New West Feature

Yellowstone Elk Study Points to Lasting Effects of a Hotter, Longer Summer

By Shauna Stephenson, 3-09-11

	An elk in Yellowstone foraging on what remains green. A new study shows the period of less-nutritious brown forage is getting longer. Photo by Flickr user Daniel Burton.

A study commissioned to find out why there are fewer elk in Yellowstone National Park ended up shedding light on the longterm effect climate change is having on the park’s famed wildlife.

Researchers have found that the green-up period – the time from mid-April through late July when snow finally melts and meadows start to fill with wildflowers – is gradually becoming shorter, leading into the brown doldrums of summer earlier. (Updated: A link to a website about the study is here.)

The study, conducted by Wyoming Game and Fish and the Cooperative Fish and Wildlife Research Unit at the University of Wyoming, found that between 1989 and 2010, the green-up period has been compressed by 40 percent, or by about 27 days over the past 21 years. At the same time, July temperatures, which used to average about 66 degrees, have increased 8 degrees since 1989, which coincides with times of drought and reduced snowpack.

This shortening of the green-up means lower quality food for ungulates such as elk and deer, which have an appetite for and receive the most nutritional benefit from plants when they first start to emerge. As the summer continues, the plants tend to lose some of their nutrition and appeal.

The shorter season gives ungulates less time to build the necessary fat reserves that get them through harsh winters. Additionally, it may help to explain why elk numbers are sagging in the area, says Arthur Middleton, a Ph.D. student based in the Cooperative Fish and Wildlife Research Unit at the University of Wyoming and coordinator of the Absaroka Elk Ecology Project.

“The way I think about it,” Middleton says, “and maybe the best way to put it pretty bluntly, is summer is when these animals make their living. That’s when they gain all the fat that we’re worried about them losing too quickly in the winter. If they’re not gaining as much as they need or could, (winter) doesn’t matter. The relative importance of what goes on in winter is lessened.”

To be fair, this type of shift is affecting more species than elk, and seems to be more widespread than Yellowstone National Park.

According to a 2002 study published in “Climate Research,” the average start of the growing season in Europe has shifted by eight days from 1989 to 1998. A study in 2000 in the “International Journal of Climatology” found that, in North America, it has shifted by five to six days from 1959 to 1993.

And if you think a couple days here and there don’t matter, think again. Species that base their reproduction on the seasons are finding themselves in a confusing state all over the globe. Migrating caribou in Greenland are showing up to their calving grounds too late to catch the most nutritious forage, critical in maintaining the health of their young. In the past, their migrations were timed to coincide with green-up, but warmer temperatures are causing forage to bloom earlier. Consequently, populations are declining.

The same is true for the flycatchers in Europe, which base their migration on light cues rather than temperature. With spring starting earlier, the birds are mistiming their migration. By the time they arrive to breed, the food resources they need for reproduction are already waning.

A similar trend for the elk in Yellowstone is starting to emerge. The park’s Clark’s Fork herd and can be divided into two groups: the nonmigratory herd of about 2,500 to 3,000 elk and the migratory herd of about 1,500 to 2,000 elk, which move from winter grounds near Cody, Wyoming, into Yellowstone near the headwaters of the Lamar River for their summer range, according to Wyoming Game and Fish.

When migratory herd numbers started to decline about two decades ago, researchers began looking for explanations. Most recently, biologists from Game and Fish and the University of Wyoming began studying variables that include pregnancy rates, nutrition and predator impacts.

Some would point fingers at wolves, citing their reintroduction in 1995, as the demise of ungulate herds in the West. To be fair, predators (both wolves and bears) do play a very significant role in reduction. But it’s not the only role.

It would also be easy to point fingers at wildlife managers or private landowners for gumming up the whole system. Unfortunately the issue is not that simple.

The science of predator-prey relationships is complex, but a few reasonable observations can be made. Prey in poor shape because of inadequate nutrition are more vulnerable to predators. Second, prey in poor shape tend to produce fewer offspring. For example, if there are 1,000 new elk calves, it’s not a big deal for a bear or wolf to have a snack, but when there are only 50 calves, predation has more impact.

The lynch pin in this scenario, the issue that is frequently overlooked because it does not grab attention like a pack of wolves, is the role of habitat.

The problem with the migratory portion of the Clark’s Fork herd seemed to be wrapped up in calf-retention rates—how many calves survived each year. But calf numbers were down in part because cow elk weren’t getting pregnant, says Middleton. In fact, only 68 percent were getting pregnant, significant when you consider it’s rare for numbers to drop below 90 percent.

Researchers began looking at Normalized Differential Vegetation Index, or NDVI, which is data NASA collects by satellite to measure patterns of foliage greenness. By comparing years of data, they found the time forage was staying green had been compressed by 40 percent over the last 21 years.

“It’s fairly crude information, but at the same time, if we can detect changes with a kind of blunt instrument floating in space, it’s probably going to mean something to foraging elk on the ground,” Middleton says.

The shortened green-up could explain the drop in pregnancy rates. Bob Garrott, director of the fish and wildlife management program at the ecology department of Montana State University, says nutrition plays a huge role in an elk’s ability to become pregnant. Poor nutrition can delay sexual maturity in elk herds, delaying their first calf production. It can also lower the age at which an elk will stop producing offspring, he said.

“The effects on first years of sexual maturity are really important because there are a lot of animals in a herd that are at that age,” he says. “The average age of first-calf production of two, versus three or four, has a big impact on a herd.”

Which holds true to the demographics of this particular herd, says Middleton. Clark’s Fork elk are not breeding frequently until they are 4 to 5 years old and may stop at 11 to 12, which is a much shorter window than average. On good range, it is not unheard of for yearling elk to get pregnant and adult elk are known to reproduce after the age of 12, although, like humans, it is less likely as they age. In general, they typically stop reproducing around age 15.

Similar research being conducted just south of Yellowstone echoes the Wyoming team’s findings regarding summer range. Hall Sawyer, a research biologist for Western EcoSystems Technology Inc., has been following mule deer herds for more than a decade. He’s found that unlike elk, mule deer seem to be more adept at hitting forage when it is at its peak nutritional quality.

“In the past, we thought of (that migration) as a conveyor belt where they get on and move from winter (range) to summer (range),” Sawyer says. “What we’re finding is that’s really not the case.”

Instead, deer have a series of stopover sites, moving quickly through certain areas to optimize the best nutrition; they camp out for a few days until it’s time to move to the next site. How they know when and where hasn’t been determined. But, without fail, the studies prove deer are showing up in the most nutritionally beneficial area, every time.

Traditionally, mid-April-through-July green-up has coincided with the time ungulates need the most nutrition, when they are nursing calves. For instance, a cow elk’s nutritional needs increase by almost 150 percent when she is nursing a calf.

“Summer sets the bar for the rest of the year,” Middleton says. “The rest of the year is about hanging onto what happened and what was achieved during the summer.”

And for some species like elk, making summer count is going to become more and more difficult as temperatures continue to rise.

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