Recovery Plan for the Northern Spotted Owl - DRAFT

For these two reasons, Thomas et al. (1990) concluded that species welldistributedacross their range are less prone to extinction than species confinedto smaller portions of their range. Third, range reduction around thefringes of a species' geographic or elevational range could have serious consequencesbecause these areas are often the sites of the most rapid adaptationswithin a species. Eliminating the fringes of the range might reduce the evolutionarycapability of the species. Fourth, the elimination of the geographic orelevational fringe portions of a species' range might be considered unwise inthe face of possible widespread climatic changes, especially where the directionand magnitude of those changes are uncertain. For example, some scientistsbelieve that global warming could result in some local cooling points in thePacific Northwest rather than a universal warming effect (Smith 1990). If theclimate cooled, it could place increasing importance on the southern parts ofthe range and on low elevation habitats. If the climate warmed, it could placeincreasing importance on the northern extent of the range.Emphasis should be placed on managementfor clusters,or local population centers, of owls in large habitat blocksrather than for individual pairs.Empirical evidence and modeling show that clusters of 15 to 20 breeding pairshave much higher persistence rates than small, isolated clusters. Theseclusters, or local population centers, can be defined as groups of breeding owlswhere pairs have overlapping or nearly overlapping territories. The evidenceand rationale supporting this principle are described in detail in Thomas et al.(1990).One of the advantages of local population clusters is that they can provide for apopulation structure that can sustain itself for many generations. This contrastswith extremely small local populations, composed of two or fewer pairs,that are highly susceptible to local extinction (Diamond 1984). In order torealize this advantage, the local populations must be large enough to holdmultiple breeding pairs, and also support juveniles, subadults, and "floaters."Floaters are nonbreeding individuals without established territories. It isthought that they serve as ready replacements for birds that die or vacate theirterritories for other reasons (Thomas et al. 1990). This ready replacement ofbirds in breeding territories should help maintain the populations within thelocal population centers.Within each local population center, it is critical to provide for stable or improvinghabitat conditions. This will reverse the trend of increasing fragmentationof habitat which has been experienced in most areas across the range. Fragmentationof habitat is associated with lowered spotted owl densities, decreasedproductivity of spotted owl populations (Bart and Forsman 1992),increasing susceptibility of forest stands to windthrow, decreasing success ofjuvenile dispersal, and possibly increased competition with barred owls andpredation by great horned owls (Thomas et al. 1990).For a strategy based on local populations to be successful, those local populationsmust be capable of acting as sources of surplus owls for the species'metapopulation. A source area is one that has a positive rate of populationincrease and is capable of contributing individuals to the metapopulation.Local populations might cease to act as sources if they are too small or if theyoccupy highly fragmented habitat (Thomas et al. 1990). It is important to notethat each local population does not have to act as a source each year. It isexpected that there will be some variation across populations and across years,and that a portion of the local populations would not act as sources in some101