Indicator: Marsh Patch Size

Aug 10, 2020
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This indicator is a continuous index of based on the size of freshwater and saltwater marsh 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 continuous index is a simple, objective way of quantifying the functional value of different patch sizes.

In addition, patch sizes of fresh marsh were ranked with knowledge of marsh bird habitat relationships. Over time, a decrease in patch size will correspond to marsh degradation and wetland loss. Brown and Dinsmore (1986) tested bird responses to wetland patch size in interior fresh marshes. They showed 10 species were not present in wetland patches of < 5 ha, but were included in greater patch sizes. Examples of these species include least bittern, northern pintail, and northern shoveler. Several other species had strongly increased abundance with patches > 5 ha. The patch size class > 5 ha and ≤ 20 ha is inclusive of marsh bird home ranges: King rail in fresh marsh have known home ranges of 7.7-16 ha (Pickens and King 2013); least bittern home ranges are highly variable, but Bogner and Baldassarre (2002) reported a mean of 9.7 ha and Moore (2009) showed a mean wetland area of 10.9 ha where least bittern were detected. Brown and Dinsmore (1986) showed a species richness– wetland area relationship with the greatest species richness near 20-30 ha. For king rail, Drew and Collazo (2014) also had support for a model with a patch size class of > 20 ha for predicting species occupancy. Similarly, Pickens and King (2014a, 2014b) showed water-level management, often at a scale > 20 ha, and habitat measured at a 100 ha scale, corresponded to king rail abundance.

Since the index is dependent on the NLCD, future monitoring is feasible. A decrease in patch size will be an early sign of marsh degradation and/or wetland loss.

Input Data

2016 National Land Cover Database (NLCD) (2016 NLCD)

Mapping Steps

1) Pull out emergent herbaceous wetland class from 2016 NLCD.

2) Calculate patch size using the ArcGIS Spatial Analyst-Region Group function with a 4-neighbor rule.

Final indicator values

The final indicator is continuous. Values range as follows:

High: 6,729 ha patch

Low: 0 (<1 ha patch)

Known Issues

–We used a 4-neighbor Region Group. This may underestimate the size of marsh patches.

–This indicator does not differentiate between freshwater and saltwater marsh. Some species have preferences for one over another.

Disclaimer: Comparing with Older Indicator Versions

There are numerous problems with using South Atlantic indicators for change analysis. Please consult Blueprint staff if you would like to do this (email

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.

Bogner, H. E., and G. A. Baldassarre. 2002. Home range, movement, and nesting of least bitterns in western New York. Wilson Bulletin 114:297-308.

Brown, M., and J. J. Dinsmore. 1986. Implications of marsh size and isolation for marsh bird management. The Journal of wildlife management:392-397.

Drew, C. A., and J. A. Collazo. 2014. Bayesian networks as a framework to step-down and support Strategic Habitat Conservation of data-poor species: A case study with king rail (Rallus elegans) in Eastern North Carolina and Southeastern Virginia, Prepared for the United States Fish and Wildlife Service Raleigh Field Office.

Homer, Collin G., Dewitz, Jon A., Jin, Suming, Xian, George, Costello, C., Danielson, Patrick, Gass, L., Funk, M., Wickham, J., Stehman, S., Auch, Roger F., Riitters, K. H., Conterminous United States land cover change patterns 2001–2016 from the 2016 National Land Cover Database: ISPRS Journal of Photogrammetry and Remote Sensing, v. 162, p. 184–199, at []

Moore, S., J. R. Nawrot, and J. P. Severson. 2009. Wetland-scale habitat determinants influencing least bittern use of created wetlands. Waterbirds 32:16-24.

Pickens, B. A., and S. L. King. 2013. Microhabitat selection, demography, and correlates with home range size for the king rail (Rallus elegans). Waterbirds 36:319-329.

Pickens, B. A., and S. L. King. 2014a. Linking multi-temporal satellite imagery to coastal wetland dynamics and bird distribution. Ecological Modelling 285:1-12.

Pickens, B. A., and S. L. King. 2014b. Multiscale habitat selection of wetland birds in the northern Gulf Coast. Estuaries and Coasts 37:1301-1311.

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2020-08-15 04:00:00 (lastRevision Date), 2020-08-15 04:00:00 (Release Date), 2019-08-15 04:00:00 (Start Date), 2021-08-15 04:00:00 (End Date)
Rua Mordecai(Point of Contact), Amy Keister(Point of Contact), Hilary Morris(Point of Contact), U.S. Fish and Wildlife Service(Point of Contact), Southeast Conservation Adaptation Strategy (SECAS)(Point of Contact), 2020-08-15(lastRevision), 2020-08-15(Release), Indicator: Marsh Patch Size,,
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South Atlantic Blueprint

The South Atlantic Conservation Blueprint is a living spatial plan to conserve natural and cultural resources for current and future generations in the face of future change. It spans parts of six states, from Virginia to Florida, including U.S. waters to 200 miles offshore. The Blueprint...