Ecological Knowledge and Environmental Problem-Solving Concepts and Case Studies (1986) / Chapter Skim
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17. Conserving a Regional Spotted Owl Population
17. Conserving a Regional Spotted Owl Population
Pages 227-247
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From page 227... ...
The survival of spotted owls is of increasing concern, because their required old-growth forest habitat is being reduced and fragmented by logging. The planning for spotted owl management described here is based on theoretical population genetics and ecology, as well as on quantitative natural history observations on reproductive ecology, dispersal, and foraging behavior.
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From page 228... ...
Timber harvests in the last few decades have removed nearly all the easily accessible lowland old-growth forest, and the much-reduced spotted owl population now exists primarily in rugged, mountainous terrain. A major purpose of the national forests is to sustain yields of different 228
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From page 229... ...
Hence, the problem is to manage logging and related activities so that remaining old growth will support long-term survival of spotted owls in all national forests in the Pacific Northwest. Four general ecological issues are most important: · It is necessary to determine the habitat characteristics required for the survival and successful nesting of individual pairs of owls.
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From page 230... ...
In addition, the regional population must be sufficiently large and well distributed to withstand severe environmental fluctuations and reductions and even extinction of local populations. These requirements are being studied with a general model of regional population dynamics and with demographic values determined for the spotted owl from field studies and from the literature on better-studied species.
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From page 231... ...
Points of view range from the position of some environmental interest groups, that too much old growth has already been cut and that habitat management criteria (particularly minimal SOMA characteristics) are inadequate, to the view of some representatives of the timber industry, that current management guidelines are too stringent and that the current estimate of minimal acceptable population size is too large.
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From page 232... ...
Hence, spotted owls appear to have a very low reproductive potential and thus poor ability to recover from reductions in population size. Spotted owls in the Northwest appear to be nonmigratory, and radiotelemetry has shown that adults move over extremely large home ranges in the course of a year.
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From page 233... ...
Long-Term Population Viability Studies of population viability have focused on how and why the risk of extinction increases as populations become smaller and more isolated from other populations. Interest has been not only in the fate of local populations, but also in how long-term viability of a regional population or an entire species can be affected by changes in the size, makeup, and distribution of its constituent local populations.
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From page 234... ...
Population productivity can decrease as a result of random changes in fertility rate, litter size, sex ratio, death rate, immigration rate, and so on. Behavioral dysfunction occurs in some species below a threshold population size, often disrupting breeding.
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From page 235... ...
To predict random genetic change in a population of average size N when the population characteristics depart from the ideal, an effective population size, Ne' is often used in lieu of the actual census size.
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From page 236... ...
Figure 1 shows that, assuming a generation time of 2 years for the spotted owl, a regional population of 500 or more will be sufficient to provide protection against genetic deterioration for many centuries. It is assumed that the inbreeding coefficient calculated for the regional population also applies
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From page 237... ...
increases as function of effective population number, Ne, and number of generations. At low Ne, F approaches dangerous extent of inbreeding in fewer generations.
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From page 238... ...
The population is becoming increasingly fragmented, as a result of continued timber operations, and some individual forest populations might be close to the lower limit of adequate short-term demographic resilience-about 50-100 individuals. Planning is required now, so that the regional population does not decrease below the size necessary for long-term survival.
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From page 239... ...
Censuses have been performed on all national forest land and evaluated in conjunction with information from adjacent lands to identify areas that might become effective biological reserves for the species, regardless of ownership and prevailing land use. · Step 3: Habitat requirements for spotted owls and the best distribution of habitat within and between individual forest populations are determined, and the results lead to development of habitat capability models that describe the full range of habitats over which the species occurs (Nelson and Salwasser, 19821.
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From page 240... ...
Habitat capability for each forest can be estimated with the habitat management criteria for owl pairs described above and with censuses of owls and maps of projected habitat distribution. The formulas presented earlier can be used with population data on spotted owls to estimate Ne both for individual forests and for the regional population (Formulas 4 and 5)
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From page 241... ...
Local populations that are not isolated from others are assumed to experience the same inbreeding coefficient as the regional population as a whole (Figure
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From page 242... ...
Key assumptions in current planning involve dispersal behavior of juveniles, the nature of the owls' dependence on stands of old-growth forest for reproduction and survival, and the adequacy of population sizes for maintaining demographic resilience. These assumptions are being studied by a number of agencies (Ruggiero and Carey, 19841.
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From page 243... ...
But metapopulation and other genetic models are only as good as the values that go into them. In spite of recent research on spotted owls, there is little assurance that the values of variables used in the models are accurate.
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From page 244... ...
1980. Habitat Utilization by Spotted Owls in the West-Central Cascades of Oregon.
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From page 245... ...
1981. Minimum population sizes for species conservation.
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From page 246... ...
The spotted owl management plan constitutes one of the first attempts to incorporate metapopulation modeling into usable management guidelines. But, as pointed out by Salwasser, even the basic issue of whether demographic or genetic constraints are more critical is being hotly debated, and only very general rules of thumb for determining minimal population sizes are available.
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From page 247... ...
1981. Minimum population sizes for species conservation.
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