Wetland Patch Size
This layer is one of the South Atlantic LCC indicators in the estuarine ecosystem. It is an index based on the size of estuarine patches.
Reason for Selection
Larger, better connected wetlands are positively associated with fish (Meynecke et al. 2008), shrimp (Turner 1977), and marsh birds. In particular, seaside sparrow (Benoit and Askins 2002, Rush et al. 2009), saltmarsh sharp-tailed sparrow (Benoit and Askins 2002), and marsh wren (Rush et al. 2009) have been associated with marsh area effects. Other species are expected to be limited based on home range size. In addition, wave attenuation is increased with wetland area (Shepard et al. 2011). Over time, a decrease in patch size will correspond to marsh degradation and wetland loss. As a wide variety of marsh patch sizes correspond to species and ecosystem services (noted above), the quantile index is a simple, objective way of quantifying the functional value of different patch sizes. Since the index is dependent on the NLCD, future monitoring will be relatively easy. A decrease in patch size will be an early sign of marsh degradation.
Input Data
-- National Wetlands Inventory (NWI) (accessed 15 May 2014) - "Estuarine and Marine Wetland" class
-- National Land Cover Database 2011 (NLCD 2011) - "Emergent Wetland" class
-- For the Atlantic coast, we used an NWI update by The Nature Conservancy's South Atlantic Marine Bight Assessment. Salt marsh, salt marsh impoundments, and tidal flats corresponded to estuarine marsh.
Mapping Steps
Indicators that have not changed since Blueprint 2.0 were initially computed, or in the case of existing data, resampled to 1 ha spatial resolution using the nearest neighbor method. For computational reasons, we then used the Spatial Analyst-Aggregate function to rescale the resolution to 200 m. The aggregate function avoided loss of detail by taking the maximum value of each cell in the conversion (e.g., species presence).
1) The NLCD emergent wetland class was masked with NWI's "Estuarine and Marine Wetland" category to distinguish estuarine marshes from freshwater marshes.
2) The Spatial Analyst-Region Group function was used to delineate individual patches using an 8-neighbor rule. The method defines patches by clumping into individual patches all wetland cells directly adjacent or diagonal to each other.
3) The wetland patches were reclassified into five quantiles of wetland sizes (in hectares):
0 = 1-328 ha (low)
1 = 329-1,228 ha
2 = 1,229-3,087 ha
3 = 3,088-6,088 ha
4 = 6,088-15,154 ha (high)
Defining the Spatial Extent of Ecosystems
This indicator has been clipped to the estuarine ecosystem. Visit the Blueprint 2.0 ecosystem maps page for an explanation of how each ecosystem’s spatial extent is defined
Known Issues
-- Does not account for variations in marsh vegetation composition.
-- The ability of National Wetland Inventory (NWI) data to distinguish estuarine marsh from freshwater marsh is relatively unknown.
-- This indicator has not been expanded to cover areas newly added to estuarine marsh with the improved ecosystem maps of Blueprint 2.1. This resulted in a small number of no data pixels, < 37 total, throughout the coastal area of the South Atlantic.
Indicator Overview
The South Atlantic ecosystem indicators serve as the South Atlantic LCC's metrics of success and drive the identification of priority areas for shared action in the Conservation Blueprint. To learn more about the indicators and how they are being used, please visit the indicator page. Check out the Blueprint page for more information on the development of the Blueprint, a living spatial plan to conserve our natural and cultural resources.
Literature Cited
Benoit, L.K., Askins, R.A., 2002. Relationship between habitat area and the distribution of tidal marsh birds. The Wilson Bulletin 114, 314-323.
Conley, Mary and Netwon, Robert. Weaver. 2015. Coastal Ecosystems. in Conley, M, M.G. Anderson, L. Geselbracht, eds. The South Atlantic Bight Marine Ecoregional Assessment. The Nature Conservancy, Eastern U.S. Division, Boston, MA. < http://easterndivision.s3.amazonaws.com/Marine/SABMA/FINAL_DRAFT_SABMA_coastal_ecosystems_090415.pdf>.
Homer, C.G., Dewitz, J.A., Yang, L., Jin, S., Danielson, P., Xian, G., Coulston, J., Herold, N.D., Wickham, J.D., and Megown, K., 2015, Completion of the 2011 National Land Cover Database for the conterminous United States-Representing a decade of land cover change information. Photogrammetric Engineering and Remote Sensing, v. 81, no. 5, p. 345-354
Meynecke, J.O., Lee, S.Y., Duke, N., 2008. Linking spatial metrics and fish catch reveals the importance of coastal wetland connectivity to inshore fisheries in Queensland, Australia. Biological Conservation 141, 981-996.
Rush, S.A., Soehren, E.C., Woodrey, M.S., Graydon, C.L., Cooper, R.J., 2009. Occupancy of select marsh birds within northern Gulf of Mexico tidal marsh: current estimates and projected change. Wetlands 29, 798-808.
Shepard, C.C., Crain, C.M., Beck, M.W., 2011. The protective role of coastal marshes: a systematic review and meta-analysis. PLoS One 6.
Turner, R.E., 1977. Intertidal vegetation and commercial yields of penaeid shrimp. Transactions of the American Fisheries Society 106, 411-416.
U.S. Fish and Wildlife Service 2014. National Wetlands Inventory - Wetlands. <http://www.fws.gov/wetlands/>.
