Assessing the extent to which roadless areas complement the conservation of biological diversity

We assessed the extent to which inventoried roadless areas (IRAs) on USDA Forest Service lands contain biophysical features that complement the conservation reserve network (e.g., national parks, designated wilderness areas, and wildlife refuges) in the United States. We compared the percentage of land area in IRAs and conservation reserves across three geographic divisions (Alaska, East, and West), 83 ecoregions, 10 elevation zones, and 1 1 land-cover classes. We also summarized variation in the size class distribution of IRAs. Of the 83 ecoregions evaluated in the United States, 28 have > 12% of their total area in conservation reserves. If IRAs are considered with conservation reserves, the number of ecoregions exceeding the 12% threshold increases from 28 to 32. When only national forest land in the ecoregions is considered, the area of designated wilderness exceeds 12% in 18 of the 45 ecoregions summarized. If IRAs are considered along with designated wilderness, the number of ecoregions exceeding the 12% threshold increases from 18 to 32. On national forest land below 1500 m, 10% is designated wilderness and 18.5% is IRA. Above 1500 m, 20% is designated wilderness and 31.5% is IRA. If IRAs are considered along with designated wilderness, all land-cover classes (except water in the West) would exceed the 12% threshold in Alaska and the West. In the East, only the evergreen forest class would exceed the 12% threshold when IRAs are considered with designated wilderness. Of 462 polygons of designated wilderness in the national forests, 353 are < 20 250 ha, and 25 areas exceed 101 175 ha. If adjacent IRA land is considered along with national forest wilderness, the number of areas < 20 250 ha decreases to 295, and the number of areas > 101 175 ha increases to 45. These results highlight the contribution that IRAs could make toward building a representative network of conservation reserves in the United States. Including these areas as reserves would expand ecoregional representation, increase the area of reserves at lower elevations, and increase the number of areas large enough to provide refugia for species needing large tracts relatively undisturbed by people. Even with the addition of IRAs, many ecoregions in the United States would remain underrepresented in the reserve network, especially in the East.