WHAT THE SCIENCE SAYS . . .
California Spotted Owl
THE CALIFORNIA SPOTTED OWL: A Technical Assessment of Its Current Status
Summary of major factors of concern in habitats of California spotted owls in the Sierra Nevada, reasons for those factors and their impacts on the owls.
|Factor||Reason(s) for the factor||Impact on spotted owls|
|Decline in abundance of very large, old trees||Selective logging of the largest trees from stands||Loss of owls preferred nest sites|
|Long recovery period for spotted owl habitat after logging||Selective logging of the largest trees from stands||Less of total landscape in suitable owl habitat at any given time|
|Ingrowth of shade-tolerant tree species, creating unnaturally dense stands with ground to crown fuel ladders||Selection harvest; aggressive fire suppression; sheep grazing, which created ideal seedbeds for conifer germination late last century||Increased threat of stand-destroying fires|
|Excessive build-up of surface fuels||Aggressive fire suppression over the last 90 years, leading to higher densities of trees, more competition for space and water, so a higher death rate of trees||Increased threat of stand-destroying fires|
|Loss of large-diameter logs from the decaying wood source on the ground||Intentional fires by sheepherders; selective logging of largest trees; piling and burning logs after logging; domestic fuel-wood removal||Potential decline in flying squirrel densities via loss of fungi that are a dietary staple for the squirrels|
|Decline snag density||Selective logging of the largest trees from stands; salvage logging; fuel wood removal||Loss of potential nest sites|
|Disturbance and/or removal of duff and topsoil layers||Sheep grazing; mechanical disturbance from logging equipment, skid trails, and so on; increase surface fuel that burn hot enough to destroy the duff layer||Potential decline in flying squirrel densities via loss of fungi that are a dietary staple for the squirrels|
|Change in composition of tree species (fewer pines) and black oaks, more firs and incense cedar||Selective logging of the largest trees, particularly pine species, from stands; aggressive fire suppression||Some loss of next sites|
Is the Sierran population declining? We cannot be certain. Failure to detect significant declines in the two Sierra study areas must be interpreted cautiously, because the power of both tests was very low. We know nothing about the normal, long-term fluctuations of spotted owl populations in the Sierra Nevada. If the California spotted owl has experienced gradual declines n habitat quality in these mountains, the effects may be subtle and difficult to detect. Because we lack adequate, historical inventories of spotted owls in the Sierra Nevada, we have no basis for comparison with our current knowledge. Their current distribution and abundance, however, do not suggest that they have declined in their overall distribution in the Sierra Nevada or that they have declined markedly in abundance within any forest type.
Selective logging of the largest trees from the most productive sites inthe Sierra Nevada has resulted in significant changes in diameter distributions of trees, leaving relatively few very old, large trees that are clearly selected by the owls for nesting. Consequently, we are far from comfored by results from the demographic studies. Before reaching a final conclusion on this matter, we need to continue studies until the power of their tests on lambda is greatly increased.