In Denali, caribou are the main prey source for wolves, but they also prey on moose and Dall sheep. These animals (with hooves) are often called ungulates. How easy it is for wolves to catch and eat ungulates is related to how easy it is to catch them. We call this their vulnerability to predation. Vulnerability of caribou to predation is largely related to total snow fall (Mech et al. 1998). In a winter with above-average snowfall, it is more difficult for caribou to move and find food. The extra effort that it takes for the caribou to survive might affect their health and breeding success. It is also harder for them to run and escape predators. On the other hand, in winters with low snow cover, it is easier for caribou to move, find food and escape predators.
Denali is a very large park and our wolves have large territories—an order of magnitude larger than in other areas of the United States. As such, finding wolves for ground-based observations can be challenging.
Additionally, we strive to reduce the impact of human activities to wolves. This is especially important during sensitive periods (such as during breeding and pup rearing). Areas around known den sites or rendezvous sites are also closed to hiking access, as per our wolf management plan.
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Wolves exist at a wide range of densities throughout their range. It is not uncommon for wolf populations in arctic regions to have densities less than 5/1000 km2 (reviewed in Fuller et al. 2003).
When estimating prey biomass in an area, it is important to remember that not all prey biomass is available to the wolves as food. The more prey available to wolves, more wolves will live in a certain area. While this is a useful trick to know, even this is not always accurate. Sometimes wolf densities can still vary even with similar prey availability (Fuller 1989).
Sometimes scientists cannot predict why more or less wolves are reproducing, dying, or moving around. Differences in the environment can affect wolf populations in unexpected ways, usually by influencing prey availability. Disease can also have an effect. Wolves tend to be well adapted to most diseases, but outbreaks of rabies and canine parvovirus have caused decreases in wolf populations.
Counting wolves in a large area is difficult. In Denali's wolf program, scientists use aerial tracking methods to count every wolf in every pack for a complete population census. Usually, one or two wolves in each pack wear radio-collars, which help scientists to locate the packs. Once the packs are located, pilots and observers count how many wolves there are and what they are doing. Depending on where the wolves are, it can be hard to see and count all of the wolves, especially during the summer when vegetation can obscure resting wolves.
Additionally, there are many reasons it can be difficult to maintain contact with the packs. Wolves die, move in and out of packs, form new packs, or split into multiple packs. Pilots and biologists search for tracks to find packs without collared members and obtain counts of those packs when possible. Snow conditions and weather often make locating new packs difficult until the spring months.
To make things more challenging, wolves do not always travel and hunt together with their pack mates. During one aerial observation, a pilot might see three wolves traveling together with no other wolves in sight. The next day the very same pack members may be seen traveling as a group of seven. Counting wolves as they travel through thick timber can challenge even the most experienced observer. In addition, changes in weather, especially in early winter months, can make it difficult to see tracks and wolves.
Studies tell us that wolf populations can handle an annual harvest of 28 to 50% of the population. However, this is a large range and not without controversy and on-going debate. For example, when wolves were greatly reduced due to wolf control the population rebounded quickly to density near that at the beginning of control. However, other evidence says that harvest of wolves may be harmful and cause population decreases.
It is not clear what effect harvest has on the Denali wolf population. Many variables may have a role. Density dependent populations will have a better chance of bouncing back after mortalities. If a population is more isolated, fewer wolves will be migrating between populations to buffer mortalities. Finally, if harvest affects breeding members of the population, it may harm the chances of reproductive success.