Appendix A. Habitat capability index for Marbled Murrelet (Brachyramphus marmoratus).
Developers: Thomas A. Spies, Michael T. McGrath, and Brenda C. McComb
Reviewers: S. Kim Nelson, Gary Roloff, Lee Folliard, Dominick DellaSala
Murrelets in the Pacific Northwest typically nest on moss-covered, large-diameter limbs or platforms created by normal tree growth in large conifers (76 cm dbh; Quinlan and Hughes 1990, Singer et al. 1991, Hamer and Nelson 1995, Nelson 1997). However, in the Oregon Coast Range, Nelson and Wilson (1999) have recorded differential habitat use in coastal stands in the Sitka spruce (Picea sitchensis)-zone vs. stands dominated by Douglas-fir (Pseudotsuga menziesii) elsewhere in the Coast Range. Nests in Douglas-fir stands were more typical of other Marbled Murrelet nests in the Pacific Northwest: generally in multi-story older-aged stands (>100 yr old), occurring in trees >76 cm dbh, and on large, moss-covered limbs or in tree deformities. However, in the Sitka spruce and western hemlock (Tsuga heterophylla) dominated stands, nests have occurred within single-story stands (>60 yr old), in remnant western hemlock trees (range: 49177 cm dbh) with enlarged limbs created by dwarf mistletoe infestations. For nests occurring in the Coast Range, nest-site characteristics differed from the surrounding landscape primarily with respect to moss and lichen coverage, mistletoe infestation, and canopy layers (Nelson and Wilson 1999). Thus, nest-site modeling for this species in Oregon must account for the vegetative zone under consideration and tree-level attributes.
Although the landscape surrounding nest sites has not been demonstrated to be utilized by the species, the surrounding landscape could be very influential with respect to nest predation risk from corvids (Andren 1992). The combination of increased forest fragmentation (as defined by increased interspersion of divergent forest age classes and forest openings) and decreasing patch size serve to increase the risk of nest predation by forest generalist predators (e.g., corvids; Andren 1992). Hence, a greater percentage of area surrounding occupied (i.e., displaying nesting behavior or flying under the forest canopy; after Ralph et al. 1993) Marbled Murrelet sites in western Washington was found to be in old growth and large sawtimber, compared with sites where the species was not detected (Raphael et al. 1995).
Habitat capability index
The index is calculated as:
where HCI = habitat capability index, f = the focal pixel, NCI = nesting capability index (Eq. A.2), and LCI = landscape capability index (Eq. A.6).
Nesting capability index
All metrics for this index are calculated for a focal pixel at the center of a 3 × 3 “moving window.” This moving window of pixels averages conditions for the 0.5625 ha surrounding the “focal” pixel (i.e., 3 × 3 pixels). The averaging is done to: (1) smooth inter-pixel variation; and (2) provide a “patch” or “stand” level summary, which is consistent with the scale of the stand modeling and stand inventory data. The index is calculated as:
where NCI = nesting capability index, f = focal pixel, i = pixel, S1 = mistletoe “nest” tree index (Eq. A.3), S2 = platform tree index (Eq. A.4), and S3 = canopy heterogeneity index (Eq. A.5).
Mistletoe nest tree index
The mistletoe nest tree index for coastal stands located in the Sitka spruce zone is sensitive to the presence of western hemlock trees capable of providing a nesting platform for Marbled Murrelets (Fig. A1). Because of the small size of this tree species in the coastal spruce-zone, an infection of dwarf mistletoe allows the limbs of trees between 50 and 75 cm dbh to swell large enough to support a murrelet nest. Limits and slope changes in the index correspond to the range and mean of observed tree densities for this size class surrounding Marbled Murrelet nests in the coastal spruce-zone. Nelson and Wilson (1999) provided micro-site densities of 15, 58, and 120 trees per ha between 50 and 75 cm dbh, our stand-level estimates are 9, 29, and 70, respectively.
The index is calculated from the following equation:
If tshe50-75 < 9 then S1 = 0.0
|If 9 ≤ tshe50-75 < 29 then S1 = (tshe50-75 * 0.035) - 0.315|
|If 29 ≤ tshe50-75 < 70 then S1 = (tshe50-75 * 0.00732) + 0.4878|
|If tshe50-75 ≥ 70 then S1 = 1.0|
where tshe50-75 = density (trees per ha) of western hemlock between 50 and 75 cm dbh and
S1 = mistletoe “nest” tree index.
|FIG. A1. Relationship between nest tree index and density of potential hemlock mistletoe trees.|
Platform tree index
In Douglas-fir dominated stands within the Coast Range, Marbled Murrelets place their nests on large-diameter, moss-covered limbs (i.e., tree limbs >10 m in height and >10 cm in diameter). These limbs typically occur on trees ≥76 cm dbh (Hamer 1995, Nelson and Wilson 1999). Hence this index (Fig. A2) evaluates the potential of nesting opportunities within a stand. Micro-site densities were 30 and 60 trees per ha for trees ≥75 cm dbh, our stand level estimates are 11 and 35, respectively. The index is calculated from the following equation
|If 11 ≤ tph75 < 35 then S2 = (0.04167*tph75) - 0.45833||
|If tph75 ≥ 35 then S2 = 1.0;|
|Else S2 = 0|
where tph75 = density (trees per ha) of trees ≥75 cm dbh and S2 = platform tree index.
|FIG. A2. Relationship between index score and density of potential platform trees.|
Canopy heterogeneity index
Murrelet’s typically nest stands in mature or old growth stands, which have high levels of canopy heterogeneity. We use the diameter diversity index (DDI) McComb et al. (2000) to characterize canopy heterogeneity (Fig. A3). Canopy heterogeneity indicative of mature stands (i.e., ≥100 yr old) has been found to have DDI ≥6.4.
|If DDI ≤ 5.0 then S3 = (0.04 * DDI)||
|If 5.0 < DDI ≤ 6.5 then S3 = (0.33333 * DDI) - 1.46667|
|If 6.5 < DDI ≤ 7.5 then S3 = (0.3 * DDI) - 1.25|
|If DDI > 7.5 then S3 = 1.0|
where DDI = index for tree diameter diversity within a stand and S3 = canopy heterogeneity index.
|FIG. A3. Relationship between index score and diameter diversity index.|
Landscape capability index
The potential effects of predation on nests are modeled through the landscape capability index (Fig. A4). The index values range from 0 to 1 with 0 being low capability (high potential influence of predation) and 1 being high capability (low predation influence). Potential predator habitat is defined using the following generalized CLAMS patch types:
Any pixel containing any of these features is classed as source habitat for murrelet nest predators. Distance effects are evaluated at two spatial extents, 050 m and 50100 m. The literature suggests that most predation occurs in the first 50 m but some effects may occur up to 100 m. The index is calculated from the following equation:
[max = 1; min = 0]
where LCI = landscape capability index for the focal pixel, prop1 = proportion of source habitat to source nonhabitat 050 m from the focal pixel, and prop2 = proportion of source habitat to source nonhabitat 50100 m from the focal pixel. The scores from the two zones are averaged together with the following weighting factors: 1.0 for 050 m and 0.25 for 50100 m. Within each distance zone away from the source habitat, the proportion of source habitat to source nonhabitat will affect the potential index score according to the following relationship.
|FIG. A4. Relationship among index score and proportion of predator habitat at different distances from focal pixel.|
Andren, H. 1992. Corvid density and nest predation in relation to forest fragmentation: a landscape perspective. Ecology 73:794804.
Hamer, T. E. 1995. Inland habitat associations of Marbled Murrelets in western Washington. Pages 163175 in C. J. Ralph, G. L. Hunt Jr., M. G. Raphael, and J. F. Piatt, editors. Ecology and conservation of the Marbled Murrelet. USDA Forest Service General Technical Report PSW-152.
Hamer, T. E., and S. K. Nelson. 1995. Characteristics of Marbled Murrelet nest trees and nesting stands. Pages 69-82 in C. J. Ralph, G. L. Hunt Jr., M. G. Raphael, and J. F. Piatt, editors. Ecology and conservation of the Marbled Murrelet. USDA Forest Service General Technical Report PSW-152.
McComb, W. C., M. T. McGrath, T. A. Spies, and D. Vesely. 2002. Models for mapping potential habitat at landscape scales: an example using northern spotted owls. Forest Science 48:203–216.
Nelson, S. K. 1997. Marbled murrelet. In A. Poole and F. Gill, editors. The birds of North America, no. 276. The Birds of North America, Inc., Philadelphia, Pennsylvania, USA.
Nelson, S. K., and A. K. Wilson. 1999. Marbled murrelet habitat characteristics on state lands in western Oregon. Unpublished 19971998 annual report. Oregon Cooperative Fish and Wildlife Research Unit, Oregon State University, Department of Fisheries and Wildlife, Corvallis, Oregon, USA.
Quinlan, S. E., and J. H. Hughes. 1990. Location and description of a Marbled Murrelet tree nest site in Alaska. Condor 92:10681073.
Ralph, C. J., S. K. Nelson, M. M. Shaughnessy, and S. L. Miller. 1993. Methods for surveying Marbled Murrelets in forests. Technical Paper 1. Pacific Seabird Group, Marbled Murrelet Technical Committee.
Raphael, M. G., J. A. Young, and B. M. Galleher. 1995. A landscape-level analysis of Marbled Murrelet habitat in western Washington. Pages 177189 in Ralph, C. J., G. L. Hunt Jr., M. G. Raphael, and J. F. Piatt, editors. Ecology and conservation of the Marbled Murrelet. USDA Forest Service General Technical Report PSW-152.
Singer, S. W., N. L. Naslund, S. A. Singer, and C. J. Ralph. 1991. Discovery and observations of two tree nests of the Marbled Murrelet. Condor 93:330339.