Ecological connectivity (current flow) across 11 western states

The ecological connectivity (i.e., current flow) model was implemented using Circuitscape software and is the cumulative result based on “all-to-one” mode and the landscape resistance surface desceribed below. Specifically, individual protected area (PA) centroids (described below) were connected to ground and 1 Amp of current was injected in the remaining centroids. The resultant data layer represents the sum of these estimates across all PAs and potential ecological connectivity across the entore PA network in the western US. The model output units are  amperes and reflect the relative density of current passing through a given pixel (i.e., nodes or resistors). Current passing through these nodes or resistors can be used to predict expected net movement probabilities for random walkers moving or dispersing through corresponding raster nodes or edges (McRae et al. 2008).

The PAs used to create this dataset were defined using land management designations from the U.S. PA Database v1.3 (USGS 2012). The dataset included only those PAs that were designated within IUCN categories I-IV and that were ≥ 20.2 km2 in size, as this is the federally mandated minimum size for wilderness areas in the US (Wilderness Act 1964). Immediately adjacent PA polygons were combined and geometric centroids (i.e., single pixels, constrained to polygon interiors) were derived to represent each unique polygon (n = 1043 centroids).

The landscape resistance layer was created by combining information on human modification of the landscape and percent slope using the equation R = (H + 1)^10 + s/4 + 1, where H represents the degree of human modification and s is percent slope. The degree of human modification (H) of the western landscape circa 2010 was quantified using methods from Theobald (2013), with scores ranging from 0.00 (unmodified) to 1.00 (completely converted) using multiple data layers including land cover, transportation, housing density, and oil and gas well density. To account for possible movement processes that avoided relatively large elevation changes or steep terrain (e.g., crossing over mountain ranges or through deep valleys), a penalty for areas with steep slopes was added, following Theobald et al. (2012). Lastly, data from the National Hydrography Dataset Plus (USGS 2008) was used to assign all rivers with annual mean flow > 1000 cubic feet per second a resistance of 1000 to reflect their role as barriers to movement for many terrestrial organisms.

A flow-based model of ecological connectivity, such as this one, can provide a flexible, yet theoretically grounded and robust method for identifying the location of new protected areas wherever data on landscape resistance can be derived. Moreover, complementary estimates of flow centrality and effective resistance can be combined to understand how new PAs might both facilitate and enhance ecological connectivity over administratively complex landscapes. Our methodology and the ‘wall-to-wall’ nature of this model can help to illuminate the conservation context and significance of public and private lands. These data can easily be clipped and summarized to examine patterns of connectivity within specific jurisdictions or planning areas (e.g., states, LCCs). These data also can be used to bolster and strategically site new land designations that more effectively protect—and truly network—biodiversity and ecological processes.

Additional methodological and application details can be found in Dickson et al. (in press).

Full metadata can be viewed upon download in the file named 'metadata1_original.xml'