Js. Meyer et al., INFLUENCE OF HABITAT ABUNDANCE AND FRAGMENTATION ON NORTHERN SPOTTED OWLS IN WESTERN OREGON, Wildlife monographs, (139), 1998, pp. 5-51
Current management for the northern spotted owl (Strix occidentalis ca
urina) is largely driven by metapopulation models or individually-base
d models that assume the success of juvenile dispersal in a fragmented
landscape is a primary factor determining the future existence of spo
tted owls in the Pacific Northwest. We tested hypotheses about fragmen
tation by comparing sites known to be occupied by spotted owls with ra
ndom sites to determine if relationships existed between landscape ind
ices and spotted owl presence and productivity in western Oregon. From
a total of 445 known spotted owl sites within the Bureau of Land Mana
gement's (BLM) checkerboard patterned lands in western Oregon, we rand
omly selected (1) 50 long-term data sites to determine if landscape ch
aracteristics influenced site occupancy or reproduction, (2) 50 random
owl sites to evaluate possible biases in the long-term data sites, an
d (3) 50 random landscape locations for comparison with the 50 random
owl sites. BLM staff classified from aerial photographs the mosaic of
forest successional stages within a 3.4-km-radius circle surrounding e
ach of the 150 sites. From these mosaics, we calculated several indice
s of landscape characteristics and forest fragmentation for 0.8-, 1.6-
, 2.3-, and 3.4-km-radius circles. Results were combined with data on
occupancy and reproduction to test the null hypotheses that landscape
characteristics did not affect site location, site occupancy, or repro
ductive success of spotted owls. Landscape indices did not differ betw
een long-term owl sites and randomly-selected owl sites, indicating li
ttle bias in our sample of long-term data sites. Landscape characteris
tics at random owl sites differed significantly from those at random l
andscape locations. Differences were greatest for 0.8-km-radius circle
s sur rounding the study sites, suggesting that site selection by spot
ted owls may be most strongly affected by landscape characteristics wi
thin a 0.8-km-radius circle (less than or equal to 200 ha). Statistica
lly significant differences also were found for radii up to 3.4 km, bu
t most of those differences did not contribute significant new informa
tion beyond the differences existing in the core area of the circles.R
andom owl sites contained more old-growth forest, larger average size
of old-growth patches, and larger maximum size of old-growth patches t
han occurred in random landscape locations, for all circle radii (P <
0.01). Additionally, random owl sites contained less young-age forest
within 0.8-km-radius circles than did random landscape locations. Howe
ver, amount of clearcut forest did not differ between random owl sites
and random landscape locations. None of the forest fragmentation indi
ces except size of old-growth patches was strongly related to site sel
ection, none was strongly related to frequency of occupation of sites,
and only fractal dimension was moderately related to reproduction. In
stead, the major influences of landscape pattern were related to amoun
t of habitat. Amount of habitat dominated in resource selection probab
ility functions (RSPF) for western Oregon, and these RSPF's can be use
d to predict the probability that a given landscape mosaic will be a s
uitable spotted owl site.