Moose Calf Mystery Solved: Too Many Wolves

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Mech study shows a steep increase in wolf numbers coincided with a sharp decline in moose calf survival

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.