Methods for estimating wolf abundance

METHODS FOR ESTIMATING WOLF ABUNDANCE

Counting Wolves, Not a Perfect Science

Many wolf managers and biologists around the country finish their annual count of wild wolves to determine whether populations are meeting management objectives in the spring of the year. Wolf populations are generally counted by the states or by the recovery areas, depending on which agency is responsible for wolf management.

For example, wolves in Arizona and New Mexico are a managed recovery population protected by the federal Endangered Species Act (ESA) and monitored primarily by the U.S. Fish and Wildlife Service (USFWS). Wolves in Minnesota and Montana are state managed, and each state’s respective department of natural resources monitors and manages the wolf population within its state.

Generally speaking, wolves managed under the ESA are counted annually. However, not every state conducts an annual census. Individual states have different management objectives and can adjust their census style to fit those goals.

Minnesota conducts a wolf census approximately every five years. Minnesota is the only state in the lower 48 where wild wolf populations maintained a foothold on the landscape throughout European settlement of the United States. At the time of ESA listing, there was a stable population of 500 to 1,000 wolves in Minnesota. Within a decade, the population almost doubled, and in the 1990s it reached 2,000. A minimum of 1,600 wolves was established as a maintenance goal in Minnesota’s wolf management plan, but the wolf population in the state has remained roughly stable at around 3,000 for more than a decade. If a significant drop below 1,600 wolves were to occur, Minnesota would take action to both determine the cause and address the issue per the state’s management plan as well as the ESA delisting agreement.

Montana conducts an annual count of wolves in areas where the population is relatively new in terms of years on the landscape. Wolf populations in Idaho, Wyoming and Montana are still changing and adapting to the natural and cultural landscape since reintroduction in the mid-1990s. Counting wolves annually allows Montana to monitor population trends more accurately.

The methods used to count wolf populations also vary by state. The monitoring methods used are influenced by a number of factors:

Type of population (state managed vs. federally protected)
Topography
Vegetation/foliage
Accessibility
Visibility
Climate
Snow depth in winter
Person power (volunteer or agency staff)
Resources (ground patrol, helicopter and radio telemetry)
Collaboration with other agencies or groups
The wolf population that lives within Yellowstone National Park is highly visible, and a high proportion wear radio collars and are regularly tracked. The expansive river valleys and lack of tall vegetation allow observers frequent opportunities to see wolves and count their numbers even from miles away with the use of spotting scopes. Additionally, the park has a host of volunteer “wolf watchers” who spend significant amounts of time in the park monitoring both wolf packs and individual animals. These data help park staff collect information about the wolves living in the park and save the park valuable resources that can be applied elsewhere.

In Wisconsin, trained volunteers collect wolf sightings and complete track and howling surveys for the Wisconsin Department of Natural Resources (DNR). These volunteers log thousands of hours, which frees DNR staff to analyze and combine the data with its own radio-tracking data to produce a wolf-census report for the state.

Minnesota also combines multiple data sources to determine the number of wolves on the landscape. Due to dense vegetation and the size of the population, counting every wolf isn’t practical. Therefore, a collaborative effort is made to gather data from research entities and individuals throughout the wolf’s range. This information might come from sightings of wolves on the ground, locations taken using radio telemetry, scent posts or track surveys. Analyzing these data requires Minnesota DNR staff to incorporate a variety of factors such as road and human density, prey populations, vegetative cover, inland lakes and incidence of disease.

This method doesn’t give an exact count, but it does provide a solid number for wildlife managers to use as a starting point. A “confidence interval” describes the probability of a number being higher or lower based on the data available. In Minnesota, the confidence interval for an estimated wolf population is approximately 500. This means that if every wolf could be counted, there could be between 2,500 and 3,500 wolves. This confidence interval is in place because it is impossible to count every single wolf in Minnesota.

When a wolf census is conducted, it is done in mid- to late-winter, which is the time of year when a wolf population is almost at its lowest. A count at this time of year indicates whether or not population management objectives are being met and whether action needs to be taken.

In late spring, when pups are born, the population is at its highest. However, this is a transitory situation. Pup mortality varies considerably depending on the overall condition of the pack, availability of food resources, weather, disease and security of the den.

This high springtime number decreases throughout the year as some pups die. Additionally, adolescent and adult wolves disperse and die each year with the most common causes being accidental, incidental and depredation-control human take, starvation and intraspecific strife or wolf-on-wolf aggression, usually related to territorial disputes.

The methods used by state agencies also work for recovering populations under federal management. However, wolf populations that are federally protected and managed are under greater scrutiny from the agencies responsible for them as well as the public.

With the census data collected, managers can determine how to prudently move forward with management decisions such as increasing or decreasing protections or implementing a hunting or trapping season. Evaluation of past management practices are also measured against the outcome of a population count.

Due to the amount of data that must be collected and analyzed, most census results are released in late spring.