- [Amphibian] |
- [Aquatic Macroinvertebrates] |
- [Bald Eagles] |
- [Freshwater Mollusks] |
- [Long-term Monitoring Plots] |
- [Mule Deer] |
- [Sage Sparrow Habitat] |
- [Small Mammals] |
- [Salmon] |
- [Steelhead] |
Sage Sparrow Habitat Study
Section 10.11.2.3) C. A. Duberstein,
M. A. Simmons, and
M. R. Sackschewsky
Sage sparrows nest almost exclusively in sagebrush communities
(Petersen and Best 1985;
Rotenberry and Wiens 1989).
On the Hanford Site, the presence of sage sparrows
is used as an indicator of high quality habitat. The
Hanford Site Biological Resources Management Plan (BRMaP;
DOE/RL-96-32) quantified quality sage sparrow habitat as
having sagebrush cover greater than 10% and annual grass
cover (primarily cheatgrass [Bromus tectorum]) less than 25%.
These standards are part of the mitigation criteria for areas
that may be impacted during cleanup activities. Mitigation
involves either preserving an area or substituting another
area and restoring that area so that it meets the habitat standards
set for sage sparrow.
In 2003, a 3-year study was started to evaluate the habitat standards set forth in the management plan to determine if these standards were adequate to protect sage sparrow habitats on the Hanford Site. Three tasks were identified: (1) quantify habitat characteristics (e.g., sagebrush cover, annual grass cover, amount of bare ground) of sage sparrow territories on the Hanford Site between State Highway 240 and the Columbia River, (2) develop a computer model showing the relationship between these habitat characteristics and sage sparrow territory size, and (3) collect additional data on sage sparrow density to test this relationship. The resulting information can be used to evaluate existing habitats on the Hanford Site for mitigation and protection of the sage sparrows and their habitat.
During fiscal years 2003 and 2004, the first two tasks were completed. Sage sparrow territories and vegetative features of the territories were measured, and a model was developed relating vegetative features to territory size. The resulting model related sage sparrow territory size to sagebrush cover, annual grass and forb cover, burn history, and patchiness. The last variable is a measure of the number and size of shrub patches. These four habitat characteristics define four general habitat types: (1) mature/undisturbed, (2) mature/ disturbed, (3) recovering/undisturbed, and (4) recovering/ disturbed. Mature and recovering refer to the amount of sagebrush cover, burn history, and patchiness, with a mature habitat having more cover, a longer time, in years, since the last fire, and larger continuous areas of sagebrush (i.e., less patchiness). A recovering habitat would be one that had burned more recently, has less sagebrush cover, and many small patches of sagebrush (i.e., more patchiness). Disturbance can be expressed as a function of annual grass and forb cover with undisturbed sites having less annual grass and forb cover than disturbed sites.
From the work in fiscal years 2003 and 2004, smaller sage sparrow territories were consistently found in habitats characterized as mature/undisturbed. The largest sage sparrow territories were in recovering/disturbed habitats. All of these habitat types support sage sparrow territories; however, given trends noted in the data, mature/undisturbed habitats should support a higher density of birds than recovering/disturbed habitats.
To evaluate the assumed relationship between habitat type and density, in 2005, we measured male sage sparrow density and habitat attributes at 20 locations on central Hanford. Because males defend their territories by singing, they are easier to locate than females. At each of these locations, sagebrush cover, annual grass and forb cover, fire history, and patchiness were determined. Based on the habitat characteristics, each location was assigned to one of the four habitat types and compared to the measured male sage sparrow density. Results showed that sites designated as mature/undisturbed had a mean density of approximately 1 male sage sparrow per hectare, while recovering/disturbed habitats had a mean density of 0.3 male sage sparrow per hectare. Mature/disturbed and recovering/undisturbed had mean densities of 0.5 and 0.8 male sage sparrow per hectare, respectively (Figure 10.11.6).
Results from this study indicate that the current biological resources management plan mitigation thresholds for sage sparrow habitat may need to be revised. Nearly 30% (13 of 44) of the sage sparrow territories measured from 2003 through 2005 had less sagebrush cover than the biological resources management plan mitigation threshold of 10% sagebrush canopy cover. However, these areas all supported sage sparrow territories. In addition, over 40% of the occupied territories also exceeded the maximum threshold of 25% cheatgrass cover in the understory. Results from this study provide information to update the existing mitigation thresholds, and the model provides a method to assess the potential of shrub-steppe habitats on the Hanford Site to support sage sparrows.
