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Serving Northern St. Louis County, Minnesota

Moose & wolves

Data points to the connection between wolf density and the sharp drop in moose calf survival

Marshall Helmberger
Posted 7/26/17

A few weeks ago, I reported here on the latest thinking of researchers with the Department of Natural Resources regarding the moose situation in northeastern Minnesota, including their view that the …

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Moose & wolves

Data points to the connection between wolf density and the sharp drop in moose calf survival

Posted

A few weeks ago, I reported here on the latest thinking of researchers with the Department of Natural Resources regarding the moose situation in northeastern Minnesota, including their view that the prospects for a recovery in the moose population appear to be slim.

After publishing that story, I heard from Dr. David Mech, who most of us know has been studying the interrelationships between wolves and their prey for nearly half a century. He has previously cited research suggesting that the decline in the northeastern Minnesota moose population appears to coincide with historically high wolf densities in his study area, located mostly east of Ely.

Dr. Mech forwarded a Q & A that he had provided to International Wolf magazine about a year and a half ago, which had some very relevant information, and a more hopeful outlook on the future of moose in the region.

First, he noted that original suggestions that the moose decline was the result of warming temperatures have now been largely discounted by researchers. That’s not to say that the climate isn’t changing… that’s not in dispute. But the changes we’ve seen so far do not appear to be limiting moose numbers in northeastern Minnesota. That may eventually happen, but it does not appear to be a major factor yet, according to Dr. Mech.

A graphic included with the Q & A, which we are republishing above with permission, offers a much more compelling explanation of the real problem. The graph demonstrates a strong correlation between a significant increase in wolf densities and a dramatic fall-off in moose calf survival. Mech notes that wolf numbers had been held in check by the introduction of a new disease, canine parvovirus, but that wolves developed resistance to the disease fairly recently, which allowed their numbers to rise dramatically from a population low around the year 2000 to the highest density on record by 2009. As recently as the late 1990s and early 2000s, with wolf density still within long-term averages, the cow-calf ratio, as determined by the DNR’s annual aerial survey, was running in the 60-80 percent range. That means that roughly three-quarters of the moose cows that researchers spotted in those years had surviving calves with them in mid-to-late January when the survey is typically done.

But within five years, as wolf densities literally doubled in Mech’s study area, the cow-calf ratios collapsed. In 2002, the cow-calf ratio hit an impressive 94 percent. By 2007, it had fallen to less than 30 percent. The cow-calf ratio bottomed out at just 24 percent in 2011, just as wolf densities hit historic highs in Dr. Mech’s study area.

Scientists are always quick to point out that such correlations do not necessarily prove causation. Indeed, many things correlate that have no connection. But the connection in this case is further backed by additional data, including the DNR’s research on calf mortality, which clearly indicated that wolf predation was by far the single biggest factor.

Even DNR researchers, who had been cautious in the past about connecting the moose decline to wolf numbers, were willing to acknowledge they are playing a pretty big role, as I reported last month. And that’s one reason that DNR researchers were pretty pessimistic about the prospects for recovery.

But Dr. Mech points to a slightly more optimistic future, in part because the wolf population appears to be trending downward in the past several years. That’s not surprising. Wildlife populations fluctuate based on a number of factors and never increase forever. While the wolf trend line was straight up for several years beginning in the early 2000s, they have declined somewhat from the 2009 peak. In fact, according to Dr. Mech, the wolf population in the study area has declined by about a third since 2009, partly due to the relative scarcity of prey, primarily moose and deer.

If there is, in fact, a correlation between wolf density and calf survival, you might expect a modest improvement in cow-calf ratios from the extreme lows we saw beginning around 2006, when the moose population decline really kicked in. And, in fact, we have seen some improvement, along with an apparent stabilization in the moose population since 2014. While the calf-cow ratio averaged 31.5 percent from 2006-2013, it has improved to 37.75 percent in the four years since, which has probably helped the population stabilize. If calf survival could climb back in the 40 or 50 percent range we could well begin to see some population recovery.

What are the prospects for that? It probably depends on the wolf population, but there’s reason to think that the modest reduction in wolf numbers could continue, at least in Dr. Mech’s study area, which is in the heart of the moose range. Wolf numbers in the study area bottomed out in 2000 and then increased every year for nine years straight, which correlates to the aftermath of the 1999 blowdown. A number of biologists have noted that the flush of fresh young browse made possible by the blowdown prompted an influx of whitetail deer into the core of the moose range. This likely led to a greater incidence of brain worm and other deer-carried parasites in moose in those areas, as well as helped wolf numbers increase since their prey base had increased. It was, in effect, a double whammy for moose.

Prior to the blowdown, Dr. Mech’s study area was home to very few whitetail deer, which helped to limit the density of the wolf population and minimized the incidence of deer-borne parasites. Of course, in nature, nothing stays the same for long. The forest has grown up again since 1999, and along with the changing habitat in that region there is evidence that deer numbers are declining again. That should be good news for moose. We’ll have to wait and see, but there’s reason to think that the population decline we’ve experienced may, in part, have been a reaction to a large-scale, landscape-altering weather event and its subsequent consequences and that the population trends for all the species involved could still change trajectory. I’m willing to be an optimist, at least for now.