Natural factors include natural variability in numbers, predators, and competition (described below) and also diseases, parasites, and fire, which are described more fully under the climate change section.
There are natural variations in the numbers of caribou. This variability is thought to run in cycles, and to be influenced by the overgrazing of habitat when populations are at their peak. Traditional knowledge holders in the NWT have put the cycles of barren-ground caribou at between 10 and 60 years. Scientific studies agree that the population cycles seem to run over several decades.
This variability is not a threat to caribou all by itself, but natural variability can produce low numbers in any given herd or location, and then the low numbers may be driven even lower by other threat factors, making it harder for the caribou to rebound. The rate of change of caribou herds can be steep. For instance, the George River and Leaf River herds increase or decrease at rates of up to 15% a year. That means a herd can double in less than ten years - or its numbers can be halved.
Wolves are the main predators on the caribou populations of the Canadian Arctic. One three-year NWT study found that in three-quarters of wolves sampled, about two-thirds of their stomach contents were from caribou. Grizzly bears are another major predator, and in some cases depend heavily on caribou for their diet. They not only kill caribou, but also scavenge caribou killed by wolves or hunters. Black bears, lynx, wolverines, and even golden eagles are also among animals that will attack and eat full-grown caribou or calves. There are concerns that as other animals (such as moose and deer) move into caribou territory, they can maintain higher populations of caribou predators.
There are different views as to how much caribou compete with other animals for food sources, and whether this competition would affect the numbers of caribou, or their choice of range. According to some Indigenous knowledge sources, the presence of muskoxen in an area can reduce food available to caribou. Some Indigenous people believe that caribou do not even like the smell of muskoxen. Some scientific sources also suggest that muskox densities in some places may be detrimental to caribou because of an overlap in what they eat. There is also the potential for more overlap between northern caribou and white-tailed deer and moose. Climate change has allowed these species to expand their range, and they could be competing for some of the same food, and could also introduce new diseases to caribou.
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Behaviour is more important than thermal performance for an Arctic host–parasite system under climate change
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Large‐scale prion protein genotyping in Canadian caribou populations and potential impact on chronic wasting disease susceptibility
Aerial Survey of Muskoxen (Ovibos moschatus) and Peary Caribou (Rangifer tarandus pearyi) on Northwest Victoria Island, April-May 2015
Format: videoNatural factors