Migrating vs. Resident Elk: Who has the best strategy?
Many animals migrate in an effort to find food, a more hospitable climate, and most importantly, a place to breed. However, a herd of elk known as the Clarks Fork herd, made up of nearly 4,000 elk, are coming back from their Yellowstone National Park migration with fewer calves compared to those elks that do not migrate, which are known as resident elk.
So why are the resident elk being considered more successful compared to migrating individuals in the same herd?
A new study by the Wyoming Cooperative Fish and Wildlife Research Unit has found that the long-term decline in the number of calves produced annually by the migrating Clarks Fork herd is due to drought and increased numbers of big predators which are creating implications for these migratory animals.
These migratory elk experienced a 19 percent depression in rates of pregnancy over the four years of the study and a 70 percent decline in calf production over 21 years of monitoring by the WGFD, while the elk that did not migrate, in the same herd experienced high pregnancy and calf production and are expanding their numbers and range.
"This is one of North America's wildest and best-protected landscapes, where elk and other ungulates still retain their long-distance seasonal migrations — and yet it is the migratory elk that are struggling while their resident counterparts thrive in the foothills," said Arthur Middleton, who led this work as a University of Wyoming doctoral student and is now a postdoctoral fellow at the Yale School of Forestry and Environmental Studies.
The study shows that the hotter and dryer summer conditions of the last two decades, coincident with the long-term drought widely affecting the West, has reduced the duration of the spring period when tender new grasses are available to elk. This makes it harder for female migratory elk to find the forage they need to both nurse a calf and breed.
Another likely cause of the declining calf numbers among migrants was predation. The range is home to four times as many grizzly bears and wolves than resident elk, and both predators are known to prey on young elk calves. Resident elk get a break from high levels of predation in part because when predators kill livestock on the resident range, they are often removed by wildlife managers and ranchers.
While conservationists have been focusing on physical barriers like fences and development that can disrupt migration, this study shows that subtle changes in predator management and forage quality also play an important role in the success of migratory species.
The study is featured in a Forum section of Ecology.
Read more at the USGS Newsroom.
Elk image via Shutterstock.