Mountain caribou in British Columbia, Canada
Image provided by Cory Legebokow
Contents
Introduction
Caribou (Rangifer tarandus) -- referred to as reindeer in Europe and Asia -- are a mid-sized member of the deer family (Cervidae). In North America, caribou were historically found from the Maritime Provinces in Canada to Alaska, and from Idaho and Washington to Ellesmere Island in the Arctic North. However, their distribution has greatly receded within the past 150 years. In Canada alone, caribou have been extirpated from the Maritimes as well as 40% of their historic ranges in British Columbia, 50% in Ontario and approximately 60% in Alberta. Most of the range reduction so far has occurred at the southern limit of their distribution. For example, caribou populations in Ontario have moved northward at a rate of more than 30 km per decade. This observed range reduction has been largely attributed to disturbances caused by commercial forestry operations. More recently, this pattern has been repeated in other parts of their range. Today, caribou are designated threatened or endangered over many parts of their range in Canada by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) and their preservation is one of the most important conservation challenges.
The formal taxonomy of caribou and reindeer only recognizes 1 global species. In North America, however, caribou are often grouped into different ecotypes based on behaviour (migratory versus sedentary) and differences in habitat use. In British Columbia, deep snow populations of caribou in the interior wet-belt mountains are referred to as “mountain caribou” and are the focus of this article. These caribou are currently listed as threatened in Canada by COSEWIC and endangered by the provincial government of British Columbia.
Mountain caribou in British Columbia
The mountain caribou in the wet-belt of southeastern British Columbia are regarded as their own ecotype because their winter eating habits and habitat use are very different from all other caribou. Because of the unusually deep snowpack on their range (usually 2-5 m deep), terrestrial growing foods such as lichens and shrubs are inaccessible during much of the winter. Here, they walk on top of the deep snowpack to access abundant arboreal lichens (i.e., lichens that grow on trees) in the canopy of trees. Arboreal lichens are more abundant in old forests and thus the distribution of these caribou is closely associated with late successional forests that are usually over 140 years old. This close association makes mountain caribou populations particularly vulnerable to the effects of habitat loss as a consequence of forestry operations.
The perceived vulnerability of these caribou to forest management has resulted in substantial research to better understand the population dynamics of these caribou. Over the past 25 years alone, more than 600 individual caribou have been fitted with remote tracking devices, either radio or GPS collars. These devices allow researchers to locate individual animals to collect information on their movements, preferred habitats, and status (dead or alive). In addition, mountain caribou are surveyed from aircraft every few years when snow conditions are optimal to determine population sizes. Aerial monitoring of mountain caribou that are in high-elevation subalpine areas on deep snow is very accurate with more than 90% of collared individuals being seen without the help of the collars during surveys. Results from aerial surveys have been combined with the remote tracking data to determine the distribution and abundance of these caribou across their range in British Columbia and Idaho.
Figure 1: Distribution of deep snow populations of mountain caribou (yellow) in British Columbia, Canada (from Hummel & Ray 2008).
Mountain caribou are now the southernmost remaining caribou in North America. In 2004, Wittmer et al. delineated a total of 18 distinct populations of mountain caribou in British Columbia (Figure 1). Based on the extensive sample of collared individuals available at that time, there were no known movements of individuals between these populations. The combined range of all remaining mountain caribou populations was estimated at roughly 29,000 km2. One of these remaining populations, the South Selkirks population, crosses the border into northern Idaho thus representing the last caribou that still occur in the United States outside of Alaska. Since 2004, 2 of the distinct populations are thought to have become extinct and 1 other is known to share individuals with a larger population. Overall, the range reduction has been significant when compared to historic descriptions of mountain caribou occurrences.
Mirroring their range reduction, the total population size of mountain caribou has declined drastically over the past decades. Historically, these caribou were abundant with a few individual groups reported to consist of over 1,000 individuals about a century ago. In 2004, the combined number of caribou across all populations was estimated at fewer than1,800 individuals. Since then, more accurate survey data suggest that the total number of these caribou is approximately 1,900 animals (Table 1) with the largest remaining population, the Hart Ranges population, consisting of about 720 individuals. Nine of the remaining 15 populations consisted of fewer than 50 individuals. Distinct populations were declining at a rate of approximately 8% per year at various times in the past 20 years. Rates of decline were often highest in small populations (Figure 2) suggestive of an “Allee effect” whereby smaller populations have lower reproductive and survival rates than more dense populations in a relatively small geographic area. In stark contrast to historic group sizes, the largest of the 5565 individual groups of caribou seen by biologists since current monitoring began in 1984 consisted of 62 individuals.
Table 1: Current sizes of 15 extant populations of mountain caribou in British Columbia, Canada (modified from Hatter 2006). Population Population estimate in 2006 South Selkirks 37 Purcells-South 20 Purcells-Central extinct Nakusp 85 Duncan 9 Monashee 8 Columbia-South 29 Frisby-Boulder 19 Columbia-North 138 Kinbasket-South 2 Groundhog 30 Wells Gray (including Allan Creek*) 455 Barkerville 51 North Cariboo Mountains 267 George Mountain extinct Narrow Lake 40 Hart Ranges 717 Total 1907 * Allan Creek population shown to be connected with Wells Gray population Figure 2: Correlation between instantaneous rate of increase (r) and population size in 2002 of 15 subpopulations of mountain caribou in British Columbia, Canada (modified from Wittmer et al. 2005b). Causes of observed population declines To understand causes for observed population declines we first need to quantify the birth and death rates of distinct caribou populations to identify which component(s) of these rates are doing poorly. Wittmer et al. showed that late-term pregnancy rates were high and constant. On average more than 92% (±2%) of adult females were pregnant each year. These late-term pregnancy rates can be used to approximate a birth rate. Many studies have shown the importance of estimating sex- and age-specific survival rates (survival rates are the inverse of death rates) in ungulates (i.e., hooved mammals). In mountain caribou, survival rates of calves could easily be derived from the survey data. Because these caribou were surveyed in late winter, when caribou are usually in open, sub-alpine habitat (Mountain caribou in British Columbia, Canada) where they can be easily seen, the percentage of calves in the population can be readily assessed from a helicopter. Given that researchers know the percentage of cows that should have given birth to a calf from the pregnancy rates, any discrepancy can be assumed to be because the calf has died. While the average percentage of calves varied among populations, it usually ranged between 0 and 23% per year. When pooled over all populations, surveys suggested that on average 11.6% (±1.5%) of the populations consisted of calves .
Populations of long-lived species with polygamous mating systems such as caribou are most sensitive to variation in adult female survival. The very large sample of collared individuals allowed estimating survival rates from the encounter data. As important as documenting the survival rate, but more difficult to determine is the causes of death. After all, we must know why they are dying if we are to fix what is ailing them. Every time an individual was relocated from a plane, its status was relayed to the researchers via the remote tracking collar. If a signal indicated that the animal was dead, a detailed site investigation could help to determine the cause of mortality. Results showed that survival rates of adult females varied greatly among populations. In some populations, multiyear average survival rates were as low as 0.55. This meant that of all collared animals, 45% died over the course of one year. Such survival rates are extremely low for any large ungulate and a population with such a rate will soon be extinct. In contrast, average annual survival rates in other populations were as high as 0.96. From these data Wittmer et al. established that the large observed variation in rate of decline was due to different survival rates in populations. Populations with high adult survival rates were generally stable.
Wittmer et al. attempted to attribute the observed variation in female survival among populations to environmental variables. They determined that 2 variables are best suited to explain differences in survival rates among populations: 1) the proportion of the distinct population ranges that consists of young forest (defined as forests between 1 and 40 years) and 2) the population density. In general, survival rates declined drastically once the proportion of young (regenerating) forest available to caribou exceeded 10%. This effect was more pronounced in populations occurring at low densities.
The majority of adult caribou in all populations died from predation. Wolves, cougars, grizzly bears, black bears, and wolverine were all shown to prey on caribou in our study area. The majority of mortalities including predation occurred during summer.
Over the past 25 years of research, a clear picture of the recent dynamics of mountain caribou in British Columbia has emerged. The decline of these caribou is strongly linked to indirect effects as a consequence of commercial forestry operations. A first consequence of logging has often been an increase in the abundance of other ungulates and in particular moose and deer. Increased prey availability has subsidized predatorpopulations leading to an increased predation risk for caribou. Such a scenario where a species declines because of indirect interactions with alternate prey via a shared predator species is referred to as ‘apparent competition’. Lower adult female survival and rates of increase at low densities were also suggestive of an Allee effect, described above. Without intensive management, it appears that these caribou populations are unable to persist in areas where industrial activities alter habitat and cause a shift in predator-prey dynamics.
Conservation strategies
The implementation of conservation strategies for caribou has been slow to catch up with research recommendations in most of Canada. This is particularly true for public lands such as National Parks, where managers have shown great reluctance to adopt active management. While mountain caribou have been listed as threatened by COSEWIC, responsibility for their conservation is primarily deferred to the provincial governments.
Over the short-term, addressing the unsustainable effect of predators is a high priority if caribou populations are to be maintained. This will require implementing targeted predator removal that can be costly and controversial. Non-lethal control methods (such as female sterilization) may provide a more publicly accepted alternative but is likely only useful for some species including wolves. For other predators, such as cougars, management by increasing hunter harvest with some targeted removal may be sufficient. Reducing the numbers of moose and deer by sport hunting in certain areas has been completed and is now being monitored. It is expected that the number of predators will remain small if their primary prey can be kept at lower numbers.
Large-scale protection of caribou habitat as well as more distant areas where primary prey spend their winters may be the most secure option for caribou conservation but, with the current amount of early post-disturbance, younger forests, it will take decades before even this scale of protection would become effective. A combination of habitat protection, prey management, and targeted predator removals will likely be required. While important steps have been made in British Columbia towards achieving this goal, it remains unclear if such management will be sufficient. Thus the future of woodland caribou across southern British Columbia and even the rest of Canada remains uncertain.
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