Moose calf mystery solved: Too many wolves

Sometimes, no matter how hard you try to avoid it, the answer to your question is staring you in the face. And it appears that could well be the case as it relates to the declining moose population in northeastern Minnesota.

For several years, the Department of Natural Resources has conducted millions of dollars worth of research trying to determine why it seems our region’s moose herd is disappearing.

Since early on in the effort, researchers have focused on warmer temperatures, and that’s due in large part to the results of a 2009 study by DNR biologist Mark Lenarz, which correlated declining moose numbers in the region to rising average temperatures in January. The theory goes something like this: moose have a competitive advantage over whitetail deer in areas with extreme cold and plentiful snow, which has typified northern Minnesota for many years. But as winters have warmed (which, on average, they have), that competitive advantage is undermined. Greater deer survival leads to more competition for food and an increased prevalence of deer-borne parasites, such as liver flukes and brain worms, that weaken and often eventually kill infected moose. In addition, goes the theory, warmer winters have increased the survival of winter ticks, another parasite with a number of negative impacts to moose.

I don’t discount in any way that some of these trends may be playing a role in the moose decline.

But in the rush to demonstrate a climate-related connection, the DNR has, at least to date, downplayed a much more direct cause of the moose decline: wolves.

It comes down to Occam’s razor, the principle that the most likely answer to a question or problem is the most straightforward, requiring the fewest assumptions.

The notion that moose are declining because more wolves are in the woods to eat them is about as direct an answer as one could find. But is it true?

Indeed it is, says Dr. L. David Mech, who has studied wolves throughout a good chunk of northern St. Louis and Lake counties for decades. In a paper published several months ago, Mech provided compelling evidence that the primary cause of the moose decline is the sharp rise in wolf numbers, at least within that portion of the primary moose range that overlaps with Mech’s study area.

Mech, in his study, re-examined the Lenarz data and conclusions and found his correlations of the moose decline with warming temperatures in mid-winter to be dubious. Mech, of course, wasn’t the first to doubt this connection. While our region has certainly seen a trend towards warmer winters over the past 25 years, our winters remain far colder than many other parts of North America where moose continue to do reasonably well— and that’s a point that many have made.

Keep in mind, correlation is used in research all the time, but correlation does not prove causation, and can often be misleading. And questions certainly arise with the correlations cited by Lenarz. Why would the moose decline, for example, be linked to mid-winter temperatures? If deer survival and winter ticks were the primary issues related to a warming climate, one would expect to see the moose decline more directly linked to warmer temperatures in March and April, when deer are the most stressed (and most likely to succumb to conditions), and when winter ticks are susceptible to freezing (they drop off their moose hosts in March and April).

It’s true that mid-winter temperatures in Minnesota have been increasing (more so than during other parts of the year) and that moose are declining, but is there reason to believe the two trends are linked? It’s a stretch.

While Mech’s study relies on correlation as well, he demonstrates a remarkably strong link between rising wolf numbers in his study area and the decline in moose survival, both for adults and, particularly, for calves.

Consider the numbers. Mech compared DNR aerial moose survey results, including adults and calf ratios within his wolf study area, to the changes in wolf numbers in the same area— and the correlation was astonishing. Back in the 1990s and early 2000s, when wolf numbers in Mech’s study area ranged roughly between 50-65, moose numbers remained strong and calf-to-cow ratios were consistently above 50 percent in DNR surveys.

But beginning in 2004, the wolf population steadily increased in Mech’s study area, from 58 in 2003, to 97 by 2009. It’s hovered around the 90s ever since.

In 2003, seven of every ten cows sighted on the DNR’s aerial survey had a calf. By 2007, when the wolf population had jumped from 58 to 81, barely one-in-four cows had a living calf by the time of the aerial survey. During the same period, the percentage of calves in the moose population fell, from 28 percent to just 14 percent. Given that calves are the most vulnerable to wolf predation, their declining proportion in the population makes sense.

This trend of rapid decline in calf success cannot be accounted for through climatic change. The differences in temperatures between 2003, when calves were apparently doing quite well, and 2009, by which time their numbers had plummeted, are too insignificant to show such a direct effect.

And Mech has more data. He cited other studies, from eastern Ontario, which compared wolf densities to moose calf survival. Remarkably, Mech found that wolf densities in the Canadian study area were two-thirds that found in his Minnesota study zone. And calf survival in Minnesota was, you guessed it, about two-thirds that found in the Canadian study. It’s pretty powerful evidence that wolf densities are a much better predictor of calf survival, than temperature fluctuations. That’s especially so, since climate factors would presumably play a role in Canada as well as Minnesota.

And, most significantly, we don’t need to rely on correlation alone. While the ongoing moose calf study in the region has had its problems, what data it has collected fits exactly with Mech’s conclusions. The vast majority of our moose calves are dying in wolf attacks.

The good news is this is a wildlife management issue, which can be controlled, or at least could be controlled if anti-wolf hunting organizations hadn’t succeeded in ending the wolf hunt in Minnesota. Assuming the legal issues are resolved soon, Mech recommends that the state focus more of its wolf harvest quota in future years in the primary moose range, to give the moose population some breathing room.

The alternative is to do nothing, and to let nature take its course. In the long run, if moose continue to decline, wolf numbers will decline as well, particularly in those regions where wolves rely almost exclusively on moose in winter. But before that happens, the moose could be all but extirpated from Minnesota, and any recovery would be difficult at best.

There’s really little reason to delay. The evidence is increasingly clear. While climate factors may play some indirect roles in the moose decline (such as making moose less healthy and more vulnerable to wolf predation), wolves are the primary direct factor behind the disappearance of this northwoods icon. That’s a scientific conclusion that’s hard to refute.