America’s Last Great Places assessed three existing “portfolios” of existing or potential conservation areas in the conterminous United States, using their GIS shapefile datasets.

– National Audubon’s Important Bird Areas (IBAs)

– The Nature Conservancy’s ecoregional assessments (TNCs)

– U.S. Geological Survey’s U.S. Protected Areas inventory (PAs)

Sites with a minimum size of 10,000 acres were evaluated in each of these portfolios – approximately 1,250, 4,100, and 9,000 sites respectively.

Seven factors important for capturing and conserving natural diversity were evaluated for all sites, using GIS datasets with coverage of the lower 48 states.

Variety of environmental diversity – Thousands of ecological land units (ELUs), involving detailed combinations that include climate regime, landforms, geology and land cover. Variety for each area was used for Last Great Places analysis. Data Source: Esri/USGS

Variety of terrestrial ecosystems – 689 upland and wetland ecological systems. Variety for each area. Data Source: LANDFIRE

Habitat suitability: imperiled species – habitat suitability for over 2200 endangered, threatened or imperiled species. Mean value for each area was used for Last Great Places analysis. Data Source: NatureServe

Degree of human modification – combined degree of human settlement, agriculture, mining & energy production, and electrical infrastructure. Mean value for each area. Data Source: NASA-SEDAC

Vegetation condition – departure of ecological systems from their natural range of variability – vegetation composition, structure & ecological processes. Mean value for each area. Data Source: LANDFIRE

Ecological resilience – diverse topography, bedrock, soil and micro-climates, deemed more likely to sustain native plants, animals, and natural processes into the future as climate changes. Adjusted mean value for each area. Data Source: TNC

Conservation management status – whether land is managed for permanent biodiversity maintenance through some legal and/or institutional mechanism. Mean value for each area based on GAP status. Data Source: USGS PAD-US

The sites in each portfolio were ranked using standardized normal distributions for each of the seven factors, with a typical range of 0 to 100 and a mean of 50. The 1000 top-ranked sites across in each portfolio were then collated, using both their national and ecoregional percentile rankings, equally weighted so as to include representation from all 69 ecoregions. These 3000 sites across all three portfolios were collectively re-ranked, resulting in the final ranking of America’s Last Great Places.

There was substantial overlap among many of the sites within and across the three portfolios. We did not attempt to integrate or “resolve” the overlaps, as boundaries were very rarely identical, but rather let each site in each portfolio stand on its own accord.

To download a Word document with more detailed information on the data sources and assessment methods, please click here.