Technical Thematic Report No. 11. - Woodland caribou, boreal population, trends in Canada
Causes of decline/Importance of boreal caribou
Causes of decline
The broad-scale range recession and population declines of boreal caribou in most provinces and territories within their range are associated with human settlement and industrial resource extraction due to the loss, degradation, and fragmentation of their habitat (primarily mature coniferous forest) (Bergerud, 1974; Mallory and Hillis, 1998; Schaefer, 2003; Vors et al., 2007). Proximate causes of decline associated with landscape-level habitat change include increased predation (Bergerud, 1967; Edmonds, 1988; Seip, 1992; McLoughlin et al., 2003; Environment Canada, 2007; Vors et al., 2007; Vors and Boyce, 2009), overhunting by humans (Bergerud, 1974; Edmonds, 1988), increased risk of disease or parasite transmission from other ungulate species (Bergerud, 1974), and linear disturbance (Dyer et al., 2001; Dyer et al., 2002). Weather and climate change may affect several aspects of boreal caribou ecology by combining with other threats in complex ways that magnify the principle causes of decline.
There is wide agreement that the primary proximate limiting factor for boreal caribou populations is predation, driven by human-induced or natural landscape changes that favour early seral stages and higher densities of alternative prey (Bergerud and Elliot, 1986; Ferguson et al., 1988; Bergerud and Mercer, 1989; Seip, 1992; Cumming et al., 1996; Stuart-Smith et al., 1997; Rettie and Messier, 1998; Schaefer et al., 1999; Racey and Armstrong, 2000; Courtois, 2003; Courtois et al., 2007; Vors et al., 2007; Environment Canada, 2007; Environment Canada, 2008). Boreal caribou are closely associated with late-successional coniferous forests and peatlands (Rettie and Messier, 2000). Such habitats appear to function as refugia, away from high densities of predators and their alternate prey (Bergerud et al., 1984; Bergerud, 1985; Cumming et al., 1996; Rettie and Messier, 1998; Racey and Armstrong, 2000). Although wolves (Canis lupus) were very scarce or absent throughout most of the original distribution of woodland caribou (Cringan, 1956), logging and other industrial disturbances have increased the amount of early seral-stage forest and promoted higher densities of prey species such as moose (Alces alces) and white-tailed deer (Odocoileus virginianus), which support higher predator densities, especially wolves (Bergerud and Elliot, 1986; Seip, 1992; Stuart-Smith et al., 1997; Racey and Armstrong, 2000; Wittmer et al., 2005; Wittmer et al., 2007; Vors et al., 2007; Vors and Boyce, 2009)
Linear disturbances (for example, roads and seismic lines) that accompany industrial development in the boreal forest facilitate greater predator mobility and hunting efficiency (James and Stuart-Smith, 2000; Dyer et al., 2001; McLoughlin et al., 2003; James et al., 2004). In addition, woodland caribou tend to avoid industrial infrastructure such as roads, timber harvest cut-blocks, pipelines, oil and gas well sites, and geophysical exploration lines, all of which essentially reduce the suitability of habitat adjacent to these developments (Chubbs et al., 1993; Smith et al., 2000; Dyer et al., 2001; Lander, 2006). There is also evidence that roads can act as a partial barrier to boreal caribou movements (Dyer et al., 2002) and in some areas boreal caribou are vulnerable to mortality from vehicle or rail collisions (Brown and Hobson, 1998).
The decline of several local populations across Canada is attributed to hunting and other sources of direct human-caused mortality (Bergerud, 1967; Kelsall, 1968; Bergerud, 1974; Bergerud, 1978). For example, hunting is the most significant threat faced by boreal caribou in Labrador. Hunters from Labrador as well as Quebec target isolated boreal caribou groups as well as mixed groups or those adjacent to the unlisted migratory, forest tundra caribou ecotype (Schmelzer et al., 2004). Uncontrolled hunting has the potential to cause population declines of some local populations in Manitoba (V. Crichton, Government of Manitoba, pers. comm.).
Although the extent of hunting is poorly understood in most areas, analyses of historical population trends, data from radio-collared animals, and current demographic information indicate that hunting remains an important component of adult female caribou mortality and hence is a primary threat to some local populations (for example Dzus, 2001; Schmelzer et al., 2004). Hunting of boreal caribou is facilitated by construction of roads and other linear features and by use of off-road vehicles that permit access to previously inaccessible areas.
Weather affects several aspects of boreal caribou ecology and may combine with other threats in complex ways. Severe winters with deep snow, low temperatures, and strong winds may decrease birth rates and/or calf survival (Boertje et al., 1996; Adams and Dale, 1998). Conversely, it is possible that severe winters could provide a benefit to boreal caribou in some situations by limiting the distribution and abundance of other ungulates and predators (Environment Canada, 2007).
Climate change, particularly greater weather variability, may increase the frequency and severity of wildfires and cause more freeze-thaw cycles, freezing rain, deep snow, hot summer temperatures, and changes in the food supply (see review in COSEWIC, 2002). Changes in fire cycle may reduce the area of mature forest and alter the distribution of plant communities (Racey, 2005). Ice, snow crusting, and deep snow may impede caribou travel and limit access to ground lichens, which are a vital winter forage (Brown and Theberge, 1990). Alternatively, warming tends may allow deer and other prey species to expand into boreal caribou range, facilitating predation (Racey, 2005) and the spread of disease. Warmer and more humid summers may increase harassment by insects (Environment Canada, 2007). Caribou subject to insect harassment spend less time foraging, undertake energetically costly annoyance responses and spend more time in relatively unproductive habitats (Kelsall, 1968). Severe insect harassment could result in a negative energy balance and a subsequent reduction of body condition in the fall (Gunn and Skogland, 1997; Weladji et al., 2003). Moreover, climate change could result in the increased distribution of forest insect species that cause tree mortality (for example, mountain pine beetle, Dendroctonus ponderosae). Broad-scale forest insect invasions could cause changes to the fire cycle that diminish the quantity and quality of boreal caribou local population ranges. All of these factors could increase risk to population persistence of boreal caribou.
Although little evidence exists of disease or parasites impacting boreal caribou populations (Jordan et al., 2003), broad scale climate and habitat change may play a role in increasing the risk of disease transmission from white-tailed deer to caribou. For example, caribou are susceptible to a parasitic nematode, the brain or meningeal worm (Parelaphostrongylus tenuis) carried by white-tailed deer. The parasite is deadly to caribou (Anderson and Strelive, 1968; Bergerud, 1974). Warmer winter trends combined with landscape-scale habitat change may favour conditions for white-tailed deer range expansion to areas previously inhabited mainly by caribou. Wetter summer conditions may increase survival of the brain worm larvae in deer feces, thus enhancing potential for transmission of brain worm from deer to caribou. As landscape change increases cohabitation of deer and boreal caribou across Canada, the extent to which disease may limit caribou populations cohabiting with infected deer remain an area of research opportunity.
Importance of boreal caribou
As an ungulate species sensitive to habitat change in the boreal region of Canada, boreal caribou are significant from an ecological and cultural perspective. Caribou are a prey animal and thus support predator populations. Although the importance of boreal caribou in the diet of their predators varies across their range with population density, boreal caribou are likely to occur in the diets of all large carnivores within their range, including wolves, bears (Ursus americanus and Ursus arctos), cougar (Felis concolor) and wolverine (Gulo gulo). Other predators, such as lynx (Lynx canadensis), coyote (Canis latrans), and red fox (Vulpes vulpes), and a variety of smaller predators (such as ermine Mustela erminea) and scavengers (such as raven Corvus coraxandgolden eagle Aquila chrysaetos) benefit from the remains of caribou killed by large carnivores or other causes.
As ungulates, boreal caribou populations are capable of influencing plant species abundance; herbivory is an important community structuring process for plants (Elton, 1927). Herbivores such as boreal caribou can reduce the abundance of favoured species. In the case of caribou, where they occur on islands (for example, Slate Islands) and there are no predators, the abundance of certain plant species was reduced (Cringan, 1956; Cringan, 1957; Bergerud, 2007). However, on the mainland, the low density of boreal caribou severely limits their effect on plant species abundance.
In addition to their important ecological role in the boreal forest, boreal caribou are an integral part of the communities of First Nations people across the boreal region, and as such have held cultural, spiritual, subsistence importance for thousands of years (Schmelzer et al., 2004; Hummel and Ray, 2008; Ontario Ministry of Natural Resources, 2009a). Caribou meat has provided an important source of fat and protein to the diets of First Nations people for thousands of years. In addition to using caribou meat as sustenance, First Nations people use caribou skins for clothing, boots, tents, sleeping robes, and drums. Antlers and sinew traditionally had many uses as various tools including knives, cutlery, needles, fish hooks, sleds, kayak and canoe frames (Hummel and Ray, 2008). Caribou have also inspired many artistic expressions among First Nations communities; they are used in traditional arts and crafts and contribute to the economy of northern communities (Hummel and Ray, 2008). Caribou also have significant cultural importance to northern communities. The woodland caribou is a totem animal of Pikangikum people in northern Ontario, and is considered to be a gift from the Creator to use for survival and to enrich their lives (Whitefeather Forest Management Corporation, 2006). Among non-aboriginal Canadians, caribou symbolize the unspoiled wilderness and has been featured on the Canadian 25-cent piece since 1937 (Tesar, 2007). More recently, the declines of boreal caribou across the country have sparked conservation campaigns among non-governmental organizations across the country (Hummel and Ray, 2008).
Boreal caribou as an indicator species
Boreal caribou have particular life history characteristics that limit their resilience and increase their dependence on large patches of mature coniferous forests. They are also vulnerable to human-induced habitat change (Bergerud, 1988; Sorenson et al., 2008; Environment Canada, 2008). The status of caribou populations may therefore function as a useful symbol for the health of boreal forest ecosystems.
Due to their low reproductive rate, caribou have been considered the least resilient of North American deer. They typically first breed at a later age (more than two years old), produce only one offspring per year, and are especially vulnerable to predators (Bergerud, 1988). Accordingly, caribou generally require longer time periods to recover from population stresses.
In addition to low reproductive rates, boreal caribou occupy Canada's boreal region at low population densities, often below 0.06 caribou per km2. Over millennia, the boreal population of woodland caribou adapted to dynamic forest ecosystem conditions, in which forest fire is the dominant cause of habitat disturbance and renewal. Forest fires vary in frequency and magnitude throughout the boreal forest of Canada, and boreal caribou populations shift their range over time in response to fire-induced changes in habitat quality (Environment Canada, 2008). Consequently, local populations require relatively large ranges to compensate for portions of the range in early seral stages to avoid predators and to find suitable habitat. The median size of a local population range is 9,000 km2 (Schaefer and Mahoney, 2003). Environment Canada (2008) demonstrated a negative relationship between human-induced changes to forest composition and configuration and boreal caribou population performance, indicating that the effects of industrial resource extraction on boreal caribou are additive.
Given the boreal caribou's requirements for large areas and their low resilience to changes in forest structure and configuration, the maintenance of viable boreal caribou populations may be an indicator of healthy boreal forest ecosystems (Canadian Council of Forest Ministers, 2000). Moreover, given their need for large contiguous patches of suitable habitat, boreal caribou can serve as an umbrella species for other species with smaller habitat area requirements. Maintaining and restoring habitat for boreal caribou at the local population range has positive outcomes for numerous other forest-dwelling species, such as marten (Martes americana) (Thompson and Harestad, 1994), and certain lichens (Selva, 1994). The population dynamics of wolverine, a listed species at risk that also requires large habitat patches in the boreal forest, may be likened to that of boreal caribou, and therefore forest management that considers the spatial and habitat needs of caribou populations may also benefit populations of wolverine (Ontario Woodland Caribou Recovery Team, 2008).
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