Gap Analysis Project (GAP) habitat maps are predictions of the spatial distribution of suitable environmental and land cover conditions within the United States for individual species. Mapped areas represent places where the environment is suitable for the species to occur (i.e. suitable to support one or more life history requirements for breeding, resting, or foraging), while areas not included in the map are those predicted to be unsuitable for the species. While the actual distributions of many species are likely to be habitat limited, suitable habitat will not always be occupied because of population dynamics and species interactions. Furthermore, these maps correspond to midscale characterizations of landscapes, but individual animals may deem areas to be unsuitable because of presence or absence of fine-scale features and characteristics that are not represented in our models (e.g. snags, vernal pools, shrubby undergrowth). These maps are intended to be used at a 1:100,000 or smaller map scale. These habitat maps are created by applying a deductive habitat model to remotely-sensed data layers within a species’ range. The deductive habitat models are built by compiling information on species’ habitat associations and entering it into a relational database. Information is compiled from the best available characterizations of species’ habitat, which included species accounts in books and databases, primary peer-reviewed literature. The literature references for each species are included in the "Species Habitat Model Report" and "Machine Readable Habitat Database Parameters" files attached to each habitat map item in the repository. For all species, the compiled habitat information is used by a biologist to determine which of the ecological systems and land use classes represented in the National Gap Analysis Project’s (GAP) Land Cover Map Ver. 1.0 that species is associated with. The name of the biologist who conducted the literature review and assembled the modeling parameters is shown as the "editor" type contact for each habitat map item in the repository. For many species, information on other mapped factors that define the environment that is suitable is also entered into the database. These factors included elevation (i.e. minimum, maximum), proximity to water features, proximity to wetlands, level of human development, forest ecotone width, and forest edge; and each of these factors corresponded to a data layer that is available during the map production. The individual datasets used in the modeling process with these parameters are also made available in the ScienceBase Repository (see the end of this Summary section for details). The "Machine Readable Habitat Database Parameters" JSON file attached to each species habitat map item has an "input_layers" object that contains the specific parameter names and references (via Digital Object Identifier) to the input data used with that parameter. The specific parameters for each species were output from the database used in the modeling and mapping process to the "Species Habitat Model Report" and "Machine Readable Habitat Database Parameters" files attached to each habitat map item in the repository. The maps are generated using a python script that queries the model parameters in the database; reclassifies the GAP Land Cover Ver 1.0 and ancillary data layers within the species’ range; and combines the reclassified layers to produce the final 30m resolution habitat map. Map output is, therefore, not only a reflection of the ecological systems that are selected in the habitat model, but also any other constraints in the model that are represented by the ancillary data layers. Modeling regions were used to stratify the conterminous U.S. into six regions (Northwest, Southwest, Great Plains, Upper Midwest, Southeast, and Northeast). These regions allowed for efficient processing of the species distribution models on smaller, ecologically homogenous extents. The 2008 start date for the models represents the shift in focus from state and regional project efforts to a national one. At that point all of the datasets needed to be standardized across the national extent and the species list derived based on the current understanding of the taxonomy. The end date for the individual models represents when the species model was considered complete, and therefore reflects the current knowledge related to that species concept and the habitat requirements for the species. Versioning, Naming Conventions and Codes: A composite version code is employed to allow the user to track the spatial extent, the date of the ground conditions, and the iteration of the data set for that extent/date. For example, CONUS_2001v1 represents the spatial extent of the conterminous US (CONUS), the ground condition year of 2001, and the first iteration (v1) for that extent/date. In many cases, a GAP species code is used in conjunction with the version code to identify specific data sets or files (i.e. Cooper’s Hawk Habitat Map named bCOHAx_CONUS_2001v1_HabMap). This collection represents the first complete compilation of terrestrial vertebrate species models for the conterminous U.S. based on 2001 ground conditions. The taxonomic concept for the species model being presented is identified through the Integrated Taxonomic Information System – Taxonomic Serial Number. To provide a link to the NatureServe species information the NatureServe Element Code is provided for each species. The identifiers included for each species habitat map item in the repository include references to a vocabulary system in ScienceBase where definitions can be found for each type of identifier. Source Datasets Uses in Species Habitat Modeling: Gap Analysis Project Species Range Maps - Species ranges were used as model delimiters in predicted distribution models. <https://www.sciencebase.gov/catalog/item/5951527de4b062508e3b1e79>;
Hydrologic Units - Modified 12-digit hydrologic units were used as the spatial framework for species ranges. <https://www.sciencebase.gov/catalog/item/56d496eee4b015c306f17a42>;
Modeling regions - Used to stratify the conterminous U.S. into six ecologically homogeneous regions to facilitate efficient processing. <https://www.sciencebase.gov/catalog/item/58b9b8cee4b03b285c07ddef>; Land Cover - Species were linked to individual map units to document habitat affinity in two ways. Primary map units are those land cover types critical for nesting, rearing young, and/or optimal foraging. Secondary or auxiliary map units are those land cover types generally not critical for breeding, but are typically used in conjunction with primary map units for foraging, roosting, and/or sub-optimal nesting locations. These map units are selected only when located within a specified distance from primary map units.
<https://www.sciencebase.gov/catalog/item/5540e2d7e4b0a658d79395db>; Human Impact Avoidance - Buffers around urban areas and roads were used to identify areas that would be suitable for urban exploitative species and unsuitable for urban avoiding species. <https://www.sciencebase.gov/catalog/item/5540e099e4b0a658d79395d6>; Forest & Edge Habitats - The land cover map was used to derive datasets of forest interior and ecotones between forest and open habitats.
Forest edge <https://www.sciencebase.gov/catalog/item/5540e3fce4b0a658d79395fe>; Forest/Open Woodland/Shrubland <https://www.sciencebase.gov/catalog/item/5540e48fe4b0a658d7939600>; Elevation Derivatives - Slope and aspect were used to constrain some of the southwestern models where those variables are good indicators of microclimates (moist north facing slopes) and local topography (cliffs, flats). For species with a documented relationship to altitude the elevation data was used to constrain the mapped distribution.
Aspect <https://www.sciencebase.gov/catalog/item/5540ec40e4b0a658d7939628>; Slope <https://www.sciencebase.gov/catalog/item/5540ebe2e4b0a658d7939626>; Elevation <https://www.sciencebase.gov/catalog/item/5540e111e4b0a658d79395d9>; Hydrology - <https://www.sciencebase.gov/catalog/item/5540eb44e4b0a658d7939624>: A number of water related data layers were used to refine the species distribution including: water type (i.e. flowing, open/standing), distance to and from water, and stream flow and underlying gradient. The source for this data was the USGS National Hydrography Dataset (NHD)(USGS 2007). Hydrographic features were divided into three types: flowing water, open/standing water, and wet vegetation. Canopy Cover - Some species are limited to open woodlands or dense forest, the National Land Cover’s Canopy Cover dataset was used to constrain the species models based on canopy density. <https://www.sciencebase.gov/catalog/item/5540eca9e4b0a658d793962b>;