One of the most commonly known consequences of the large elk population throughout Yellowstone National Park is the significant decline in the recruitment of aspen (Populus tremuloides).
For example, in Yellowstone National Park, studies have found that on days when elk sense wolves nearby, the herbivores would retreat to the edge of the forest.
[6] So these snowy areas encourage elk to move to under the trees, where the branches act as a cover from the snow.
This likely could be because their calves are the ones most easily preyed upon by wolves and all elk are more likely to be killed in the grasslands than at the forest's edge where there is more cover and protection.
Come fall, the yearling is old enough to look out for itself and the vigilance of its mother decreases slightly since she no longer has to keep an eye out for predators for two bodies and can focus solely on her own protection (Lung and Childress, 2006).
Because of the habitat change from grassland to forest, a lot of the graze needed in the elks’ diet is being replaced with browse.
The bulls do not change as much of their graze time, even if wolves are in the area, because their bodies are not equipped to deal with such bad nutrition right before winter.
Even without nutrient deficiency from changes in summer foraging patterns, their body conditions deteriorate (Christianson and Creel, 2010).
This is just one reason why elk, male and female, have to rely a lot on stored fat and muscle that was gained throughout the rest of the year to meet the needed levels of dietary energy and nitrogen.
Contrary to popular belief, this large decrease is caused by aspects other than simple direct predation from wolves.
The elk also suffered from the negative effects of their “behavioral or physiological responses” to the newly introduced carnivores.
It has become apparent that other things come into play involving this drop besides just the fact that wolves are the number one predator of elk in Yellowstone.
[8] This is also supported by the fact that most calves up to six months old are sufficiently protected by their mother and the herd and are not killed by predators.
Since the calves themselves are not being killed, this suggests that the reintroduction of wolves has indirectly affected the reproductive rate of elk through the new behaviors they had to adopt in order to avoid predation.
There has also been a decline in the calf-to-cow ratio in the herds, which suggests that fewer elk than expected are giving birth.
It has been found that elk more than double the number of hours during the day that they spend vigilant and because of this they are decreasing the proportion of time spent feeding by about 19%”.