OUTDATED Indicator V 2.0: Landscapes - Low Road Density

Low Road Density
This layer is an older version of one of the South Atlantic LCC indicators in the landscapes “habitat aggregate”, which is intended to capture connections across all terrestrial ecosystems. It is an index of areas with few roads. This indicator was updated in Blueprint 2.1 to use an improved resampling method.

Reason for Selection
Large areas with few roads are favorable for conservation of numerous species, including reptiles and amphibians, birds, and large mammals. Roads can cause negative impacts by promoting dispersal of invasive species and inhibiting water flow. Road density, as well as urban expansion in areas of low road density, are straightforward to measure and monitor. Road density has been used in other broad-scale conservation planning efforts and is widely used and understood by diverse partners.

Input Data
We obtained TIGER (Topologically Integrated Geographic Encoding and Referencing) road files of the South Atlantic LCC region from The Nature Conservancy.

We developed a mask to exclude estuaries, large lakes, and reservoirs from the analysis. More specifically, the mask excluded the areas of waterbodies mapped from a 1: 1 million scale ( National Atlas of the United States 2014). Estuarine open water was excluded based on National Wetlands Inventory (NWI) data.

Mapping Steps
Indicators used in Blueprint 2.0 were initially computed, or in the case of existing data, were 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) All TIGER roads were included in the analysis. The Spatial Analyst-Line Density function was used to estimate road density within a 2 km radius of each cell (km of road per sq km of area). Importantly, the area in the calculation included large waterbodies, and the following steps explain how we accounted for this.

2) Road density was multiplied by the area of the 2 km radius circle (12.56636 sq km). The result was road length within each 2 km radius.

3) Focal statistics were used to sum the terrestrial area (see mask description in the “Input Data” section) within a 2 km radius of each cell.

4) The road length was then divided by the terrestrial area within a 2 km radius. The result was: road density = road length (km) within a 2 km radius / terrestrial area (sq km) within a 2 km radius. Steps 2-4 above helped depict barrier islands and coastal areas where the radius of circles included large bodies of open water.

5) “Low road density areas" were classified as those with < 1.5 km/sq km. Areas with a road density > 1.5 km/sq km have been correlated with skewed sex ratios in turtles ( Steen and Gibbs 2004), as excessive female mortality may occur when they are seeking nest sites. We used the ArcGIS Spatial Analyst-Region Group function to calculate low road density patch sizes by connecting patches of cells directly adjacent to each other (4-neighbor rule). A minimum low road density patch size of > 4,050 ha (10,000 acres) was targeted for conservation.

The indicator values were classified as follows:

0 = High road density (≥ 1.5 km/sq km)
1 = Low road density (< 1.5 km/sq km)

Defining the Spatial Extent of Ecosystems
Landscape and waterscape indicators were defined as features that applied across all terrestrial and aquatic ecosystems and no refined extent was needed.

Known Issues
-- Roads are a significant factor in habitat fragmentation in our region, but are not the only factor. Natural habitat can also be fragmented with anthropogenic land uses. This indicator does not address that type of fragmentation.
-- Future changes will be difficult to predict. Road shapefiles are updated regularly but lack of standardization in road digitizing across our area may make it difficult to track changes over time.

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
National Atlas of the United States, 2014. 1:1,000,000-Scale Waterbodies and Wetlands of the United States. < http://nationalatlas.gov/atlasftp-1m.html> Rolla, MO.

Steen, D. A. and Gibbs, J. P. (2004), Effects of Roads on the Structure of Freshwater Turtle Populations. Conservation Biology, 18: 1143–1148. doi: 10.1111/j.1523-1739.2004.00240.x < http://noss.cos.ucf.edu/papers/Steen%20and%20Gibbs%202004.pdf>

The Nature Conservancy, Eastern Division Maps & Spatial Data. 2012. TIGER road linework for all TNC Eastern Division states. < https://www.conservationgateway.org/ConservationByGeography/NorthAmerica/UnitedStates/edc/reportsdata/mapsdata/Pages/default.aspx>

U.S. Fish and Wildlife Service 2014. National Wetlands Inventory - Wetlands. < http://www.fws.gov/wetlands/>.

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OUTDATED Indicator V 2.0: Landscapes - Low Road Density

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