Wolves Driving Down Yellowstone Elk Numbers: Study

The numbers are from a study from Montana State University 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 elk population is the fact that wolves kill elk on a regular basis for food. But the constant danger of having your taut, fleshy neck ripped to shreds by wolf teeth have made the elk more paranoid, causing them to browse for food in more sheltered areas such as shrubs or 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, says that wolves are also 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 thatc alf 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 in turn 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 4 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

Creel and his student Paul Schuette are now trying to see if the theory of predators affecting reproduction is true for other predator-prey cycles. A study is being conducted on a Maasai Community Conservation Area in the South Rift of Kenya observing predators such as lions and spotted hyenas in relation to a large amount of prey. If proven true, the idea that predators affect the reproductive cycles of prey will change thinking about predator-prey dynamics.

The main thought on predator-prey dynamics up to now has been that a herd losing many of its members to predators would decline faster than one where predators are less prevalent. The study also covered grizzly bears, which also prey on elk and have been increasing in population in the area of the study, but concluded that this was due to geographical expansion. This is just a showcase of how important wolves are to Yellowstone National Park, and how the park affects the rest of the world. –Sean Reichard

Photo by Wallace Dayton, courtesy of National Park Service.


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