Northern spotted owl (Strix occidentalis caurina) recovery strategy: chapter 8
5. Threats to the Species
Although a few new territories have been discovered in recent years (e.g., Hobbs 2002), recruitment of young into the now small, fragmented British Columbia population is likely infrequent. Areas known to have been previously occupied, as well as surveyed areas of suitable habitat where owls have not been recorded in the past, have not been reoccupied by owls over the last few years, and no banded juveniles have been relocated in following years (Blackburn et al. 2002). Small populations are extremely vulnerable to extirpation. If the population is unable to stabilize and become resilient to the factors that caused the decline, the population will become extirpated.
Threats to the species 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. Primary factors include habitat loss and fragmentation, competition with Barred Owls and, possibly, climate change. 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 environmental, demographic, and genetic events. Although primary factors generally limit population size and may ultimately cause extirpation, secondary factors are often the proximal cause of extirpation of small populations (Blackburn and Godwin 2003).
5.1 Loss and Fragmentation of Suitable Habitat
Loss and fragmentation of habitat is widely thought to be the primary threat to the Spotted Owl throughout the Pacific Northwest (USDI 1992; Dunbar and Blackburn 1994; Gutiérrez et al. 1995). More than 10% of the historic range of the owl within the Chilliwack and Squamish forest districts has been converted to urban and agricultural areas, roads, pipelines, reservoirs, hydroelectric dams and associated reservoirs, recreational developments, and utility corridors. Continued habitat loss will likely decrease the total amount of habitat available to the owl and further fragment habitat. As well, natural disturbances (e.g., from fire, insects, blowdown) may also result in further habitat losses if they are extensive enough. Decreasing amounts of habitat and increasing fragmentation within the landscape is associated with decreased occupancy, fewer potential territories, lower productivity, lower survivorship, and lower dispersal success. Patches of suitable habitat must be close enough to allow owls to use and move among them. If fragmentation is too great, owls may be unable to efficiently use the suitable habitat that may be available to them. In addition to habitat loss, conversion of old stands to young stands may impede dispersal of owls, depending on the spatial configuration of the landscape, because young stands may act as barriers. If such constraints on dispersal occur, then some areas of suitable habitat, although they are large enough to support owls, may not be occupied.
In British Columbia, clearcut logging typically has reduced stand level structural diversity in logged areas. More recent forest management practices may provide better management of biodiversity values, including provisions for maintaining more structural diversity in logged areas both at the stand-level (e.g., Wildlife Tree Patches and Riparian Management Areas), and at the landscape level (Old Growth Management Areas, Ungulate Winter Ranges, and indirectly through Visual Quality Objectives). However, these management practices do not provide large enough habitat patches to support breeding pairs of Spotted Owls. As well, rotation lengths between successive harvests may be shorter than is currently thought to be required to achieve suitable habitat conditions for owls (i.e., shorter than 100 years). This could result in the equivalent of a permanent loss of habitat because the habitat would be retained as unsuitable over the long term
Connectivity among subpopulation clusters is considered essential to maintain a population’s viability (Lamberson et al. 1994.) Connectivity with populations in the United States 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 United States and Canada is no longer likely in the lower Fraser River valley from Vancouver to close to Chilliwack. Any dispersal between owl subpopulation clusters in British Columbia and Washington is likely restricted to the Skagit River Valley.
5.2 Range Expansion of Barred Owls
In the 1960s, the Northern Barred Owl (Strix varia varia) expanded its range westward and southward and began to overlap the range of the Spotted Owl in British Columbia (Campbell et al. 1990; Dunbar et al. 1991) and the United States (Hamer 1988; Gutiérrez et al. 1995). Barred Owls thrive in a variety of forest types and seral stages and can adapt to more varied food sources than Spotted Owls.
Competition with Barred Owls is thought to be a primary threat to Spotted Owls through increased competition for habitat and prey, and perhaps as a consequence of hybridization and predation (Wilcove 1987; Carey et al. 1992; SOMIT 1997a). Territorial interactions have been observed between Barred Owls and Spotted Owls, and Barred Owls have been found at nest sites formerly occupied by Spotted Owls (Hamer et al. 2001; Kelly et al. 2003). Kelly et al. (2003) suggested that Barred Owls sometimes displaced Spotted Owls from their territories if Barred Owls occurred within 0.8 km of a Spotted Owl territory centre. Those Spotted Owls that were not displaced continued to maintain their normal reproductive output, but regional reproductive output was lower because there were fewer Spotted Owls (Kelly 2002). However, a study in Oregon found that although Barred Owls had taken over some Spotted Owl territories, the population of Spotted Owls had not declined (Forsman et al. 2002b).
Competition for food between Barred Owls and Spotted Owls seems likely because diets greatly overlap (76% in one study in western Washington), food is limiting in some years, and Barred Owls have moved into much of the Spotted Owl’s range (Hamer et al. 2001).
In British Columbia, the extent of the potential for competition is illustrated by the discovery in the early 1990s that Barred Owls were four times more abundant than Spotted Owls within the range of the latter species (Dunbar and Blackburn 1994). Most Barred Owls detected during Spotted Owl surveys in the province tended to occur along valley bottoms near riparian habitats; Spotted Owls tended to occur at mid- and upper-elevation areas where most of old forest remains (Blackburn and Harestad 2002). A preliminary analysis of Barred Owl territory occupancy within Spotted Owl survey areas between 1992 and 2001 indicated a decline in Barred Owl numbers similar to that recorded for the Spotted Owl (Blackburn and Harestad 2002). The authors suggested several scenarios to explain the similar population declines, including a lack of displacement competition, because the similar declines suggest other factors were simultaneously affecting both species. Conversely, the results may suggest that both species were declining due to competition for the same resources.
To further complicate the issue, hybridization between Barred Owls and Spotted Owls has been recorded (Hamer et al.1994). 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). 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). Despite the extensive sympatry of these two species, the genetic isolating mechanisms that separate Spotted Owls and Barred Owls are thought to be effective enough to maintain hybridization at this very low incidence (Hamer et al. 1994).
Kelly et al. (2003) suggested two possible scenarios if present trends continue: (1) Barred Owls displace Spotted Owls and the latter species becomes extirpated, or, (2) some form of equilibrium is reached where both species coexist. Currently, the potential impact of Barred Owls on Northern Spotted Owls in British Columbia is unknown. Potential for competition and/or hybridization between the species is of particular concern in British Columbia because both could have serious consequences by reducing the already small pool of breeders.
5.3 Climate Change
Climate change may threaten Spotted Owls if it negatively affects prey species (e.g., declining abundance and availability), weather (e.g., more rain 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 severe winters), and disease. For example, insect outbreaks are strongly affecting forest health in Special Resource Management Zones near Lillooet Lake (D. Heppner, pers. comm.). Recent increases in insect outbreaks have been linked with climate change (Dale et al. 2001).
On the other hand, climate change may improve habitat and other environmental conditions for Spotted Owls if conditions change to mimic those now found in southern parts of its range where owl densities are higher. Therefore, the level of threat from climate change remains undetermined as it relates to Spotted Owls in British Columbia.
5.4 Environmental, Demographic, and Genetic Stochasticity
Populations are vulnerable to extinction from many factors. When populations become small or isolated, for whatever reason, they can become vulnerable to extirpation through environmental, demographic, or genetic stochastic (random) events (Caughley and Gunn 1995). Stochastic events, therefore, tend to be additive in that they may contribute to factor(s) that originally caused the population’s decline.
Environmental stochasticity refers to periodic variation in conditions (e.g., wildfires, wind, forest diseases, and floods) and the effect these have on a population. Typically, environmental factors are of concern if they have the potential to eliminate habitats and the populations that they support. Catastrophic loss of habitat due to fire is a pertinent example of an environmental factor that has the potential to affect a small population or subpopulation of Spotted Owls. This issue is particularly relevant because fire suppression in some dry-site forests has changed the structure and species composition of forests in such a way that they are vulnerable to stand-replacement events. Although environmental stochasticity can affect all sizes of populations, the effects are higher in smaller populations or those already affected by other factors.
Demographic stochasticity in small populations means that changes in population size from one year to the next are more related to pure chance than age-specific survival and reproduction. That is, population size varies between years, but when the population is small, this variation has more chance of causing extirpation.
The third way random events negatively affect small populations is through loss of genetic variability. Once a population is reduced to below a certain threshold, random genetic drift will result in some alleles being lost by chance in the transfer of genetic material from one generation to the next (Caughley and Gunn 1995). The lost alleles may be related to adaptation to certain conditions; their loss would increase the species’ risk of extinction.
5.5 West Nile Virus
The West Nile virus is a disease of concern throughout the owl’s range. Originally the West Nile virus was known only from Africa, West Asia, and the Middle East. This virus was first isolated in the Western Hemisphere in New York in 1999, and has since spread rapidly across North America (Canadian Cooperative Wildlife Health Centre 2003). The geographic range of the West Nile virus may already include British Columbia because it was documented in Washington and Saskatchewan in 2002, and Alberta in 2003 (Helen Schwantje, pers. comm.).
Wild birds are the usual host of this virus; currently it has been detected in 138 species of dead birds in North America. Infected birds include several species of owls, although not the Spotted Owl as yet. Although birds infected with West Nile virus can become ill or die, most infected birds survive and become carriers of the virus (Centers for Disease Control and Prevention 2003).
The effect West Nile virus will have on Spotted Owl populations in British Columbia is difficult to predict. However, given the current low population, any added negative factor could significantly increase the chance of extirpation.
5.6 Human Disturbance
This owl’s docile nature and low density suggests that most recreational activities are probably not a threat. Only those activities directed specifically at the birds or very close to active nests are likely to disturb the birds. In southern Utah, Mexican Spotted Owls (Strix occidentalis lucida) were unlikely to flush at distances of 24 m or more from hikers (Swarthout and Steidl 2003).
The most likely human disturbance of Spotted Owls in British Columbia is related to activities that produce very loud noises. These sources of potential disturbance might include logging activity, blasting (e.g., during road construction or operation of a rock quarry), or low flight of jets or helicopters. Delaney et al. (1999) reported substantial rates of flushing in response to chainsaw and helicopter noise in close proximity to Mexican Spotted Owls during the nesting and non-nesting seasons in New Mexico. At all distances less than 60 m, chainsaws caused a higher flushing rate than helicopter noise at the same distance. Spotted Owls did not flush when these disturbances occurred more than 105 m away. Measures of Spotted Owl productivity did not differ significantly between control sites and those exposed to chainsaw or helicopter noise, although productivity was slightly lower at noisy sites (Delaney et al. 1999).
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