Ecosystem Cores and Corridors developed by TerrAdapt for the Washington Habitat Connectivity Action Plan (WAHCAP). Tiers refer to habitat quality and intactness, where Tier 1 represents the highest quality and most intact cores and corridors in the state, and the thresholds are gradually loosened for Tier 2 and then Tier 3. This statewide network of cores and corridors was developed by combining ecosystem connectivity models representing temperate lowland forest, mesic montane forest, xeric montane forest, and shrubsteppe ecosystems.

Within each ecosystem, we mapped 3 different ‘tiers’ of habitat and connectivity reflecting a range of sensitivities to suboptimal climate, vegetation, and exposure to the human footprint. The Tier 1 models represented specialists within the ecosystem that are highly sensitive to suboptimal conditions. The Tier 3 models represented more generalist species within the ecosystem that have far greater tolerance for suboptimal climate, thrive in a more diverse mosaic of vegetation types, and have greater tolerance of human modified landscapes. The Tier 2 models represented an intermediate degree of tolerance. Together, the 3 tiers allow for a diverse suite of selection behaviors to be represented, and facilitates comparison of areas that are only likely to be used by specialists versus areas on the urban fringe that may be compatible with species that have higher tolerances. These tiers were nested such that the lower tiers included and expanded upon the higher tiers. We developed a map layer that was flattened to display only the highest quality category. For example, all areas that were Tier 1 Cores were also Tier 2 and 3 Cores, but the flattened map displayed them as Tier 1 Cores because that was the highest value.