Northern spotted owl (Strix occidentalis caurina) recovery strategy: chapter 9
6. Habitat Identification
6.1 General Habitat
Spotted Owls use a wide variety of habitat types and forest stand structures throughout their range (USDI 1992). Habitat selection by Spotted Owls is likely influenced by prey abundance and accessibility, availability of nest and roost structures, and thermoregulation and protective cover from predators and inclement weather (Barrows and Barrows 1978; Forsman et al. 1984; Carey et al. 1992; Hanson et al. 1993; Forsman and Giese 1997). In general, the Spotted Owl is closely associated with mature and old coniferous forests that exhibit uneven-aged, multi-layered, multi-specied canopies that contain numerous large trees with broken tops, deformed limbs, and large cavities; numerous large snags; large accumulations of large woody debris; and canopies open enough to allow owls to fly within and beneath (Forsman et al. 1984; Thomas et al. 1990; USDI 1992).
Forest structure is a more reliable variable than actual forest age when defining suitable Spotted Owl habitat. Comparatively younger forests (less than 100 years old) with residual elements (e.g., snags, legacy trees) from previous stands may be used by Spotted Owls, whereas stands of similar age that lack the residual elements appear to be marginal or unsuitable (Buchanan et al. 1999). As a general descriptor, high-quality habitat in British Columbia tends to be in forests older than 140 years and moderate quality habitat occurs in forests 100 to 140 years old (SOMIT 1997a,b).
Within their range on the southwestern British Columbia mainland, Spotted Owls are found in the Coastal Western Hemlock (CWH) and Interior Douglas-fir (IDF) biogeoclimatic zones, and at the lower extent of the Mountain Hemlock (MH) and Engelmann Spruce–Subalpine Fir (ESSF) biogeoclimatic zones (Meidinger and Pojar 1991; SOMIT 1997a). Within this range, Spotted Owl habitat shifts from a wetter maritime ecosystem in the southwest to a drier submaritime ecosystem in the northeast (SOMIT 1997a; See Appendix 5). The elevational limit of suitable Spotted Owl habitat is thought to be below 1370 to 1500 m, depending on geographic location.
Limited research has been done on the habitat requirements of the Spotted Owl in British Columbia. For the most part, habitat definitions for British Columbia have been extrapolated from research conducted in Washington State (SOMIT 1997a).
6.2 Critical Habitat
Under the federal Species At Risk Act (SARA), critical habitat means “the habitat that is necessary for the survival or recovery of a listed wildlife species…”(SARA, 2003). Studies to define and describe critical habitat should be conducted consistent with SARA. These studies should include reviewing definitions of suitable habitat (nesting, roosting, foraging, and dispersal), developing habitat supply models, and assessing the amount and spatial configuration of habitat needed to recover the species. Current provincial standards for the Spotted Owl identify a surrogate definition for nesting, roosting, and foraging habitat as coniferous forest stands older than 100 years with dominant trees taller than 19 m, which are found below elevations of 1370 to 1500 m, depending on geographic location (SOMIT 1997a).
The amount and spatial distribution of critical habitat for Spotted Owls have not yet been defined by the SORT. Recommendations regarding the amount and distribution of critical habitat required to recover the population in British Columbia will be included in the Habitat Action Plan. During development of recovery action plans, the SORT has made interim management recommendations designed to retain options for recovery planning (Appendix 1). Although a final and complete definition of critical habitat has not been developed, critical habitat comprises several components, and some of these components are known. Thus, the SORT recommends that a partial definition of critical habitat be employed to approximate the minimum requirements for survival habitat. Survival habitat is defined as the habitat thought to be the minimum amount and distribution needed to maintain the current population size (ROMAN 2003). Given the extremely low number of Spotted Owls left in British Columbia and the risk of extirpation occurring prior to implementation of the recovery action plans, it would be prudent to consider all suitable habitat within currently occupied Long-term Activity Centers (see section 18.1.3) to be critical habitat. Our rationale is that maintaining all known birds will be crucial to recovery efforts and that suitable habitat in these areas will more than likely be included in the final critical habitat designation. This partial definition of critical habitatshould be extended to all known currently occupied sites and any newly discovered sites found during recovery planning. For this purpose, and this purpose only, “currently occupied” is defined as having Spotted Owls (pairs or singles) present during the immediately previous or current breeding season. This definition is premised on the assumption that well-designed, scientifically defensible inventories are conducted.
Until critical habitat is fully defined, for planning and management purposes, habitat should be characterized according to the surrogate definition of suitable habitat (SOMIT 1997 a). If a Spotted Owl is confirmed as occupying a site that does not meet the surrogate suitable habitat definition, the area should be considered suitable Spotted Owl habitat. This partial definition uses best current information and applies the precautionary approach as stated in the 1996 Accord for the Protection of Species at Risk, signed by the provinces (in ROMAN 2003).
6.2.1 Nesting Habitat
Spotted Owls do not build their own nests, but depend on naturally occurring or previously constructed nest sites. Nest sites include broken treetops, tree cavities, abandoned raptor nests, mistletoe “brooms,” and debris accumulations captured in clusters of branches (Forsman et al. 1984; Dawson et al. 1986). Spotted Owl nests are found primarily in large-diameter trees (e.g., more than 50 cm dbh). In Oregon, most nests were in tree cavities (64%), with the remainder on broken-top platforms (Forsman et al. 1984). In Washington, cavities tend to be used for nests west of the crest of the Cascade Range (Forsman and Geise 1997), and platforms and abandoned hawk nests tend to be used more frequently in the drier interior (Buchanan et al. 1993). In British Columbia in 2002, two nests on the east side of the Cascade Range were located in the lower crowns of Douglas-firs; one in an abandoned Northern Goshawk nest and the other in an abandoned nest of either a Red-tailed Hawk or Common Raven (Hobbs 2002).
A variety of trees are used for nesting throughout North America, suggesting, to a certain extent, that the presence of structures (i.e., cavities or platforms) is more important than tree species in nest site selection (Forsman and Giese 1997). In British Columbia, Spotted Owl nests have been found in the cavities of living western redcedar (Thuja plicata), western hemlock (Tsuga heterophylla), and living and dead Douglas-fir (Pseudotsuga menziesii)(C. Lenihan, pers. comm.). Pairs may re-use the same nest each year or use alternate nest sites (Gutiérrez et al. 1995).
6.2.2 Foraging Habitat
Spotted Owl foraging habitat is characterized by high canopy closure and complex forest structures (Gutiérrez et al. 1995). Owls tend to concentrate their foraging in old or mixed-age stands of mature and old trees, but use a wider variety of habitat for foraging than for nesting or roosting (Thomas et al. 1990). Telemetry studies in Oregon and Washington indicate that old forests are superior habitat for Spotted Owl foraging and roosting. Mature stands were found to be less suitable habitat than old forests, young stands provided marginal habitat, and clearcuts were unsuitable habitat (Thomas et al. 1990). Telemetry studies in Oregon suggest that old forest was the only stand type selected for foraging and roosting out of proportion (1.5 times more) to its general availability in the landscape (Forsman et al. 1984; Carey et al. 1990; Carey et al. 1992).
Although for flying squirrels, the quality of old forest habitat compared with 80-year-old second growth is uncertain in British Columbia (Ransome and Sullivan 2003), Spotted Owls forage mainly in older stands (Hamer et al. 1989; Forsman et al. 1984; Carey et al. 2002). This is likely because of structural characteristics of the stands, such as the multi-storied canopy that provides thermoregulation opportunities and stand openness that makes prey more available.
In Washington, median annual home ranges of Spotted Owl pairs were larger in the moister western forests (3321 ha, 67% of the suitable habitat) than the drier eastern forests (2675 ha, 71% of the suitable habitat; Hanson et al. 1993). Other studies in Washington found home ranges of 2100 to 4000 ha (reviewed by Gutiérrez et al. 1995). Preliminary home range size estimates from telemetry studies in British Columbia appear to be consistent with home range size estimates from Washington (Blackburn and Godwin 2003). The large home ranges used by Spotted Owls may be a response to the low density of the owl’s principal prey species and the amount of habitat needed to find sufficient food to sustain the owl (Carey et al. 1992).
6.2.3 Roosting Habitat
Roost sites in Washington and Oregon are mainly (88%) in old-forest stands (Carey et al. 1992). Roosts are typically in areas of relatively dense vegetation with high canopy closure and a multi-layered canopy. During summer, roosts are usually in cool, shady areas on the lower third of slopes (Blakesley et al. 1992). Owls respond to variation in temperature by moving within the canopy to find favourable microclimates (Barrows 1981). Roost site selection varies seasonally, with cooler aspects (north, northeast, and east) favoured in the summer, and warmer aspects (south and southwest) favoured in the spring and fall (Carey et al. 1992). Forsman (Stan Sovern, pers. comm.) found that, in the East Cascades, some owls will move upslope to upper ridge areas in winter to escape (apparently) cold air trapped in drainages.
6.2.4 Dispersal Habitat
Spotted Owls in British Columbia evolved in a landscape that contained large amounts of structurally complex forest habitat. The fact that the landscapes have changed means that landscape-level dispersal conditions have also changed. The quality of dispersal habitat present on the landscape is likely an important factor in the survival of dispersing birds (see section 4.1.3). Clearly, high-quality dispersal conditions will facilitate dispersal, whereas poor conditions may impede the dispersal process. Habitat-related factors that influence the suitability of dispersal conditions include both stand-level (e.g., snag abundance, canopy closure, coarse woody debris) and landscape-level (habitat amount and distribution, topography, presence and types of barriers) attributes. Large non-forested valleys are known to act as barriers to dispersal in western Oregon between the Coastal and Cascade ranges, but dispersal does occur in some broad, forested foothills between the same ranges (Forsman et al. 2002a). Although dispersing owls used a fragmented mosaic of various-aged forests, clearcuts, roads, and non-forested areas (Forsman et al. 2002a), there is no information available on the minimal conditions needed to facilitate dispersal. A number of definitions of dispersal habitat (both stand- and landscape-level) have been developed, but none of these have been evaluated (J. Buchanan, pers. obs.). An evaluation of a simulated dispersal landscape on private industrial forestlands in Washington indicated a number of conditions that would likely result in ineffective dispersal (WFPB 1996).
6.3 Habitat Protection
In British Columbia, the majority of Spotted Owl habitat is located on lands within Provincial Crown Forests and protected areas. The Spotted Owl Management Plan (SOMP) provides habitat protection for Spotted Owls within the Chilliwack and Squamish forest districts. When SOMP was approved in 1997 (SOMIT 1997a), the extent of the species’ range in the Cascades Forest District was unknown, and therefore, this forest district was not included in the original plan.
Within the Chilliwack and Squamish forest districts, approximately 1 170 000 ha currently exist as forested area capable of supporting Spotted Owls (MSRM Database, 2003). Capable forest includes both currently suitable habitat and unsuitable habitat that could grow over time into suitable habitat conditions. Of the total currently capable forested area, approximately 33% falls within areas under SOMP. This includes 16% (184 000 ha) as protected areas; 13% (152 700 ha) as Crown Forests (in Resource Management Zones – RMZs); and 4% (46 200 ha) as Greater Vancouver Regional District (GVRD) watersheds (Figure 4). Collectively, these three jurisdictional areas are referred to as Special Resource Management Zones (SRMZs) under SOMP. Within protected areas and GVRD, forests are typically managed under SOMP to maintain 100% of the area as suitable habitat over the long term. Within 15 of 17 RMZs (141 100 ha) on Crown Forests, forests are managed under SOMP to maintain 67% of the area as suitable habitat over the long term. The other two RMZs (about 11 600 ha) are not currently managed for Spotted Owls, but are to be considered in the future for full inclusion under SOMP. Within significant portions of these three jurisdictional areas, the habitat retention targets have not been attained and will require many more decades of forest growth.
Figure 4. Total and actual capable forest area managed under the Spotted Owl Management Plan within the three land jurisdictions in the Chilliwackand Squamish forest districts.
Approximately 939 800 ha of currently capable forested area in the Chilliwack and Squamish forest districts falls within Crown Forests. Of this, 477 300 ha (51%) is considered as “contributing” to the Timber Harvesting Land Base (THLB) on which timber resource extractions may occur. RMZs for Spotted Owls overlap 81 800 ha (17%) of this total “contributing” THLB, of which 36 900 ha (45%) falls within Forest Management Area under SOMP where heavy Volume Removal may eventually occur (clearcuts with some structural retention). The other 44 900 ha (55%) of “contributing” THLB falls within Long-Term Owl Habitat Areas under SOMP where Light Volume Removal (about one-third volume) may occur to create and/or enhance suitable habitat. On the remaining THLB outside of RMZs, forests are managed to integrate resource extraction with other resource values (e.g., Old Growth Management Areas) that will benefit Spotted Owls mostly by providing dispersal habitat.
Under SOMP, the provincial government decided that Spotted Owls found after June 1995 would receive no formal protection unless they were located within SRMZs. Between 1992 and 2002, at least 64 occupied Spotted Owl territories were discovered (Table 1). Thirty-seven (58%) of these fall within the protection of SRMZs. Eight of them (that were found before June 1995) are called Matrix Activity Centres (MACs) and are managed to be phased out over the next 50 years at a rate similar to habitat recruitment within SRMZs. These MACs are managed to offset predicted timber supply and forestry employment impacts. Since 1995, 19 new Spotted Owl territories have been discovered outside of SRMZs within the Chilliwack, Squamish, and Cascades forest districts. These 19 territories remain unprotected. Of the 64 Spotted Owl territories discovered since 1992, the recent population decline suggests that one-third, or possibly fewer, are thought to be currently occupied by the species.
Table 1. Known Spotted Owl locations in British Columbiaoccupied at least once between 1992 and 2002. (Blackburn and Godwin 2003)
|Known Owl Locations
|Squamish Forest District
|Chilliwack Forest District
|Cascade Forest District
|Not Managed by SOMP
6.4 Habitat Trends
It is estimated that approximately 67% (about 881 000 ha) of the total historic capable forested areas (about 1.32 million ha) occurred historically as suitable habitat (forests older than 100 years) in the Chilliwack and Squamish forest districts (Blackburn and Godwin 2003). Since the mid-1800s, timber harvesting for urbanization, agriculture, and resource extraction has occurred in the Chilliwack and Squamish forest districts, with almost the entire area of former forest lands in the lower Fraser River Valley converted to non-forest uses (approximately 150,000 ha). At present, it is estimated that about 639 000 ha (48% of total historic capable forested area) of suitable habitat currently exists within the two forest districts. This represents a 242 000 ha (28%) reduction in the amount of suitable habitat thought to occur historically, of which about one-third of this has been permanently lost from the lower Fraser River Valley (Blackburn and Godwin 2003). The overall impact of habitat loss has been somewhat offset by the recruitment of suitable habitat from maturing young forests (Figure 5).
In the future, the amount of suitable habitat is predicted to stabilize at about 565 000 ha (43% of total historic capable forested area) within the two forest districts. In comparison to historic levels, the future level of suitable habitat represents a 316 000 ha (36%) decline in the total amount of habitat once thought to be available to the Spotted Owl (Blackburn and Godwin 2003). Although the amount of total habitat will continue to decline, habitats within SRMZs and some protected areas will increase over time and could provide better habitat conditions for Spotted Owls than is currently available.
Figure 5. Proportion of suitable habitat (forest older than 100 years) in the Chilliwackand Squamish forest districts. (Blackburn and Godwin 2003)
6.5 Recovery Habitat
Recovery habitat is defined as the habitat required by a species to achieve and sustain a viable population (ROMAN 2003). The recovery goal for the Spotted Owl is to provide enough suitable habitat, spatially distributed in a way that it can support and sustain a minimum of 250 mature owls throughout its natural range (see section 13). For the Spotted Owl, recovery habitat includes both existing suitable habitat and potential habitat into which the species could recolonize. In addition, it includes dispersal habitat necessary to enable successful dispersal and establishment of young birds to new or vacant habitats.
The total amount and distribution of recovery habitat needed to meet the recovery goals is currently unknown, but its delineation will be one of the first priorities of the SORT in their development of a Habitat Action Plan. Theoretically, using 3200 ha per breeding territory and assuming that 250 mature owls is equivalent to 125 breeding pairs, about 400 000 ha of suitable habitat would be required. The existing Spotted Owl Management Plan currently manages 363 000 ha of habitat. It may appear that adding protection for an additional 37 000 ha outside of the current plan’s coverage would be enough to meet the goal and fulfill the requirement for recovery habitat. However, this is a gross oversimplification because the number of individual adults cannot accurately be determined by doubling the number of breeding pairs, and the amount of habitat required for recovery cannot simply be a total number of hectares, but must address the issues of connectivity, fragmentation, and elevational constraints across the range of the owl. As well, numbers of owls may be influenced by percentages of habitat available in the landscape and by percentage of territory overlap between neighbouring owls. Furthermore, of the 363 000 ha in the current plan, only about half of it is currently suitable, with recruitment and enhancement of second growth stands required to increase this amount. The existing management plan (SOMIT 1997a) suggests that the amount of suitable habitat would not begin to increase for several decades, after which it is hoped that numbers of Spotted Owls would also begin to recover.
6.6 Survival Habitat
Survival habitat is defined as the habitat thought to be the minimum amount and distribution needed to maintain the current population size (ROMAN 2003). The minimum amount and distribution of habitat required to maintain the current estimated population of less than 33 pairs is unknown at this time, but its delineation will be one of the first priorities of the SORT in their development of a Habitat Action Plan.
Although 363 000 ha of forest have been identified for management of Spotted Owls (of which at least 50% is still suitable) and other additional areas of suitable habitat exist outside these areas, the population continues to decline precipitously toward extirpation (Blackburn et al. 2002). So, although it may appear that sufficient habitat exists to accommodate the current population of owls, it is likely that the fragmentation of habitats and sparse distribution of potential breeding owls, as well as other biological limitations (section 4) and threats (section 5), have contributed to the population decline. The identification of survival habitat will need to consider these threats and limitations to the population. Stopping the population decline will likely require a combination of survival habitat conservation and other recovery actions.
6.7 Identification of Activities Likely to Result in Destruction of Critical Habitat
In general, suitable habitat will continue to be lost or degraded through human activities such as logging, mining, other resource development, urban and rural development, and development of transportation and utility transmission corridors, as well as by natural disturbances such as major forest fires and insect outbreaks.
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