Caribou are considered part of the climax biota because of their dependence on late successional forests and associated lichen forage [68,101]. Caribou use old-growth and mature coniferous stands across their range [80,97]. Woodlands with sparse overstories of black spruce-paper birch (Picea mariana-Betula papyrifera) or jack pine (Pinus banksiana) and a dominant ground cover of lichens are heavily utilized [57,79,101]. Caribou frequent peatlands, bogs, muskegs, lake shores, and other wetland and riparian areas [37,61,97].
Alaska: Black spruce and white spruce (Picea glauca) in pure or codominant stands with lichen-moss understories are heavily utilized in Alaska [57,103]. Sedge meadows dominated by water sedge (Carex aquatilis), rock sedge (C. saxatilis), and tall cottonsedge (Eriophorum angustifolium) provide year-round forage. Barren ground caribou also utilize willow stands dominated by feltleaf willow (Salix alaxensis), Barclay's willow (S. barclayi), grayleaf willow (S. glauca), tealeaf willow (S. pulchra), and Richardson's willow (S. richardsonii). Grasslands dominated by rough fescue (Festuca altaica) with birch (Betula spp.) and willow (Salix spp.) associates are frequently utilized. Bog birch (Betula glandulosa) dominates some landscapes at 3,000 to 4,000 feet (900-1,200 m), with tealeaf willow and rough fescue codominant at 3,000 to 3,500 feet (900-1,100 m) [103]. Mountains <7,900 feet (2,400 m) in Denali National Park are characterized by shrub tundra dominated by birch and willow [1,18] and alpine zones dominated by sedges (Carex spp.) [1]. High-elevation tundra in Denali National Park is characterized by mountain avens (Dryas spp.) [18]. A mosaic of spruce (Picea spp.)-dominated forests, cottonsedge (Eriophorum spp.)-dominated tundra, and riparian areas with mixed spruce and willow exists below 2,600 feet (800 m) in Denali National Park [1,18].
Canadian Arctic Archipelago: Wilkinson and others [117] identified 5 distinct caribou habitats in the archipelago. Barren uplands are characterized by arctic dryad (D. integrifolia), sedges, willows, grasses, and lichens. Sedge meadows are dominated by water sedge (C. aquatilis var. stans), white cottonsedge (Eriophorum scheuchzeri), and Fisher's tundragrass (Dupontia fisheri). Sand dune habitats are dominated by feltleaf willow, polar willow (Salix pseudopolaris), dwarf fireweed (Chamerion latifolium), pale Indian paintbrush (Castilleja pallida), and grasses. Tundra tussocks are characterized by willows, arctic dryad, sedges, and grasses. Lakes and lake edges are dominated by water sedge, pendantgrass (Arctophila fulva), and false semaphoregrass (Pleuropogon sabinei) [117].
Idaho, Washington, and British Columbia: Western hemlock-western redcedar (Tsuga heterophylla-Thuja plicata) communities are important in to woodland caribou during autumn and early winter [6,36,102]. Mixed stands of old growth Engelmann spruce-subalpine fir (Picea engelmannii-Abies lasiocarpa) are preferred in late winter [6,36]. Caribou occasionally use interior lodgepole pine (Pinus contorta var. latifolia) forests [24,36].
Alberta: Black spruce-tamarack (Larix laricina) dominates lowland fens and bogs, while uplands are dominated by white spruce-jack pine-quaking aspen (Populus tremuloides) [76].
Northwest Territories, Saskatchewan, and Manitoba: Dominant species include black spruce, white spruce, and jack pine [77,80,89,99,101]. White birch, tamarack, quaking aspen, and balsam poplar (Populus balsamifera) are common associates [77,80,89,99]. Jack pine is abundant on some upland sites. Dominant shrubs on upland sites include mountain cranberry (Vaccinium vitis-idaea), bog blueberry (V. uliginosum), velvetleaf blueberry (V. myrtilloides), and bog Labrador tea (Ledum groenlandicum). Willow, birch, mountain alder (Alnus viridis subsp. crispa), white birch, and tamarack border lakes and streams [80]. Black spruce dominates mature and intermediate bog habitats. Alders (Alnus spp.) and willows form the understory in intermediate bog and bog-forest habitats [97].
Ontario: Star reindeer lichen (Cladonia alpestris), reindeer lichen (C. rangiferina and Cladonia spp.), and spineless reindeer lichen (C. mitis)- rich forests serve as late winter habitat for woodland caribou [116].
Quebec: Alpine zones >3,300 feet (1000 m) are characterized by ericaceous shrubs, lichens, mosses, and graminoids, while subalpine zones 3,000 to 3,300 feet (900-1000 m) are dominated by open white spruce and balsam fir (Picea balsamea) forest [82].
Newfoundland: Balsam fir-dominated forests are heavily utilized [74].
Since caribou are highly mobile, the chances of caribou being directly killed by fire are minimal [33]. Fire-related deaths of large mammals are typically a result of smoke inhalation [63]. Evidence of caribou deaths via smoke inhalation was not found in this literature review. A small group of caribou was seen lying in an open area with low vegetation that was completely surrounded by fire in Alaska [43]. The group was later observed moving away from the area without any apparent harm caused by the fire [43]. At no time did the observers indicate that the caribou appeared panicked by the fire. Many caribou herds spend the fire season in tundra habitats where fire danger is lower than in taiga habitats [33], so risk of mortality from fire is low for caribou in tundra.
Starvation following the loss of forage due to fire is a potential threat. During the winter following the large fires in Yellowstone National Park in 1988, thousands of elk (Cervus elaphus) died from starvation [63]. With a long-term loss of forage following fire, major declines in caribou herd size would likely result.
Food availability influences food selection. Caribou prefer vascular plants and mushrooms but exploit other food sources when these are not available [12]. The vascular plant species most commonly eaten by caribou throughout the United States and Canada include the young buds, catkins, leaves, and/or sprouts of water sedge, water horsetail (E. fluviatile), mountain cranberry, velvetleaf blueberry, bog blueberry, arctic willow (S. arctica), sheathed cottonsedge (Eriophorum vaginatum), bog Labrador tea, northern Labrador tea (Ledum decumbens), bog birch, and leatherleaf (Chamaedaphne calyculata) [2,14,26,66,79,80,96,98,99,103,109]. Other locally important foods include arctic dryad, saxifrage (Saxifraga spp.), bog rosemary (Andromeda spp.), black crowberry (Empetrum nigrum), sheep-laurel (Kalmia angustifolia), bog-laurel (K. polifolia), spruce, jack pine, tamarack, sedges (Cyperaceae), blueberries (Vaccinium spp.), willows (Salix spp.), birches (Betula spp.), grasses, and mosses [12,14,27,66,80,96,98,103,109]. Fungi, especially Boletus spp., Coprinus spp., Lycoperdon spp., and Morchella spp., are readily eaten in late summer and fall [12,80,103]. After fall frost, caribou consume terrestrial lichens and evergreen leaves [12]. In addition to milk, calves as young as 2 weeks old ingest leaves of willows, bog Labrador tea, leaves and stems of sedges and black crowberry, and fruticose lichens, including cup lichens (Cladonia spp.) [103]. For more complete lists of caribou diets, see Bergerud [12,14], Cringan [27], Miller [80], and Skoog [103] .
Lichens are prominent in the caribou diet throughout the year, but reach greatest importance in winter [27,80,96,101]. Lichens commonly eaten are reindeer lichen, star reindeer lichen, spineless reindeer lichen, tree reindeer lichen (Cladonia arbuscula), other reindeer lichens (Cladonia spp.), cup lichens (C. amaurocraea and C. uncialis), cetraria lichen (Flavocetraria nivalis), Iceland-moss (Cetraria islandica), felt lichen (Peltigera canina), and snow lichens (Stereocaulon spp.) [2,12,14,26,80,99,101,108]. Other lichens, including witch's hair lichens (Alectoria jubata, A. sarmentosa, and A. ochroleuca) and brittle lichens (Cornicularia spp.) are locally important food sources when available [2,26]. In British Columbia, horsehair lichens (Bryoria spp.), which are highly valued as forage in the area, are more abundant on subalpine fir and Engelmann spruce than on whitebark pine (Pinus albicaulis), lodgepole pine, or alpine larch (Larix lyallii) [60].
Lichens are the primary foods of caribou in winter [27,80,96,101]. However, lichens are generally low in nutrients, and caribou often lose weight with a winter diet heavy in lichens [12,33,80]. Caribou may persist on a diet that limits or excludes lichens, since caribou are able to exploit vascular plant resources when available [14,33]. In winter, snow accumulation influences caribou diet [12,92]. By mid-April in Saskatchewan, snow hardening made it difficult for the caribou to forage beneath the snow, so arboreal lichens were the primary available food source followed by terrestrial lichens, bog Labrador tea, and other deciduous shrubs and trees [79,80]. When caribou population densities were high on the Slate Islands in Lake Superior, caribou lightly browsed mountain maple (Acer spicatum), American mountain-ash (Sorbus americana), willows, red-osier dogwood (Cornus sericea), and downy arrowwood (Viburnum rafinesquianum) in winter [27]. Woodland caribou in British Columbia forage on arboreal lichens, subalpine fir, Engelmann spruce, and western hemlock in early winter when show accumulation is rapid. Oregon boxwood (Paxistima myrsinites) and other vascular plants were eaten in early winter when snow accumulation was slow [92]. When the snow forms a hard crust in open habitats, caribou move to forests to feed on arboreal lichens [12]. During periods when snow cover was ≤20 inches (51 cm) deep, woodland caribou in British Columbia fed on grouse whortleberry (Vaccinium scoparium), cup lichens, and horsehair lichens. When snow was ≥24 inches (62 cm) deep, they almost exclusively ate horsehair lichens and possibly small amounts of witch's hair lichen [60]. Overgrazing by caribou has reduced the amount of available forage and habitat on Alaskan islands, while wildfire has reduced lichen availability on the Alaskan mainland [106].
During northward migration in Saskatchewan in mid-February, barren ground caribou fed in early morning and early evening [80]. Caribou tend to move almost continuously, even when foraging, which reduces the possibility of overgrazing a feeding area [103]. Snow softens by late winter or early spring, making it possible for the caribou to feed on terrestrial lichens and ericaceous plants under the melting snow [80]. Caribou dig craters in the snow to forage for lichens and other vegetation [12,17]. Caribou prefer to crater in soft, shallow snow [96]. Only one caribou feeds in a crater at a time, and they compete for the most preferred craters [80].
In black spruce-white birch forests, jack pine forests, and unburned tundra, caribou generally avoid burned areas ≤35 years after fire and show a preference for communities >50 years of age [57,96,97,101]. Caribou in the Northwest Territories utilized 151- to 250-year-old stands more than any other forest age class [108]. Average caribou use of black spruce forest in northern Canada was highest in >120-year-old stands and lowest in 1- to 10-year-old stands [99]. Jack pine forest and mixed-forest habitats that burned 4 to 5 years previously were significantly avoided (P<0.01) in Manitoba [97]. Woodland caribou showed a significant preference (P<0.01) for foraging and/or traveling in mature bog, intermediate bog, semiopen bog, sedge meadow, bog-forest, and jack pine forest, around lakes, and on roads that had not burned in ≥55 years in Manitoba [97]. A fire simulation model suggests that frequent fire and large fires reduce spruce-lichen habitats preferred by caribou [94]. In Alaska during winter, caribou were observed feeding along the edge of a burn in birch and ericaceous shrub-sedge communities near moraines [46]. In another Alaska study, edge habitats (<1,600 feet (500 m) of burned/unburned stand edge) were highly preferred over habitats >1,600 feet into either the stand or the burned area. Use of burned areas in Alaska was highest in November to December, but declined during late winter and spring [57]. In addition, unburned remnants and unburned stands adjacent to recent burns are used for feeding [78,79,97]. Caribou rarely forage within recent burns [78,79].
Caribou use burned areas for several reasons. For instance, Miller [79,81] reported that caribou used burned areas as refuges to escape predation. In another study, calving occurred in a recent burn adjacent to a traditional calving area in Alaska [34]. Recent burns are also commonly used during migratory and nonmigratory movements [78,81,96]. In late winter, caribou in Saskatchewan and Manitoba migrated through burned areas in long single lines [78]. Caribou also traverse burned areas between mature forest fragments and meadows [46,101]. Fire in tundra habitats removes woody debris, which facilitates travel [96]. However, burns in forested habitats may inhibit travel between unburned foraging sites. Surface fires can kill black spruce and burn off their roots, making standing snags susceptible to windthrow [70]. Windthrow in recent forest burns may hamper the movements of caribou [61,96,101].
The influence of burns on travel appears to depend on habitat characteristics. Large fires in Quebec during 1954 to 1955 appeared to block winter migration routes to the south, causing caribou to congregate in lichen-rich habitats in northern Quebec. This effect appeared to be short term [87]. Snow accumulation and hardness alters caribou movements. In Alaska, snow hardness was almost significantly greater (P=0.0731) in burned plots than in unburned plots. Fire may encourage earlier snow melt [96], which could facilitate spring migration (see Timing of Major Life History Events).
Effects of fire on caribou forage: Historically, fire was considered detrimental to caribou due to the destruction of lichen forage caused by fire [52,101]. Now, however, fire is perceived to improve the nutrient cycling and growth of lichens, sedges, shrubs, and forbs [56,96]. Fire reduces lichen availability, but enhances short-term productivity and quality of vascular plants such as sheathed cottonsedge, bog Labrador tea, and mountain cranberry [61,96,97]. The short-term increase in vascular plants enhances summer ranges, but the decrease in lichen availability is detrimental in winter ranges [97]. Late summer regeneration of sheathed cottonsedge following a midsummer tundra fire in Alaska provided food for a caribou herd moving through the burned area in late October [62]. The use of herbaceous vegetation, including sheathed cottonsedge and horsetail (Equisetum spp.), was limited in another study in Alaskan tundra, although availability increased after recent fire [96].
Lichens are typically consumed by fire, including surface fires, so limited food is available to caribou during early successional stages after fire [61,70]. Frequent fire may delay the regeneration of forests that support lichen growth or convert a burned area into tundra, which may not support lichen growth [61]. Fire affected caribou forage availability but not selection in the Alaskan tundra [96]. Changes in arboreal lichen biomass and availability were affected by high- and low-severity fire and clearcutting in Idaho, Washington, and British Columbia. No arboreal lichens were found on sites that experienced high-severity fire or clearcutting during the previous 40 years. Higher arboreal lichen biomass was found at high-severity burn sites aged 41 to 80 years than on clearcut sites of similar age. Arboreal lichen biomass in low-severity fire sites was higher 41 to 80 years after fire than in low-severity sites 1 to 40 years after fire [36].
Lichen regeneration following fire depends on many factors including burn patchiness, intensity, severity, extent of the burn, prefire vegetation, seral stage, and climate [61,115]. Reindeer and cup lichens (Cladonia spp.) are virtually absent until a recovering habitat reaches midsuccession [99,101,116]. Lichen regeneration, including reindeer lichens, cup lichens, felt lichens (Peltigera spp.), and arboreal lichens, takes 30 to120 years or more depending on the species [2,36,78,79,80,99,101,108]. Available forage of shrubs and lichens on average is highest in 51- to 120+-year-old stands and lowest in 1- to 10-year-old stands [99,108]. Stands <60 years old may have standing crops of lichens similar to 120-year-old stands [80]. Fire at the landscape level maintains a diverse mosaic of vegetation and successional stages in forested ecosystems, which overall contributes to the availability of lichens [61].
Fire is necessary in the landscape to maintain lichen forage availability over the long term [97]. Caribou response to fire is influenced by the duration of lichen recovery and availability of alternate feeding sites [61]. In forests >130 years old, terrestrial lichens are replaced by feathermosses, including mountain-fern moss (Hylocomium splendens) and Schreber's moss (Pleurozium schreberi), and vascular plants such as mountain cranberry [24,75,81,97] due to increasing tree density, canopy closure, and litter accumulation [24,75,81]. Fire destroys thick masses of sphagnum mosses (Sphagnum spp.) and feathermosses (Hylocomium spp.) and removes accumulated litter, allowing lichens to regenerate [3,81,93,97,100].
Feltleaf willow is a preferred caribou browse plant and a common associate in Alaska and the Canadian arctic [103,117]. Feltleaf willow is a fire-adapted species that sprouts from the root crown following top-kill by fire [86,115,119]. Feltleaf willow produces abundant, wind-dispersed seed that is important in colonizing burned areas [112,115].
Fire regime: Caribou habitats in taiga generally experience moderate to long fire-return intervals, while tundra habitats rarely burn. Summer fires are rare in northern Canada because of the heterogeneous landscape of wet and dry tundra and rock barrens. Thus, barren ground caribou are typically only affected by fires in their forested winter habitats [100]. The fire season in the Northwest Territories is mid-June to mid-August [41,56]. The fire season in interior Alaska is 1 April to 30 September, with most fires occurring May to July [40,112]. Fires in black spruce/lichen forests in the Northwest Territories and interior Alaska are primarily lightning-caused [41,50,71].
Black spruce-birch forest has the highest fire frequency of any forest type in interior Alaska [114]. Estimated fire-return intervals in the black spruce-birch ecosystem vary from 50 to 200 years [51,115]. Fires occur every 50 to 70 years in black spruce-white spruce/bog birch/reindeer lichen communities in interior Alaska [40]. Heinselman [51] estimates a fire-return interval of 130 years for open black spruce/reindeer lichen forest and 100 years for closed-canopy black spruce forest. Mean fire-return intervals in lowland black spruce forests on the Kenai Peninsula, Alaska, range from 89 to 195 years [4,72]. Black spruce-birch communities experience high-severity, stand-replacing fires. These communities are highly flammable due to the abundance of ericaceous shrubs, the prevalence of dead, low-hanging branches on the black spruce trees, which are often covered with highly flammable epiphytic lichens, and to the thick moss and lichen mats that cover the forest floor and become highly flammable after periods of low rainfall [70,71,113].
White spruce is also a predominant species in caribou habitat [57,103]. Fire frequency in white spruce forest types is generally 60 to 200 years [84]. Some white spruce forests located in floodplains are >300 years old in Alberta [51].
Jack pine is an important stand component for caribou in eastern Canadian forests [76,80,99,101]. Estimates of fire-return intervals in jack pine forests are generally <50 years [48]. In northern Ontario, major fire events occur every 5 to 30 years in jack pine forests [73]. The mean fire-return interval for jack pine forests in the Athabasca Plains in northern Saskatchewan and northeastern Alberta is 38 years [20]. Upland ridges and ridge complexes that lack natural fire breaks burn most frequently. Jack pine forests that burn more frequently than every 5 to 10 years become pine barrens [31]. Lichen mats develop within 40 years and support fire in jack pine forests [20].
Balsam fir habitats are also utilized in eastern Canada [74,82]. Balsam fir is usually rare or absent for the first 30 to 50 years after fire, but establishes thereafter under the canopy of its seral associates [5,35,42].
Engelmann spruce-subalpine fir forests provide prime habitat for endangered woodland caribou in the Columbia Mountains [6,36,55,102]. Engelmann spruce-subalpine fir forests usually develop in cool, moist locations with an average fire-return interval of ≥150 years [7]. Moist, mid- and high-elevation subalpine fir habitat types experience stand-replacing fires at intervals of ≥90 years [7,105].
The following table provides fire regime information that may be relevant to caribou. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Fire regime information on vegetation communities in which caribou may occur. Fire regime characteristics are taken from the LANDFIRE Rapid Assessment Vegetation Models [65]. These vegetation models were developed by local experts using available literature, local data, and expert opinion as documented in the PDF files linked from the Potential Natural Vegetation Groups listed below.Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics Percent of fires Mean intervalCaribou populations are declining in North America [27,101,111]. Many factors may play a role in the caribou decline. Some possible explanations include poor physical condition of females, loss of habitat due to logging and fire, an increase in energy expenditure due to increased movements, delayed births, overgrazing, high predation, overhunting, and unusually harsh winter conditions leading to high snow accumulations [11,23,27,70]. The loss of winter habitat and lichen forage by wildfires and escaped campfires is commonly cited as a primary cause for the decline of caribou [11,68,70,99]. Gray wolf predation is also a cause of high mortality in some populations [68]. Bergerud [15] speculated that an increase in predation or parasitism may account for population declines after fire rather than a loss of lichen forage following fire. However, evidence that major declines in caribou populations could be attributed to predation or disease has not been conclusive [27]. Overhunting is detrimental to small caribou populations [11,27,70]. Declines along the United States/Canadian border are largely attributed to overhunting and habitat changes caused by logging and fire [11,49]. The primary causes of caribou mortality in the Selkirk Mountains are predation, other natural causes, and hunting [111].
A review by Cumming [29] outlined management guidelines for maintaining caribou populations. Adequate winter habitat includes forest and muskeg fens that provide lichen forage, which is crucial for winter survival. Habitat must provide adequate supplies of other foods and protection against predation. Predator control may be necessary near major calving grounds [29].
Lichen cover can be maintained or improved by silviculture, whereas fire often has a negative effect. Percent cover of reindeer lichens, cup lichens, and arboreal lichens is typically higher in logged plots than in burned plots [36,116]. Reindeer lichen and cup lichen recovery was greater in a lodgepole pine forest in British Columbia 15 years after a winter harvest than in burned stands in the area. Lichen cover following summer harvest was comparable to burned stands in the area. Partial cutting can maintain arboreal lichen loading over the short term. In the same study, results indicated 80% to 90% of lichen loading remained in Engelmann spruce-subalpine fir forest 2 years after tree harvest [25]. In Ontario, reindeer lichens were observed 2 years after logging [116].
Caribou have a negative response to clearcut logging. Woodland caribou in Alberta maintained an average distance of 0.75 mile (1.2 km) from recent cut blocks [104]. In Ontario, clearcuts were avoided for 12 years after harvest within traditional caribou winter habitats [30]. Courtois and others [22] suggested that protecting large mature forest blocks, concentrating tree harvesting activities to large management blocks, and maintaining corridors connecting large forest blocks would benefit caribou. Smith and others [104] also recommended leaving core caribou habitat intact, limiting the number of fragmented stands created by timber harvesting, and creating large cut blocks to mimic the effects of large-scale fires and minimize edge effects that may promote the expansion of other ungulate species. Small-scale timber harvest promotes an increase in moose populations, which in turn leads to an increase in gray wolf populations. Large-scale timber harvest could benefit caribou by keeping moose and gray wolf populations low [53]. In winter, woodland caribou in British Columbia show a preference for Engelmann spruce-subalpine fir forests with low basal area, moderate timber volume, and moderate slope that are >5,000 feet (1,525 m) elevation. Woodland caribou move through lower elevations, however, to reach the high-elevation habitats. Logging could be possible at lower elevations in this habitat, which would reduce conflicts with caribou [107]. For more detailed information on land management recommendations, see Courtois and others [22].
Disturbances in caribou habitats have potentially detrimental effects on caribou populations. Caribou avoid railways, roads, and human settlements. Rail and road systems that bisect caribou range may inhibit seasonal movements [2]. A study in Ontario documented an increase in caribou mortality near logging roads via an increase in predation, poaching, and train and traffic accidents. These increases were related to, but not directly caused by, logging activities [30]. Limiting road access and recreation, such as snowmobile use, would benefit caribou [107]. Unexpectedly, an increase in traffic through Denali National Park in Alaska has not caused any noticeable effects on caribou abundance, distribution, or behavior. Individual caribou may become habituated to traffic while others avoid roadways [18].
The effects of petroleum development on caribou are uncertain. A traditional calving ground near an active Prudhoe Bay oil field shifted approximately 12 miles (20 km) south of the oil field as the herd grew over time. The herd's shift in utilized habitat may have been influenced by petroleum development, but this is uncertain [47]. The use of traditional calving grounds may reduce calf mortality due to reduced predation and higher-quality forage [19]. Shifts away from traditional calving grounds may lead to greater calf mortality and a decline in population size.
Cameron and others [19] advise using caution in when developing oil fields in caribou habitat. After construction of an oil field access road through a caribou calving ground in Prudhoe Bay, Alaska, caribou density declined significantly (P=0.05) within 0.6 mile (1 km) of the road. Relative caribou use of areas adjacent to the road also significantly declined (P<0.02), in apparent conjunction with an increase in surface development. Caribou densities increased significantly (P=0.04) 3 to 4 miles (5-6 km) from the road [19], indicating a possible shift in habitat utilization. In another study on Prudhoe Bay oil fields, male caribou were observed within 1.2 miles (2 km) of oil field infrastructure during the postcalving season. However, calves were primarily observed 4 to 5 miles (6-8 km) from oil field infrastructure during the postcalving season, although calves were observed closer to infrastructure in some years [28]. These results suggest that oil field development generally has a negative affect on caribou calves and calving females, but more research is needed to make the association clearer.Known predators of adults and calves in North America include gray wolf, grizzly bear, American black bear (Ursus americanus), mountain lion (Puma concolor), wolverine (Gulo gulo), coyote (Canis latrans), arctic fox (Vulpes lagopus), Canada lynx (Lynx canadensis), golden eagle (Aquila chrysaetos), bald eagle (Haliaeetus leucocephalus), and common raven (Corvus corax) [9,11,12,15,58,59,74,83,85,89,103]. Polar bear (Ursus maritimus) may prey on caribou in areas where they cooccur, but no confirmed kills have been observed [16]. Another potential predator is the red fox (Vulpes vulpes fulva) [103].
Gray wolves are the primary predator of adult caribou [76,103]. Fire may lead to an increase in gray wolf populations, especially when other prey species such as moose (Alces alces) increase after fire [15,53].
Caribou outside the Columbia Mountains: Caribou primarily occupy boreal and subboreal forests in North America [24,74,99]. Herds in northern Canada and northern Alaska summer in arctic tundra and winter in boreal forest [99]. Major tree species within caribou habitat include black spruce, white spruce, jack pine, balsam fir, Engelmann spruce, subalpine fir, lodgepole pine, tamarack, and white birch [6,24,74,101]. In Saskatchewan and Manitoba, caribou foraged exclusively in and around spruce stands in mid- to late winter [78]. In Manitoba, barren ground caribou use upland semiopen to open black spruce stands most heavily for midwinter foraging. During northward migration in Saskatchewan in mid-February, barren ground caribou feed primarily on uplands in semiopen to dense black spruce and in isolated white birch stands [80]. Mature (≥70 years) balsam fir forests are used extensively by woodland caribou in Quebec [82].
Caribou distributions within these habitats are influenced by site characteristics and associated vegetation. Caribou frequent peatlands, bogs, muskegs, lake shores, and other wetland and riparian areas [37,61,80,97]. A mosaic of habitats, such as old-growth forest uplands and mature lowland forest adjacent to wetland and riparian areas, are important for feeding and other activities during mid- and late winter in Manitoba and Saskatchewan [80,97]. Habitat heterogeneity is primarily caused by fire [80]. Caribou spend most of their time on level or gently sloping land [81,103]. High-elevation hilltops and ridges are frequently used by caribou throughout the year [81,82,103]. High-elevation habitats include grasslands [103], alpine habitats characterized by ericaceous shrubs, lichens, mosses, and graminoids, and open subalpine white spruce-balsam fir forest. Woodland caribou in Quebec favored barren habitats with a large component of bare ground in alpine and subalpine zones >2,300 feet (700 m) [82].
Favored calving grounds for herds near the ocean include gently rising plains and hills >1,200 feet (370 m) in elevation [58]. A calving area in Alaska was characterized by subarctic, mesic, and wet-sedge meadows dominated by Bering Sea sedge (Carex nesophila), purple marshlocks (Potentilla palustris), and field horsetail (Equisetum arvense). Calving habitat in Greenland was characterized by warm, dry, south-facing slopes populated by Bellardi bog sedge (Kobresia myosuroides), weak arctic sedge (C. supina), grayleaf willow, and dwarf birch (B. nana) [88]. Caribou in Newfoundland calved in mature forest and then moved to barrens 2 to 4 days after calving. Extensive scrub habitats interspersed with bogs and barrens were used for calving as well in Newfoundland [74]. Uncharacteristically, females from a nonmigratory herd in Saskatchewan did not use a specific calving location from year to year, but the females did utilize the same general calving area [90]. A herd in Alaska was displaced from its traditional calving area and used a recent burn for calving instead. The traditional area was completely snow covered during the calving period, while the burned area was snow free. Nearby treeless, snow-free unburned areas were generally avoided [34].
Snow depth and hardness may influence caribou movements and foraging habits more than stand age [78,101]. To find food in winter, caribou favor habitats with reduced snow cover, including western and southern aspects and windy mountaintops [12,106]. Caribou selectively travel and feed in areas with shallow snow, which may explain why stands at least 40 years old are utilized more than younger stands [78]. Caribou dig feeding craters at sites with soft, shallow snow in both burned and unburned sites in Alaska [96]. In early winter, barren ground caribou movements are not yet restricted by snow depth or hardness [80]. Mountain pine beetle (Dendroctonus ponderosae) attacks can affect caribou movements by killing trees and increasing windthrow. Snow depths may increase in areas with reduced canopy cover due to windthrow caused by mountain pine beetle kills. As a result, caribou may abandon once-preferred habitats and utilize areas where predation risks may be higher [21]. Increases in snowfall over several seasons can lead to population declines within caribou herds [1].
Home range: Caribou home range size is highly variable because some herds are migratory while others are not [69,80,90,103]. Average home range sizes of male and female caribou in Newfoundland were 84.0 km² and 89.8 km², respectively [74]. The range of a woodland caribou herd in Labrador was estimated at 25,000 km² [17]. The general home range of a woodland caribou herd in Ontario was roughly 160,000 km² [39]. The primary home range of an Alaskan herd covered roughly 45,000 km² [103].
Endangered woodland caribou in the Columbia Mountains: Caribou in the Columbia Mountains primarily inhabit Engelmann spruce-subalpine fir and western red cedar-western hemlock forests >4,000 feet (1,200 m) in elevation [111]. The following table describes the characteristics of habitats utilized by woodland caribou in the Columbia Mountain ecosystem in northern Idaho, northeastern Washington, and southeastern British Columbia.
Characteristics of woodland caribou habitat in the Columbia Mountains Season Major habitat characteristics Tree size Basal area Canopy cover Lichen density Understory cover Road density Other characteristics Early winter mature to old-growth western redcedar-western hemlock and Engelmann spruce-subalpine fir forests >20 cm DBH ≥50 m²/ha >50% high ...* ... 1,346-1,677 m elevation; 16%-30% slopes; southern aspects, highly productive stands; average windthrown tree density 7.4/ha [6,55,91,102,107,111] Late winter old-growth Engelmann spruce-subalpine fir and western hemlock forests; ridgetops or upper slopes; also subalpine zones ... 2.3-17.2 m²/ha 26%-50% high ... ... >1,526 m elevation; moderate slopes; northern aspects; low tree density; stem densities 741 to 1235/ha [6,102,107] Spring mature western redcedar-western hemlock-Engelmann spruce forests; forest openings and cutovers adjacent to mature stands; closed canopy forests of various ages <10 cm, 21-25 cm DBH <2.3 m²/ha, <45.9 m²/ha variable low ... ... low to midelevation; highly productive stands [6,38,102,111] Calving mature to old-growth western redcedar-western hemlock and Engelmann spruce-subalpine fir forests; old noncommercial forests; calving females usually secluded ... ≤34.4 m²/ha low high ... lower than other times of the year 1,346 m elevation; low tree density; often snow covered [55,102,111] Summer western redcedar-western hemlock and Engelmann spruce-subalpine fir forests; partial cuts, pole stands, and old-growth; high meadows adjacent to subalpine forest ... 17.3-34.4 m²/ha variable high >60% ... average elevation 1,400-1,700 m; northern and eastern aspects, relatively flat terrain at a fine scale; highly productive stands [6,38,102] Fall mature to old-growth western hemlock; high meadows adjacent to subalpine forest >20 cm DBH >45.9 m²/ha variable high high ... shift to lower elevations following frost [38,102] Rut ... >25 cm DBH >45.9 m²/ha >70% ... ... 1.3 km/km² high concentration of snags (>247/ha) [102] *No data.Johnson and others [55] identify preferred woodland caribou habitats in the Selkirk Mountains of Idaho, Washington, and British Columbia, and describe potential management conflicts due to human activity and development in the woodland caribou habitats. Prime habitat in the region includes lightly stocked stands with seral and mature Engelmann spruce-subalpine fir and western redcedar-western hemlock stands with <40% crown cover, especially in areas where lakes, bogs, and fens are present. These habitat types provide lichen forage during winter months as well as shrubs and forbs during other times of the year. Within these types, lightly stocked stands on steep southern aspects are vulnerable to fire. Any logging or fire activity within these stands would likely be detrimental to the caribou population [55].
Engelmann spruce-subalpine fir forest >5,000 feet (1,524 m) with >40% crown cover is another highly preferred habitat in the Selkirk Mountains. Western redcedar-western hemlock stands over 4,500 feet (1,346 m) within the spruce-fir type are used in early winter for feeding, movement corridors, and calving sites. These stands provide cover in late fall, while fallen trees within this habitat provide lichen forage. According to Johnson and others [55], irregularly shaped clearcuts <40 acres (16 ha) in size and <33% of the original forest size may be removed in a single drainage without serious harm to caribou populations in this habitat [55].
In the Selkirk Mountains, caribou frequently use sites adjacent to lakes, bogs, and fens for foraging in late summer and fall. Disturbances such as logging, camping, and road traffic near these water sources may be detrimental to caribou using those sites. Limiting the number of roads and utility corridors through preferred habitats would be highly beneficial to caribou in the Selkirk Mountains [55].
Woodland caribou in British Columbia frequent alpine-rock, lodgepole pine/reindeer lichen-cup lichen, and midelevation mixed spruce-fir-pine (Picea-Abies-Pinus spp.) habitats [54]. Western hemlock habitats are most important for woodland caribou in Idaho and British Columbia during autumn and early winter. However, habitats with western hemlock are largely avoided at other times of the year. Open canopy (10%-25%) is also favored during all seasons in stands without a western hemlock component [102]. Periodically in winter, caribou climb to the high ridges they more typically use in summer. These vertical movements are likely influenced by snow accumulation. Deep snow accumulation at high elevations forces caribou down to mature lowland forests. Caribou return to upper elevations if the snow pack hardens sufficiently. When snow softens in spring, caribou are again forced to lower-elevation forests. They move from low-elevation forests into snow-free alpine habitats in May and June and remain for most of the summer. Woodland caribou in this area inhabit dense, lowland forests that are roughly 4,000 feet (1,200 m) below their summer range during part of each winter. In lowland areas, woodland caribou favor flat, poorly drained areas interspersed with open bogs, meadows and ponds, and mature forests near the open ice of lakes [38].
Caribou in the Selkirk Mountains return to the same early winter habitats year after year [111]. Early winter habitat in the Selkirk Mountains is characterized by closed-canopy Engelmann spruce-subalpine fir and western hemlock-western redcedar on moderate slopes, with high densities of windthrow and arboreal lichens, at 4,000 to 6,200 feet (1,200-1,900 m) elevation [102,111]. Early winter is considered the most critical time for woodland caribou in the Selkirk Mountains because availability of suitable habitat is limited, rapid snow accumulation covers vascular plants used for forage and makes movement difficult, and arboreal lichen availability is low [91,111]. The accumulated snow hardens in late winter, and caribou are able to walk on top of the snow and more easily reach arboreal lichens in the forest canopy [111].
Caribou declined after a series of fires that greatly altered the landscape in British Columbia. Fire reduced 60% to70% of this caribou habitat. However, the decline in the caribou population was not noticed for several years following the fires. Caribou avoided burned areas that had been utilized before the fires [38].
The scientific name of caribou is Rangifer tarandus L. It is a member of
the deer family, Cervidae [8,118]. Subspecies in North America include [44,118]:
Rangifer tarandus caribou (Gmelin) woodland caribou
Rangifer tarandus dawsoni Thompson-Seton Dawson caribou (presumed extinct)
Rangifer tarandus groenlandicus (L.) barren ground caribou
Rangifer tarandus pearyi J. A. Allen Peary caribou
Originally, an estimated 3 to 5 million caribou roamed North America. The estimated caribou population in North America as of 1980 had declined to 935,000 [12]. The size of individual herds can fluctuate widely over time [12,103,106]. Thus, the population fluctuations of a single herd may not reflect the overall dynamics of the species.
Caribou migrate between summer and winter habitats. Spring migration begins as early as mid-February and is typically completed by June [58,69,80]. Early spring thaws allow caribou to migrate to calving grounds early, while late-melting snow packs can delay migrations for a full month [103]. All adult males as well as females that have not successfully bred begin the spring migration in June, often when pregnant females and their yearlings have already reached the calving grounds [69]. A sudden decrease in caribou movements occurs during calving [39]. Winter migrations commence by late September or October. Breeding in migratory herds occurs early into the winter migration [103]. Caribou often follow the same migration routes year after year [58]. For herds summering in northern tundra, forested wintering grounds are up to 800 miles (1,300 km) away. Herds in mountainous areas may move from alpine tundra in summer to forests at lower elevations in winter [10] instead of undertaking long-distance migrations.
Other seasonal movements are common as well. Midsummer migrations were observed in Northwest Territories herds beginning around mid-August and possibly ending sometime in September [58]. Movements during summer are attributed to harassment by black flies (Simuliidae), bot flies (Oestridae), and mosquitoes (Culicidae) [103]. Caribou move to cool shady forests, windy hilltops, and snow and ice fields to reduce insect attacks. Caribou continue moving and running if they cannot escape insects [12]. Some herds migrate to new areas throughout winter as well [58,80]. In Manitoba, winter movements were most consistent during the coldest periods [80].
Caribou show strong site fidelity to calving areas [39,103]. Summering ground use is somewhat variable, but the same general areas are often used repeatedly [103]. The locations of wintering sites are highly variable, although some areas are used year after year [29,39,103].
Caribou reach sexual maturity at 16 to 17 months of age [69,89], but yearlings rarely mate [12,103]. Females begin mating at 28 to 41 months of age [12,15,103]. Most males do not breed successfully until they are 4 to 5 years old [69].
The breeding season ranges from late August to late October or at the beginning of the winter migration [12,103]. Males are polygamous and travel with small bands of females and their calves during the rut [10,69]. Gestation lasts 225 to 235 days [10,12,103]. Parturition takes place in May and June in most herds [1,10,11,12,69,88,90], with a maximum range between late April and early July [10,58]. Females from northern herds typically calve later than those to the south [10]. Females give birth to 1 calf [10,12,15]. Births are highly synchronized, with up to 90% of calves in a herd being born during a 5- to 15-day period [1,12,88]. Post and others [88] suggested that regardless of predation pressure, calving synchrony and timing are largely influenced by the emergence of edible plants.
Calves are highly vulnerable to predation, which is the most common cause of calf mortality [12,15]. Calf mortality is typically 14% to 77% during the first year [12,67,74,89]. In areas with high densities of gray wolves (Canis lupus) or grizzly bears (Ursus arctos horribilis), calf mortality can exceed 90% [12]. Weather conditions also influence calf production and survival [15]. Winter calf survival in a Peary caribou herd in the Northwest Territories was highest in years with deep, hard snow. However, in the same study, mild winters and less snowfall led to an increase in calf production the following year [67].
Annual adult mortality is also influenced by predator densities [12]. Natural adult mortality generally ranges 4% to 16% annually [12,74,76,89]. Adults in a small population of woodland caribou in British Columbia suffered unusually high mortality, with an average annual rate of 24% [59]. The maximum lifespan of caribou is around 12 to 16 years [32,69,103].
