Linkage Networks’ Synthesis Analysis
The focal species and landscape integrity approaches identify habitat concentration areas (HCAs) and core habitats, respectively, and areas of the landscape important for connecting them. As in the Statewide Analysis and the Analysis of the Columbia Plateau Ecoregion (WHCWG, 2010, 2012), we consider that a linkage network consists of the combination of all the HCAs and the linkages modeled for a particular focal species, or core areas and modeled linkages for landscape integrity (WHCWG, 2010). These networks are useful individually, informing decisions pertaining to maintaining and restoring connectivity for the particular focal species (or species considered similar enough in terms of habitat and movement needs), or for maintaining connectivity between relatively intact habitat areas. However, to explore the overall patterns of connectivity across the region, and identify those areas important for maintaining and restoring connectivity for all species of interest—which are too many to model individually—it is important to synthesize the information contained in each linkage network across focal species and landscape integrity. We therefore developed a composite of the six resulting linkage networks, to explore broader-scale patterns of connectivity from the Cascades to the Coast, with the goal of identifying priority areas for connectivity conservation or restoration.
This composite is a map that simply overlays all five focal species networks and the landscape integrity network, and highlights those areas where the greatest number of linkages in the linkage networks overlap. The scientific guidance for how wide linkages need to be to functionally connect two core areas is still uncertain. The team took a fairly conservative approach to defining linkage widths, and though these decisions appear to make sense, they are somewhat arbitrary. In this analysis, linkages for each focal species were defined as extending up to 5 km (3.1 mi) of cost-weighted distance from the least-cost path, while the landscape integrity linkages extended up to 150 km (93.2 mi) of cost-weighted distance from the least-cost path. These values defined linkages of similar geographical width by compensating for the higher resistance values used in the landscape integrity model. In addition to providing a basis for a synthetic product to explore connectivity across the Cascades to Coast region, these six linkage networks were also the basis for prioritizing linkages across the main fracture zones—areas of reduced permeability between HCAs or core areas—in the region.
Prioritizing Linkages Across Key Fracture Zones
Whether to inform efforts to conserve and enhance connectivity for one species or for the landscape overall, there is a need to identify specific linkages that provide the best opportunities for maintaining or improving wildlife’s ability to move in response to current needs and future changes. It is important, however, to identify such priority linkages across the whole landscape, rather than focus on a handful of highest priorities that may co-exist in the most fragmented portion of the region. It is also important to identify priority linkages for each species, as well as those areas that are part of priority linkages for multiple species. The approach we took to prioritizing linkages was applied in two ways. First, we identified priority linkages for each focal species, and then these species-specific linkages were synthesized into a composite map (analogous to the linkage networks’ synthesis analysis). Second, we first identified the main fracture zones across the whole Cascades to Coast region, and then prioritized linkages across each of these main fracture zones.
For each focal species and for landscape integrity, highways are central landscape features where permeability is reduced, leading to the occurrence of fracture zones. We therefore used highways as the focus for identifying priority linkages connecting different HCAs within each linkage network. In this way, we identified nine major fracture zones along highways, which divided the landscape into 14 habitat blocks. Along each major fracture zone we identified the locations where least-cost paths in each network cross the highway, and then prioritized these crossing locations based on the following criteria:* Linkage length: the length of the linkage’s least-cost path (LCP), where shorter lengths were considered higher priority.* Cost/Euclidean ratio: dividing the linkage’s LCP cost-weighted distance length by the Euclidean distance between the connected HCAs provides a measure of permeability of the linkage. More permeable linkages (lower Cost/Euclidean ratio) were considered higher priority.* Linkage centrality: The circuit flow centrality calculated by the Centrality Mapper tool in Linkage Mapper (McRae, 2012) provides a measure of how central a particular linkage is to the whole network. Linkages with higher centrality were considered higher priorities.* Value of connected HCAs: Linkages connecting higher habitat value HCAs, quantified as the HCA area weighted by habitat suitability, were considered higher priority.
Using non-parametric rankings, we identified the five highest priority linkages within each fracture zone for each linkage network (five focal species’ and the landscape integrity network). For each of these priority linkages, we delineated a polygon that captured the segment of the linkage (5 km [3.1 mi] cost-weighted distance from the least-cost path) that crossed the relevant highway, plus a 1-mile (1.6 km) buffer along that highway. Similar to how we developed the composite map of linkage networks, we overlapped these priority linkage polygons to identify priority crossing areas important for multiple species and landscape integrity.
Focusing on those polygons that were priority linkages in two or more linkage networks, we categorized them based on traffic volume (a surrogate for the risk wildlife face) and the potential for loss of connectivity. We consider priority polygons with high traffic volumes as priorities for enhancing or restoring connectivity. Priority polygons with low traffic volumes are priorities for conserving existing connectivity. Priority polygons with medium traffic volumes may need further evaluation to see if they currently function as effective linkages, and therefore should be conserved, or if their functionality is compromised (for some or all species), and investments to improve connectivity are needed.
Geospatial Data Products
The Washington Connected Landscapes Project: Cascades to Coast Analysis produced the following geospatial data layers for the synthesis analysis: (1) a raster dataset depicting the overlapping linkage networks for the five focal species and landscape integrity and (2) a vector dataset depicting priority crossing areas intersecting major highways in the region.
The data layers are provided in multiple file formats that include: (1) an ArcGIS Pro file geodatabase, (2) ArcGIS Pro layer files (.lyrx), (3) ArcGIS Pro layer packages (.lpkx), and (4) shapefiles and TIFFs. Metadata files for each data layer are provided in html and xml formats.
The GIS data layers are also available in interactive maps that can be accessed on DataBasin in the gallery “Washington Connected Landscapes Project: Cascades to Coast Analysis.”
See the Washington Connected Landscapes Project: Cascades to Coast Analysis Report for a detailed account of the background and modeling information for the synthesis analysis.