(Editor’s note: This story has been updated to include a revised formula, y = 96x – 266.)
Colorado is planning to introduce wolves into the state. Can we predict how many wolves will eventually occur in Colorado? We all know that predictions are uncertain, like rain, the stock market and horse races. But scientists make predictions, called hypotheses, and then test them as a part of their work. I previously calculated the number of wolves that could be in Colorado following their introduction.
In these analyses, considering elk as the primary prey, I hypothesized 2,017-3,326 wolves could occur in Colorado, considering the land west of Interstate Highway 25 (I-25) as wolf habitat; or 672-1,108 wolves when considering a smaller area of one-third of the land west of I-25 as wolf habitat.
Another hypothesis can be made using data on wolf population growth in Montana. Wolves were naturally dispersing into northern Montana in the early 1990s, and 66 wolves were introduced to Yellowstone National Park and central Idaho from 1995-96. Wolves are currently dispersing into Colorado from Wyoming, and an introduction of about 30-75 wolves into Colorado is being planned, so the situations in Montana and Colorado are comparable.
See Figure 1 for what happened in Montana. The wolf population increased from about 50 in 1998 to a maximum of 1,256 in 2011. In 2009 and 2011, hunting and trapping of wolves began and the population leveled off at about 1,100-1,200 wolves through 2021.
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Now consider only the numbers of wolves in the graph as the population grew from 1998-2011, which reflects growth of a population not managed with hunting and trapping. To predict wolf population numbers, I did a statistical regression to estimate the straight line in the graph and its equation (y = 96x – 266), in which Y is the number of wolves and X is the number of years since 1998. This line and equation reflect the wolf population growth without hunting and trapping.
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• To predict the number of wolves in 2021 (24 years from the first data in 1998):
96 x 24 – 266 = 2,038 wolves in 2021.
• To predict the number of wolves in 2031 (34 years from the first data in 1998):
96 x 34 – 266 = 2,998 wolves in 2031.
The prediction is that the number of wolves would increase from 1,256 in 2011 to 2,038 in 2021 and to 2,998 in 2031. This is an increase of about 1,000 wolves in Montana every 10 years.
But that’s not what happened. The predicted number of wolves for 2021 (2,038) is almost twice the actual number in 2021 (1,144) in the graph. The difference appears to be wolf management with hunting and trapping that began in 2009-11. In other words, the wolf population was growing rapidly in Montana, and management with hunting and trapping stabilized the population.
Will these numbers for Montana apply to Colorado? In a general sense, yes, in that a wolf population will increase in number and over geography if there is prey available and the population is not controlled with hunting and trapping. We can hypothesize that a wolf population in Colorado beginning with about 50 wolves from natural immigration and an introduction will result in about 1,000 wolves in 10 years, 2,000 wolves in 20 years and 3,000 wolves in 30 years.
Research shows that a reasonable prey rate is about 17 elk per wolf per year, so we can also hypothesize wolves will kill about 17,000 elk/year after 10 years, 34,000 elk/year after 20 years and 51,000 elk/year after 30 years.
These estimates of wolf numbers and prey are uncertain, as with all predictions, but they are based on observed numbers in Montana and can be used to formulate hypotheses for Colorado and adjust management accordingly. Other factors need to be considered such as prey species (elk, deer and moose) and wolf expansion and emigration out of Colorado (e.g., to Utah, Arizona and New Mexico). Wolves also kill considerable numbers of livestock in Montana (see the References online) and estimates for Colorado should be made for proactive management to prevent livestock losses.
The data from Montana show that wolves in Colorado will increase in number if habitat is available. Management methods of hunting, trapping and removal of wolves preying on livestock can stabilize wolf numbers, maintain viable wildlife populations and protect livestock. — Dr. Matthew Cronin
(Matthew A. Cronin is a scientist with Northwest Biology Company LLC in Bozeman, MT. He can be contacted at croninm@aol.com.)
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