Wolves Driving Down Yellowstone Elk Numbers

A new study shows that the wolves of Yellowstone National Park are affecting elk populations in very surprising ways. Since 1995, elk populations have been declining. It was estimated in 1995 that elk populations in the northern range numbered somewhere between 17,000 and 19,000. In the three winters prior to 2008, the estimated elk population has dropped to somewhere between 6,279 and 6,738.

The numbers are from a Montana State University (MSU) study showing that the presence of wolves has caused elk to alter their behavior towards the point that elk are birthing fewer calves. The obvious observation of the effect of wolves on the elk population is the fact that wolves kill elk on a regular basis for food. But the constant danger of being attacked by wolves has made the elk more paranoid, causing them to browse for food in more sheltered areas such as near shrubs or under low tree branches. This, in turn, means that elk being hunted by wolves are eating less and receiving less nutrition, according to the study from Scott Creel, ecology professor at MSU.

Creel, who co-authored the paper with former students John Winnie Jr. and David Christianson, said that wolves are causing a decline in elk calves being born. Two studies monitored radio-collared elk calves and discovered that few of them were killed by wolves in the first six months of their life. That does not explain how wolves were affecting elk population. After studying the calves, Creel concluded that calf numbers were low immediately after the birth pulse, suggesting that decline in the birth rate was a part of the population decline. The birth pulse is a time in spring when most elk birth their calves.

Researchers also found that elk facing the threat of predation had low levels of progesterone, a hormone required to support pregnancy. This discovery raised the question of why progesterone levels were low and, in turn, how the wolves were involved.

There are two theories as to why the elk affected by wolves had low progesterone levels. One is that the elk were suffering from chronic stress, which caused the release of the hormone cortisol into the system. Cortisol helps free up energy for fight or flee, but too much of the hormone causes the immune and reproductive system to shut down. The other theory is that the elk were not receiving enough nutrition because they were hiding in nutrient poor forests, in comparison to open meadows where food is abundant.

The researchers at MSU studied fecal and urine samples for over four years and found no basis for the chronic stress theory but did discover that elk preyed upon by wolves had lower progesterone levels compared to elk with no wolves present. This shows that the elk are trading their posterity for their own survival.

Reprinted in part from Yellowstone Insider

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