Spotted owl (Caurina subspecies) COSEWIC assessment and status report: chapter 5

Habitat

Most previous habitat suitability descriptions have been extrapolated from research in Washington (Hanson et al. 1993, SOMIT 1997a). There are two general ecosystems used by Northern Spotted Owls in Washington and British Columbia: wetter ecosystems on the west slope of the Cascade Range and drier ecosystems on the eastern slopes of the Cascade Range. In British Columbia, Northern Spotted Owls occupy habitats found largely within 3 broad ecosystems: maritime Coastal Western Hemlock (CWH), submaritime CWH, and Interior Douglas fir (IDF) biogeoclimatic zones (Meidinger and Pojar 1991, SOMIT 1997a).

Very recently, considerable effort has been expended on developing a habitat model for British Columbia. The Canadian Spotted Owl Recovery Team (CSORT; Chutter et al. 2007) has grouped habitat types into three ecologically based subregions: maritime, sub-maritime or continental. In British Columbia, high-quality habitat tends to be in forests >200 years old, at elevations below 1200 m and with large amounts of large diameter and tall trees (Chutter et al. 2007). The most recent habitat model and supply analysis for British Columbia is provided by Sutherland et al. (2007).

Throughout its range, the Northern Spotted Owl occurs in mixed coniferous forests with an uneven-aged, multi-layered canopy that includes numerous large trees with broken tops, deformed limbs and large cavities, and numerous large snags and accumulations of logs and downed woody debris (Thomas et al. 1990; USDI 1992). In moister parts of the range, these habitat characteristics are found naturally in late seral and old-growth forests. In drier parts of their range, including British Columbia, owls have been observed in some younger forest stands where structural components typical of old-growth forests have been created by disturbances such as fire, wind or selective logging (Dunbar and Blackburn 1994; Buchanan et al. 1995).

Forest structure is more important than actual forest age when defining suitable Northern Spotted Owl habitat. In the southern part of its global range (northern California), Northern Spotted Owls can be found in younger-aged stands (< 100 years old) than further north in Washington and British Columbia. Longer growing seasons, different tree species and good site conditions in southern parts of the owl’s range can provide old-growth-like structural components of owl habitat (e.g., large diameter trees and snags, multilayered canopies, and large woody debris) sooner than similar aged forests further north (Forsman 2003).

Figure 2. Former distribution of the Northern Spotted Owl in British Columbia (Chutter et al. 2007).

Figure 2. Former distribution of the Northern Spotted Owl in British Columbia

Breeding habitat

Northern Spotted Owls primarily use old-growth trees for nesting, either in old-growth stands or in remnant old-growth patches (Thomas et al. 1990; Forsman and Giese 1997; Ripple et al. 1997). Nest sites are typically located in dense, multi-layered, older forests with 85-90% canopy closure (Gutiérrez et al. 1995). Occasionally some of these characteristics are lacking, particularly in the drier, eastern parts of their range (Buchanan et al. 1995).

Northern Spotted Owls do not build their own nest, but depend on naturally occurring nest sites. Nest sites include broken treetops, tree cavities resulting from heart rot, abandoned raptor nests, mistletoe brooms, squirrel nests, and debris accumulations (Forsman et al. 1984; Dawson et al. 1986; Fenger et al. 2006). In Oregon, most nests were in tree cavities (64%) with the remainder on platforms, and average height above ground for nests was 27 m (range 10-55 m) (Forsman et al. 1984). A variety of different trees are used for nesting, suggesting the presence of structures (i.e., cavities or platforms) is more important than tree species in nest site selection (Forsman and Giese 1997). Pairs may reuse the same nest each year or use alternate nest sites (Gutiérrez et al. 1995; Hobbs 2004, 2005). In Washington, cavities tend to be used for nests on the coast and platforms tend to be used in the drier interior (Forsman and Geise 1997).

In British Columbia, Northern Spotted Owl nests have been found in the cavities of living western red cedar (Thuja plicata), western hemlock (Tsuga heterophylla) and Douglas-fir (Pseudotsuga menziesii) as well as in the top of Douglas-fir snags and on platforms in Douglas-fir trees. Most nests (n=12) were in hollows on top of broken tree stems (n=6), cavities at broken branches (n=3), in mistletoe (n=1) and in an old Northern Goshawk nest (n=2) (Manley et al. 2004; Hobbs 2004, 2005). The first nest ever reported for North America in a black cottonwood (Populus balsamifera trichocarpa) was found in British Columbia in 2004 (Hobbs 2005). Nests ranged from 5.1 to 38 m in height above ground. Nest trees in BC ranged from 50-131 cm dbh in live (n=6) or dead trees (n=5) (Manley et al. 2004).

In British Columbia, nests sites were found in four Biogeoclimatic variants: Southern Dry Submaritime Coastal Western Hemlock, submontane very wet maritime CWH, Wet Warm Interior Douglas-fir, and Cascade Dry Cool Interior Douglas-fir. Nests were at elevations ranging from 368-1130 m, were on steep or moderately steep slopes (mean 53% slope). In Interior Douglas-fir zones nest slopes had southeast to southwest aspects. In Coastal Western Hemlock zones aspects were north to east. Nine nests were in older forest structural stages, and two nests in the IDF were in mature age classes. Crown closure ranged from 15-70% with a mean of 48%. Decaying wood and rock were dominant substrates at nest patches (Manley et al. 2004).

Foraging habitat

Northern Spotted Owl foraging habitat is characterized by high canopy closure and complex structure (Gutierrez et al. 1995). Owls tend to concentrate their foraging in old-growth or mixed-aged stands of mature and old-growth 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 suggest that old-growth forests are superior habitat for Northern Spotted Owl foraging. Mature stands were found to be less suitable habitat than old-growth, young stands provided marginal habitat and clear-cuts were unsuitable habitat (Thomas et al. 1990; Forsman et al. 1984; Carey et al. 1990; Carey et al. 1992).

Northern Spotted Owls forage mainly in older stands (Carey et al. 2002). Prey may or may not be more abundant in old-growth stands versus second growth as studies have shown contradictory or no data for primary prey species (Ransome and Sullivan 2003; J. Hobbs pers. comm.). However, the structural characteristics of old growth stands, such as multi-storied canopy which provide thermoregulation opportunities and stand openness likely makes prey more available in these situations.

Roost habitat

Spotted Owls have a narrow thermal neutral zone (Ganey et al. 1993; Weathers et al. 2001) and appear to select sheltered roosts to avoid poor weather, summer heat, predation and harassment by corvids and passerines (Barrows and Barrows 1981; Barrows 1981; Forsman 1980; Forsman et al. 1984, Herter et al. 2002; Courtney et al. 2004). During warm weather, Spotted Owls seek roosts in shady recesses of understory trees and occasionally will even roost on the ground (Barrows and Barrows 1981; Forsman et al. 1984; Gutiérrez et al. 1995). In winter, they roost relatively high in large trees near the trunk with overhanging branches to shelter themselves from precipitation.

Roost sites in Washington and Oregon were mostly in old-growth forest stands (88%) but some were in mid-seral forests (11%) (Carey et al. 1992). Roosts were typically in areas of relatively dense vegetation with high canopy closure. 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 and multi-storied stands facilitate this movement (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).

In British Columbia, Northern Spotted Owls tend to roost during the breeding season in larger conifers near nests within stands of older age classes but may occasionally use thickets of younger conifers (Hobbs 2002). Multi-year roost site fidelity has been confirmed in British Columbia (J. Hobbs pers. comm.)

Dispersal habitat

Juvenile owls are obligate dispersers that leave their parent’s territory in the fall. To be successful, dispersing juveniles must find habitats that provide protection against predators and inclement weather, and provide available prey to sustain dispersing individuals. Old and mature forests are thought to provide ideal conditions for dispersal (Hanson et al. 1993). However, dispersing owls use a fragmented mosaic (unlike habitat used for other functions) of various-aged forests, clearcuts, roads, and non-forested areas (Forsman et al. 2002a).

Dispersal of juveniles tends to occur in random directions and juveniles travel variable distances (91% < 50 km; range 0.6-111 km) before settling on territories (Forsman et al. 2002a). Breeding-age owls also occasionally disperse, especially young females who have lost their mate. These movements may be to new territories or back and forth movements between alternate territories (Forsman et al. 2002a).

The quality (stand structure, degree of fragmentation, topography) of dispersal habitat is likely an important factor in survival of dispersing birds (Lamberson et al. 1992; Miller et al. 1997). Large non-forested valleys (e.g., such as the Lower Mainland of British Columbia in its current state) are known to act as barriers to dispersal (Forsman et al. 2002a).

In British Columbia, radio-telemetry data from 7 dispersing juveniles confirmed their use of old forest habitats during that life stage (Hobbs 2004, 2005). This study also showed that dispersing juveniles were able to negotiate their way through some sub-optimal habitats such as early seral forest, severely burned areas, and across large waterbodies (Hobbs 2005), although US research showed decreased survival with increased use of fragmented forest for dispersal (Lamberson et al, 1992).

Habitat trends

Historical trends

Before European settlement, the amount of suitable habitat in southwestern British Columbia likely remained relatively stable notwithstanding habitat manipulations by First Nations peoples and by natural disturbances. Blackburn and Godwin (2003) estimated that about 67% (881,000 ha) of the 1,320,000 ha historic forested areas was suitable habitat (forests older than 100 years) in the Chilliwack and Squamish Forest Districts Since the mid-1800s, timber harvesting for urbanization, agriculture and resource extraction has occurred in this area. Almost all of the forest in the lower Fraser River Valley has been converted to non-forest uses. 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 1/3 is permanently lost from the lower Fraser River Valley (I. Blackburn pers. comm.). Very recently, CSORT has stated there is 534,442 ha of suitable habitat (282,427 ha of nesting habitat and 251,995 ha of foraging habitat (Chutter et al. 2007); however, only 395,000 ha are in large enough patches to be suitable habitat for Northern Spotted Owls.

Changes to the distribution and quality of habitat in the landscape have a greater impact than the mere % loss of habitat. Although some impact of habitat loss has been offset by the recruitment of suitable habitat from maturing young forests much of this recruitment has occurred in constrained forests (e.g., protected areas, inoperable or inaccessible areas, high elevation forests), while most of the habitat loss has occurred at lower elevation along valley bottoms (I. Blackburn pers. comm.). The current distribution of suitable habitat includes small habitat patches, fragmented and isolated habitats, and landscapes with low densities of habitat; factors that may make these habitats unsuitable for Northern Spotted Owls. The general loss of lower elevation habitat has limited Northern Spotted Owls, in some areas, to using mid- to high-elevation habitats or excluded them entirely (I. Blackburn pers. comm.).

Future habitat trends

The amount of suitable habitat in British Columbia continues to decline through harvesting of old forest, mostly through industrial logging operations but also from urban and infrastructure expansions. In the future, the amount of suitable habitat is predicted to stabilize at about 565,000 ha, a further 12% decline in suitable habitat (Figure 3). In comparison to historic levels, the future level of suitable habitat represents a 314,000 ha (36%) decline in the total amount of habitat once thought to be available to the Spotted Owl (I. Blackburn pers. comm.). The CSORT expects a >20 year long lag time before area of habitat available begins to increase and is more suitably distributed for Northern Spotted Owls (Chutter et al. 2007).

Figure 3. Estimated trend in Northern Spotted Owl habitat in the Squamish and Chilliwack Forest Districts (Figure from Blackburn and Godwin 2003).

Figure 3.   Estimated trend in Northern Spotted Owl habitat in the Squamish and Chilliwack Forest Districts (Figure from Blackburnand Godwin 2003)

Habitats within some Spotted Owl Management Zones (Special Resource Management Zones: SRMZs) and some protected areas will increase and provide better habitat conditions for Spotted Owls than currently available if current protection is maintained or improved. Due to logging history, 44 of 101 Long Term Activity Centres (LTACs) are <67% suitable for Northern Spotted Owls and require habitat recruitment to meet government policy requirements (Chutter 2004). As well, current forest practices (small and dispersed cutblocks) may provide for greater quantities of low elevation habitats for over-wintering and dispersal in the future (I. Blackburn pers. comm.). However, in some SRMZs and other Northern Spotted Owl “protected areas” with >67% suitable habitat logging continues (as of November 2006).

Habitat continues to decline in the USA as well and the current draft recovery plan for Spotted Owls includes allowances for logging of suitable habitat (USFWS 2007; Wildlife Society 2007).

Habitat protection/ownership

Protection of habitat is widely recognized as essential to preventing species extirpation and extinction (see Accord for the Protection of Species at Risk) (Chutter et al. 2007). Loss and fragmentation of habitat is recognized as a primary threat to the Northern Spotted Owl throughout the Pacific Northwest (USDI 1992; Dunbar and Blackburn 1994; Gutiérrez et al. 1995), and has been identified similarly in the Canadian Spotted Owl Recovery Strategy (Chutter et al. 2004); therefore, habitat protection is essential.

In British Columbia, the majority of Northern Spotted Owl habitat is located on Crown lands within provincial forests and protected areas. In 1997, the Spotted Owl Management Plan (SOMP) was released as government policy. Under SOMP (SOMIT 1997a,b) 363,000 ha were identified for the management of Northern Spotted Owls. Approximately 159,000 ha (44%) of this is located within parks and other protected areas including Provincial Parks (E.C. Manning, Golden Ears, Garibaldi, Cypress, Skagit Valley, Stein, Mehatl and Birkenhead Lake), Cascade Provincial Recreation Area,the Greater Vancouver Water District, and Ecological Reserves (Chilliwack River and Skagit River Cottonwoods). The remainder (204,000 ha) of habitat occurs within the Provincial Forest in the Chilliwack and Squamish Forest Districts.

Recent work (with different habitat model variables) has determined that only 99,675 ha in protected areas is actually suitable for Northern Spotted Owls. About 340,000 ha of unprotected suitable and restorable (within 20 years) habitat is available to breeding owls (Chutter et al. 2007).

Areas managed for Northern Spotted Owls include 19 Special Resource Management Areas (SRMZs) (Figure 4) which are further subdivided into 101 LTACs. Each LTAC represents the approximate home range of one pair of owls (~3,200 ha): targets are set to maintain 67% of this area as suitable habitat. Harvest is allowed within LTACs with greater than 67% suitable habitat and some silvicultural activities are allowed in LTACs for enhancement purposes (Chutter 2004); although in 2003 CSORT recommended no harvest in LTACs that have been occupied since 1997 (Chutter et al. 2004).

SOMP addresses some Northern Spotted Owls that occur outside of protected areas and SRMZs through management of eight 3,200-hectare Matrix Activity Centres (MACs). These eight MACs are designed to be phased out as protected areas for owls over the next 50 years as additional habitat (older forests) in LTACs is recruited and offsets predicted timber supply and forestry employment impacts. Nineteen sites within SOMP boundaries discovered after 1995 (11 in the Squamish and Chilliwack Forest Districts and all eight sites in the Cascades Forest District) are not offered any protection under the SOMP. However, three sites in the Cascades Forest District are receiving interim SOMP LTAC-equivalent protection as WHAs (J. Hobbs pers. comm.).

Figure 4. Spotted Owl Management Areas: Special Resource Management Zones, ProvincialParks, Matrix Areas and unprotected areas, 2003. Map source: Biodiversity Branch, BC Ministry of Environment, Victoria, BC.

Figure 4.   Spotted Owl Management Areas: Special Resource Management Zones, ProvincialParks, Matrix Areas and unprotected areas, 2003.

Of the 72 sites reported as active at least from 1992 to 2005, 43 receive some form of long-term habitat protection within the boundaries of SOMP as they occur in parks/protected areas (11), the GVRD watersheds (5), or existing SOMP LTACs (27) which fall outside parks and the GVRD (Chutter et al. 2007). Of the 13 active sites detected in 2005, eight are located within parks/protected areas (Chutter et al. 2007; Hobbs 2005). More significantly, from an owl habitat management perspective, eight of the 11 sites known to exist in parks/protected areas since 1992 remained active in 2005, compared to just one of 32 in LTACs outside of parks (Chutter et al. 2007).

Two forest companies holding Crown forest tenures within the range of the Spotted Owl have voluntarily deferred logging in Spotted Owl SRMZs in response to concerns for Northern Spotted Owls. These deferrals remain indeterminate as both companies are awaiting further action/direction from government (Chutter et al. 2004). Two additional proposed LTACs were being protected as the involved forest companies had deferred logging (Chutter et al. 2007). BC Timber Sales, however, has taken control of some of the deferred areas and continues to allow logging within some SRMZs and other known Spotted Owl sites (J. Hobbs pers. comm.).

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