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Serving Northern St. Louis County, Minnesota

Don’t confuse proximate cause with ultimate cause


As an animal ecologist who has followed the wolf-moose relationship here in northern Minnesota since 1971, I must differ with Marshall Helmberger’s views about wolf predation on moose, published on 10 April in the Timberjay.  Not only has Mr. Helmberger fallen prey to the correlation-causation problem, despite having noted the problem, he has failed to understand the difference between proximate and ultimate causes.

Wolves and moose have co-existed in what is now northern Minnesota since shortly after the glaciers retreated some nine millennia ago.  During this long period of co-existence, most moose probably died by being killed by wolves but, nonetheless, the moose population persisted.  When a moose got sick, it was probably killed and eaten by wolves because it was easy to catch.  When a moose was plagued by winter ticks, it was probably killed and eaten by wolves because it was easy to catch.  When a moose calf had a mother unable to defend it well, it was probably killed and eaten by wolves because it was easy to catch.  In all these cases, the proximate cause of death for a moose was wolf predation but the ultimate cause was disease or injury or something that weakened a moose.

After the long period of co-existence, why are wolves now killing enough moose possibly to cause extinction of the population?  The general answer is that something must have changed.  One hypothesis is that wolves have evolved new abilities to kill moose, allowing them to kill more moose than they could in the past.  Research on wolves in northern Minnesota is ongoing, both through the U.S. Geological Survey and through the Natural Resources Research Institute in Duluth.  If some new hunting adaptation has appeared, we should have learned about it.  In addition, we have no evidence from elsewhere that wolves have evolved such new abilities.  Thus, our best information suggests that wolves have no new abilities to cause the moose population in northern Minnesota to decrease to low levels.  Although wolves are the proximate cause of many moose mortalities, they do not appear to be the ultimate cause of a decrease in the moose population.  Killing and eating many moose may, however, have caused the local increase in wolf numbers in David Mech’s study area.  What is surprising is that many moose are dying now without having been killed by wolves.

Another general hypothesis for the moose population decrease in northern Minnesota is that something has changed for moose.  Some evidence does support this hypothesis.  Limited research suggests that foraging by moose may be limited by hot temperatures during summer.  Other research suggests that vegetation changes, caused by climate change or forest management, may affect the abilities of moose to forage efficiently.  These and other pieces of evidence suggest that changing conditions for moose in northern Minnesota may be making them more vulnerable to predation by wolves than they have been historically.

Thus, contrary to Mr. Helmberger’s assertions, the best evidence suggests that the most common, proximate cause of death for moose in northern Minnesota, wolf predation, is not the ultimate cause of the moose population decrease.  The only way to learn what the ultimate problems are for moose, and to learn whether management can alleviate the problems, is to do research on moose, especially, but also on wolves, which is what people from the DNR and other agencies are doing.

Roger A. Powell

Department of Applied

Ecology-North Carolina

State University

Ely, Minn.