Final Project Reports
Final Project Reports
Nebraska Gap Analysis Project
University of Nebraska-Lincoln
Introduction
The Nebraska Gap Analysis Project (NE-GAP) was undertaken to assess the distribution and conservation status of biodiversity in the state under existing land ownership and management regimes. Our objectives were 1) to map actual land cover as closely as possible to the alliance level; 2) map the predicted distribution of those terrestrial vertebrates and selected other taxa that spend any important part of their life history in the project area and for which adequate distributional habitats, associations, and mapped habitat variables are available; 3) Document the representation of natural vegetative communities and animal species in areas managed for the long-term maintenance of biodiversity; 4) Make all Gap project information available to the public and those charged with land use research, policy, planning and management; 5) Build institutional cooperation in the application of this information to state and regional management activities.
Land Cover
A map of the land cover of Nebraska was prepared using primarily Landsat Thematic Mapper imagery from the period 1991-1993. Early spring and late summer dates were selected within the same year, when possible. The minimum mapping unit (MMU) for the land cover map is 30 meters, which is the spatial resolution (pixel) of Landsat 5 TM data.
The final thematic map identifies 20 different land cover classes. Agricultural fields and grasslands dominate the landscape of Nebraska covering almost 40% of the state. The second most identifiable feature is the Sandhills Upland Prairie class (23%) found throughout the Nebraska Sandhills. Five woody vegetation classes cover 3% of the State. These classes are usually found along riparian corridors and canyons. Ponderosa Pine Forests and Woodlands are found along the Pine Ridge in northwest Nebraska and the Niobrara River. Juniper woodlands (mainly cedar) are increasing across the state due to the suppression of wildfires. Juniper woodlands are concentrated in valleys, canyons, and other protected lowlands and are usually mixed with deciduous woody vegetation.
Open water and wetland classes cover only 2% of the State, but these features figure prominently into vertebrate species distribution. Of note are the Platte River, which cuts across the middle of the State, and the various reservoirs found across the State. Wetlands fed by groundwater are found in the Sandhills and are important for waterfowl breeding. Other wetlands are found in the Rainwater Basin of South Central Nebraska. These wetlands are fed by runoff and are utilized by birds and waterfowl during migration along the Central Corridor. Only the largest of the states wetlands are filled year-round.
Accuracy Assessment
An accuracy assessment was conducted to evaluate the results of the land cover classification using two separate sets of ground reference data collections. Producers accuracy’s for the different land cover classes ranged from 0- 81%, while users accuracy’s ranged from 7- 83%. Delineation of shrublands appears to be a weak spot in the land cover map. However, the shrubland category covered only 1824 km 2 or 0.9% of the land area in Nebraska. The dynamic nature of agriculture also poses significant problems for land cover mapping and assessment of accuracy. Many of the misclassifications evident in this class may have arisen from temporal decorrelation. Field data were gathered in the late 1990s and early 2000s, but the imagery is from the early 1990s. Land use in many agricultural areas of the state has changed during this time period.
Terrestrial Vertebrate Distributions
Potential distribution maps were developed for 332 terrestrial vertebrate species comprising 193 species of breeding birds, 78 species of mammals, 14 species of amphibians, and 47 species of reptiles. The NE-GAP wildlife habitat relationships were modeled and species range maps were generated on a grid of 40km 2hexagons. To develop models of the relationships between the wildlife species and their habitat, a database of geospatial data was developed that included a broad range of surrogate variables for habitat suitability and quality, e.g., land cover composition, aspects of climate, surficial soil texture, hydrology and terrain. This geodatabase was linked using advanced statistical modeling (a recursive partitioning algorithm) to species occurrence data obtained from biological surveys and museum voucher specimens. In the absence of a sufficient number of observations, wildlife-habitat relationship models were developed from the literature and implemented in the geodatabase.
Accuracy assessment of the models was conducted using occurrence data not previously used and focused on the omission error rate (i.e., an estimate of the frequency of incorrectly designating an area as “not habitat”). Higher omission rates indicate poorer model performance. Considered across taxa (Birds vs. Reptiles and Amphibians vs. Mammals), the median values are almost always zero, while the average omission rates range from 2.6-27.5%. This discrepancy between the average and median indicates a highly skewed distribution of model performance which indicates that omission rates are generally quite low, but a few species have poorly performing models which affect the average but not the median.
Land Stewardship
The NE-GAP project revealed profound gaps in the network of stewardship needed to cover a representative selection of Nebraska’s biodiversity. Privately owned lands comprise the majority of Nebraska’s land area, (approximately 97.4%) and >98% of land in Nebraska can be classified as belonging to land management Status class 4. Only 0.61% of Nebraska’s land area can be designated as Status 1 or Status 2 lands. The largest property-owners of these lands are The Nature Conservancy, which manages a number of preserves, and the U.S. Fish and Wildlife Service, which manages three National Wildlife Refuges in the state.
Gap Analysis
The protection status for each land cover classification was derived from a digital overlay of the land cover map with the stewardship map in a GIS. This process created an intersection between each land cover type and its representation in a management status. The largest amount of land area for the state is in active or fallow agricultural fields (39.38%), followed by Sandhills upland prairie (22.74%), then Little Bluestem-Gramma Mixedgrass Prairie cover type (15.13%). Land ownership in Nebraska is predominately private (97.2%) and the most lands are classified as Status 4. The distribution of protected areas (Status 1 & 2) includes four University of Nebraska prairie sites, one USFS wilderness area, 64 USFWS waterfowl protection areas, six USFWS wildlife refuges, 49 NGO-owned units, seven state Natural Resource District managed areas, and three NPS managed national monument sites.
Only six of the 332 modeled terrestrial vertebrate species in Nebraska had more than 1% of their predicted habitat in status 1 and 2 lands. No species had more than 10% of their predicted habitat on these protected status lands.