. In addition, if property investments accumulate at a 3% rate, the net economic value will be greater when the barriers to plant migration are removed. But if this value increases to a rate of 6%, then the optimal solution is to leave the barriers in place. This divergence highlights the trade-offs between public and private value because low marshes and open water are on public property; they are navigable waters and sit below the mean tide line.
This study shows that a salt marsh does not always lose land with increasing rates of sea level rise. The response of each individual plant community zone is more nuanced. Direct human activities and intervention in the migration process are estimated to account for the large majority of the losses that are predicted to occur this century. Rising sea levels and inflating property values will likely interact to reduce the incentive to save wetlands. The results show that the financial incentive to secure private property with barriers will increase by several orders of magnitude, given the IPCC<!–[if supportFields]> XE “Intergovernmental Panel on Climate Change (IPCC)” <![endif]–><!–[if supportFields]><![endif]–> high SLR<!–[if supportFields]> XE “sea-level rise (SLR)” <![endif]–><!–[if supportFields]><![endif]–> scenario over the low-rise scenario. In conclusion, as wetlands migrate landward, their survival is also dependent on the rate of return on property and housing investments. Local conditions and human proclivities will radically differentiate the benefit and costs of sea level rise around the world.

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