Spotted owl (Caurina subspecies) COSEWIC assessment and status report: chapter 8

Limiting Factors and Threats

Northern Spotted Owls in British Columbia are very vulnerable to extirpation because of their small population size and low densities. Factors that threaten the species, from now onwards, can be divided into primary and secondary factors (Blackburn and Godwin 2003). Primary factors are those that cause long-term sustained effects that limit the carrying capacity, or total capable population size. The primary factor is habitat loss and fragmentation, while competition with Barred Owls is a primary factor resulting from habitat fragmentation.Secondary factors can cause short-term effects in population size, but populations would normally recover soon after the influence of the factor changes to a more favourable condition. Secondary factors include stochastic environmentalanddemographic events, genetic variability, predation, disease, parasites and viruses. Although primary factors generally limit population size and may cause extirpation, secondary factors are often the leading cause of extirpation of small populations (Blackburn and Godwin 2003).

Very small and declining population

Populations (<25 individuals and 6 breeding pairs as of 2005; 17 individuals and 3 pairs in 2006) and densities are very low in British Columbia. Although a few new territories have been discovered in recent years (e.g., Hobbs 2002, 2004, 2005; Keystone 2004), recruitment of young into the British Columbia population may have ceased as no new owls have been found in areas that have been intensely surveyed the last few years (Blackburn et al. 2002; Hobbs 2002, 2004, 2005). In addition, all recently radio-tagged juveniles have been found dead (n=6) or disappeared (n=1) (Hobbs 2004, 2005). If no natural recruitment is occurring, then the population is likely doomed to extirpation within 2-5 years unless populations are augmented.

Geographic isolation of existing pairs and lack of recruitment

Small, insular populations with high variance in reproductive success have a high likelihood of rapid extirpation unless populations are connected to create a larger meta-population (Shaffer 1985; Gilpin 1991; Lamberson et al. 1994). For resident species such as Spotted Owls, connectivity among the greater population is provided by dispersing juveniles which move through the landscape to find their own territory and mate. In 2006, the lack of recruitment of young owls from known active nests into the Canadian population, the large proportion of sites occupied by single owls and the distances between known breeding pairs suggests that existing sites and owls are too isolated for a biologically feasible level of natural connectivity.

In ideal conditions about 50% of juvenile Northern Spotted Owls die before or during natal dispersal. During dispersal, Spotted Owl juveniles have an increased susceptibility to both starvation and predation (Forsman et al. 2002a). This high mortality rate, coupled with distance between unoccupied suitable habitat and territories with only single owls in British Columbia, and very low numbers, makes it unlikely that any individual dispersing owl can find a mate. Lahaye et al. (2001) suggested that settlement by juvenile Spotted Owls might be postponed in declining owl populations due to a lack of mates, which has obvious implications for management of populations in British Columbia. However, Hobbs (2005) documented dispersing juveniles from different nests crossing paths near Anderson Lake, British Columbia, which suggests there is still potential for genetic exchange and establishment of “new” pairs, if juveniles could survive.

Managing for connectivity/dispersal habitat in British Columbia seems critical if Northern Spotted Owls are to be recovered and then maintained within their historic and current range in Canada.

Competition

Many biologists believe that Barred Owls may now pose a substantial impediment to recovery, although the severity of this threat is difficult to measure (Courtney et al. 2004; Herter 2004). Barred Owls are able to thrive in a variety of forest types and seral stages and can adapt to more varied food sources; therefore, they likely are at a competitive advantage over Northern Spotted Owls in fragmented old-growth forests. One study in the Washington Cascade Range showed that the presence of Barred Owls excluded Spotted Owls, even in Spotted Owl reserve areas (Pearson and Livezy 2003). Kelly et al. (2003) predict two scenarios if present trends continue: 1) Barred Owls displace Northern Spotted Owls or 2) an equilibrium is reached where either both occur within the same landscape or each occurs in different parts of the landscape.

Although the impact in British Columbia on Northern Spotted Owls from competition with Barred Owls is unknown, it likely is similar to that found in the USA (e.g., Kelly et al. 2003). Because of the very small population in British Columbia, any case of displacement of Northern Spotted Owls by Barred Owls will have serious consequences.

Great Horned Owls also occur in Northern Spotted Owl habitats and likely compete for food and space (Gutierrez et al. 1995).

Climate and climate change

The Spotted Owl is at the northern limit of its range in British Columbia. Colder and harsher climates in British Columbia suggest lower fecundity and survivorship than in the USA. Subsequent habitat changes from logging and other development may increase thermal and food intake stresses on Spotted Owls, further decreasing habitat suitability (Main and Harestad 2004).

Weather pattern is identified as one possible cause of widespread population declines (Anthony et al. 2006), so there seems a logical link to climate change as a threat. Effects of global warming may threaten Northern Spotted Owls if negative impacts occur on prey (e.g., declining abundance and availability), weather (e.g., more rain or snow or colder mean temperatures), vegetation (e.g., changing composition and structure), environmental stochasticity (e.g., increased fire rates and intensity if less rain, more insect outbreaks due to less sever winters), and disease (e.g., more exotic diseases such as West Nile Virus). For example, the Mountain Pine Beetle outbreak has reached catastrophic epidemic proportions with virtually all lodgepole pine forests in British Columbia seemingly doomed. Insect outbreaks are also seriously damaging Ponderosa pine and Douglas-fir forests throughout south interior British Columbia, including in SRMZs near Lillooet and Carpenter Lakes. Increases in insect outbreaks have been linked with global warming (Dale et al. 2001).

On the other hand, global warming may improve habitat and other environmental conditions for Northern Spotted Owls if conditions mimic those now found in the USA. This threat, however, is unmanageable as it relates to Northern Spotted Owls in British Columbia.

Habitat loss and fragmentation due to logging

Loss and fragmentation of habitat to harvesting of old-growth forests and degradation of habitat as a result of even-aged management of forests are widely believed to be the primary long-term threats to viable populations of Northern Spotted Owl throughout the Pacific Northwest (Gutiérrez et al. 1995; Courtney et al. 2004) and in British Columbia (Chutter et al. 2007).

Historically, in British Columbia, forest harvesting by clearcut logging has reduced structural diversity in harvested areas. Today, about 31% of suitable Northern Spotted Owl habitat lies within the harvestable landbase (Chutter et al. 2007) and is, therefore, threatened. Over the next 25 years, the rate of habitat loss caused by timber harvest and natural disturbance is expected to exceed the recruitment of suitable habitat from young forests resulting in further fragmentation and isolation of habitats available to the owl. Even more sobering is the projection that habitat recruitment under reasonable scenarios is not expected to equal habitat loss until 50 years from now (Sutherland et al. 2007). Note that eight of the 11 sites known to exist in parks/protected areas since 1992 remained active in 2005. This compares to one of 32 in SOMP/GVRD LTACs outside of parks, suggesting that ecological conditions within parks and/or habitat protection measures under the Parks Act, are more conducive to continued Spotted Owl presence (Chutter et al. 2007).

Loss of habitat to logging may be less permanent than losses from urban and rural development if logged areas are managed for owl habitat in the future. Evidence from the USA shows that lands managed under the Northwest Forest Plan, which has provisions for maintaining habitat for Spotted Owls, can better maintain habitat and owl populations as population declines were lower (-2.8%) than in other lands (-5.8%) (Anthony et al. 2006).

Northern Spotted Owls tend to use lower elevation habitats during winter to possibly avoid extreme cold temperatures and deep snow packs that limit prey availability. The shift to higher elevations, due to loss of old forest at lower elevations to harvest or development may cause increased mortality caused by starvation and exposure during severe cold and wet winters (I. Blackburn pers. comm.).

Recent impacts from habitat loss may be more related to fragmentation effects such as increased predation pressure, increased competition for food and space from Barred Owls, distance between suitable habitat patches, and other factors rather than just amount of habitat lost.

Habitat loss and fragmentation due to wildfire and other natural disturbances

In the USA, wildfire has consumed more Spotted Owl habitat than logging since 1990 (Courtney et al. 2004), although the effects of fire may not be detrimental to Spotted Owls unless extensive areas are lost (Bond et al. 2002; Clark 2007). Clark (2007) has shown that Spotted Owls will use moderately and severely burned areas (> 70% over mature forest overstory removed by fire) in the USA.

In British Columbia, wildfires have also impacted significant amounts of Northern Spotted Owl habitat. For example, the 2002 Seton Fire consumed about 1,400 ha of mainly suitable Northern Spotted Owl habitat and a 2004 wildfire consumed 900 ha of good habitat near a known nest (Hobbs 2005). The adult female that resided around the 2004 burned area was radio-tagged and was not observed to use the burned habitat for the 6 months her transmitter was responding, although she was observed roosting in live trees within 10 m of the still smoldering fire edge in the late fall.

Losses of habitat from wildfire, insect infestations (mountain pine beetle, Douglas-fir beetle) and blowdown may reduce available habitat if losses are extensive (Chutter et al. 2007), and negatively impact survival of existing birds or recovery potential. Insect infestations have expanded rapidly over most of interior British Columbia, and are also very evident within the eastern and northern edges of the range of the Northern Spotted Owl in BC.

Habitat loss to other human development

In British Columbia, in addition to commercial forestry, there has been considerable loss and/or fragmentation of forest habitat from rural and urban development, especially in the Lower Mainland, the Squamish valley, and other valleys with transportation corridors. Habitat lost to urban and agricultural development is undoubtedly lost forever as habitat for Northern Spotted Owls. Development of roads, pipelines, and utility corridors, in general, contribute to habitat fragmentation. In British Columbia, losses associated with expanding water reservoirs, independent power projects, and electric transmission lines are considered significant future threats (Chutter et al. 2007). There are also potential threats from increased mine and backcountry tourism development (Pierce Lefebvre 2005).

Connectivity with populations in the USA has been compromised by human development of the lower Fraser River valley. Large unforested valleys are known to act as barriers to dispersal (Forsman et al. 2002a); therefore, dispersal of owls between the USA and Canada is unlikely in the lower Fraser River valley between Vancouver and Agassiz.

Productivity

The Northern Spotted Owl is long-lived but has very low reproductive potential due its low clutch size, irregular breeding (does not breed every year), and high juvenile mortality. Recovery of populations is strongly limited by these reproductive limitations.

Predation

Predation has not been identified as a limiting factor for Northern Spotted Owl populations overall (Gutierrez et al. 1995). At the time the Northern Spotted Owl was listed as Endangered in the USA, increased predation risk in increasingly fragmented habitats was thought to be a major threat; however, research has not borne this out (Courtney et al. 2004). However, the threat of predation is likely more important for small populations of Northern Spotted Owls, such as found in British Columbia, compared to larger populations as the loss of one individual may have a larger, if not significant, impact on populations. Predation accounts for a large portion (68.0%) of juvenile mortality (Forsman et al. 2002a).

Diseases and parasites

Little is known about diseases and parasites that affect Northern Spotted Owls (Gutiérrez et al. 1995), but it is thought that these factors may be involved in some cases of starvation or predation (Forsman et al. 2002a). A majority of dead owls examined by Forsman et al. (2002a) were infested with blood or intestinal parasites. West Nile Virus is identified as a potential threat but of uncertain effect (Courtney et al. 2004).

Hybridization

Northern Spotted Owls are closely related to Barred Owls (Gutiérrez et al. 1995). In Washington and Oregon, a total of 50 hybrids were observed between 1974 and 1999 (Kelly 2002). In British Columbia, there is one known case of a male Barred Owl breeding with a female Northern Spotted Owl (Hobbs 2004) and one case of a male Spotted Owl paired with a female Barred Owl (J. Hobbs pers. comm.). The frequency of interspecific matings is extremely low when compared to the total number of Northern Spotted Owl matings within the demographic study areas in Washington and Oregon (Kelly 2002) and the present extent of hybridization is assumed to be low (Gutiérrez et al. 1995). In general, segregation by habitat (Barred Owls use more open forest, Spotted Owls use more closed-canopied mature forest) serves to separate the two species (e.g., Herter and Hicks 2000), but these differences may be declining. Despite the extensive sympatry of these two species, genetic isolating mechanisms that separate Northern Spotted Owl and Barred Owl are thought to be effective enough to maintain hybridization at this very low incidence (Hamer et al. 1994).

Genetics

Courtney et al. (2004) did not identify any genetic issues that are significant threats to Northern Spotted Owls range-wide, but these authors did note there is concern that the small Canadian population may be at such low levels that inbreeding, hybridization, and other effects could occur (e.g., restricted gene flow in fragmented habitats and isolated populations, Barrowclough et al. 2006).

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