Thursday, October 8, 2015

The Ecological Footprint of Wolves

Since 2001, Bob Beschta has observed firsthand how aspen, cottonwoods, willows, and berry-producing shrubs have begun to recover with the return of wolves to Yellowstone twenty years ago. Scientific research has left no doubt in his mind about the beneficial—really, ecosystem-saving—presence of wolves. 

So Mary and I jumped at the chance to assist this professor emeritus at Oregon State University in his field work related to plants, elk, and wolves. His scientific wandering takes him well off trail in areas of Yellowstone where grizzlies travel. Adding two people, even if they aren’t scientists, to his group of one makes sense.

As we walk away from the trailhead and toward the distant hills where we will work, Beschta points with his hiking pole to some healthy willows and aspen growing along Glen Creek. If wolves were still absent from Yellowstone, he says, those aspen and willow would have been eaten down to the nubs by elk who could dine there for as long as they pleased. But facing wolves, or the threat of wolves, elk have changed where and how they eat.  This has allowed once-ravaged plants to reach what research shows is a critical height. If they can grow to six feet tall, elk browsing no longer threatens their existence. “All aspen and willow need is a chance,” Beschta says.

Further along the trail, he points to a large stand of colorful young aspen and willow. He stops and says that these aspen really interest him, because he sees no mature tree, no mother tree, nearby. Aspen almost always spread as clones from the roots of another tree. But they can also spread by seeds, just like cottonwoods do. Some of these aspen are already putting on their red, yellow, and orange fall colors while others still sport summer green. This variety supports his belief that they are seeded. “If they were a clone, they would all be acting the same.”

This colorful stand not only shows that aspen can grow from seed, it also reveals, he says, the ecological footprint of wolves. We have seen three different piles of fresh wolf scat on the trail so far. The presence of wolves may have kept elk from devastating this stand. As the willows and aspen have grown, beaver have settled nearby, since they love to eat both plants. They have built a dam on Glen Creek; we can see the pond just south of the trees. This pond moistens and softens an increasing amount of ground around its edges. Elk do not like feeding on soft ground because it slows their escape from wolves. But aspen and willow love moist ground, and without elk browsing fresh sprouts to within an inch of their lives, the stand has grown higher and wider. 

As we hike on, Beschta says that he has done much of his research and co-published about 25 scientific papers with another OSU scientist, Bill Ripple. Many of those papers have related to how the wolf’s influence works its way down to the plants of an ecosystem. This hypothesis, called Trophic Cascade, has had a mixed reception, and other scientists have presented alternative ideas. The most common alternative as to why Yellowstone’s aspen, willow, and cottonwood are doing better is that climate change helps them. Beschta doesn’t believe that.

“If you look at our papers, you will see that with almost every one of them we’ve done a climate assessment, always considered climate a competing hypothesis.” Time after time, Beschta and Ripple arrived at the same conclusion: there is nothing in the climate data that can explain the changes since wolves were reintroduced. The improvements in the plant communities were just too immediate to call climate the cause. 

Leaving the trail, we bushwhack across the valley floor, around and through sage, enjoying its invigorating aroma. We rock-hop across Glen Creek to reach a stand of aspen and its recruits—the thin, young trees growing below the few mature ones. Beschta goes to work. He unstraps from his backpack a sliding ruler that can measure tall trees. He selects some recruits and measures their height. At various points along each trunk he determines whether and when the recruit was browsed by elk. He yells this information out, and Mary records the data in the field notebook.

While Beschta gathers data and Mary records it, I do my part by climbing up a 65-degree slope (Beschta measured the angle as part of his research) to look for bison and elk scat. I walk a certain pattern and use two clickers to record scat. Then I give Mary the count, which provides some indication of whether elk or bison frequent this area.

After Beschta finishes measuring recruits, he surveys the scene, looking for conditions that would affect an elk’s decision on whether to dine here. Are there logs that would impede running away from wolves? Are there conifers or ridges nearby from which wolves could suddenly attack?

Standing amongst the recruits, he turns a circle, points, and says, “If I’m an elk, I can see 100 yards that way, 100 yards this way. I can see wolves. Even if they come out of the woods, I’ve got a good escape route.” 

He stands where elk may have less fear of wolves. That’s also where the recruits are shortest. The opposite is true as well: if a site has factors that could make escape or spotting wolves difficult, elk browse less and the aspen, willow, and cottonwoods grow more.  This relationship between wolves, elk, and aspen is part of what’s called an ecology of fear.

Beschta wraps up our lesson and his tools and we bushwhack to a stand of six aspen further up the hillside. As he photographs the trees for his records, he says they are at least 100 years old. He adds that some of Yellowstone’s aspen stands have been growing in the same place for thousands of years. 

He pockets his camera and takes us on a journey through time. Let’s go back, he says, to 1900 when wolves roamed Yellowstone and kept elk in check. We would be standing in the cool shade of a full aspen forest, instead of in the sun filtered through six, lonely old trees. This grove would be thick with aspen of all sizes, stair-stepping in height from sprouts to mature adults.

But moving forward to say 1940, we would find that thick forest disintegrating. Hungry elk are everywhere; wolves are nowhere—the last one was killed over a decade earlier. When adult aspens send up the sprouts intended to replace them someday, elk chomp the tender greens down to within a couple feet of the ground or less. When next year’s growing season rolls around, elk will again eat the new growth. Year after year this scenario repeats until the ravaged sprouts no longer have the strength to recover. As the young plants die, the future of Yellowstone’s aspen looks bleak.

Now let’s travel to the present where we find only aspen of two ages: mature trees that sprouted before wolves were killed off and young ones that have sprouted since wolves returned in 1995. Seventy years of aspen are missing, consumed by lazy, overabundant elk when wolves were gone. But some of the sprouts we find are at—or higher than—that magic six feet, brightening the aspens’ future.

And if we zip 100 years ahead, with wolves keeping elk cautious and in check, this grove we are standing in will again have aspen of all sizes, from tiny sprouts to huge adults in a full forest. It will look as it did 200 years before. Thanks to wolves. Beschta suggests that we should call Yellowstone’s trees “wolf aspen” because without the return of wolves, aspen would have eventually disappeared from this northern part of the park.

I ask Beschta how he came to believe so strongly in the benefit of having wolves in this—or any—ecosystem. He says that after he earned his Ph.D. in watershed management he started teaching and researching at OSU. He spent much of his career studying riparian zones—those diverse plant and animal communities that border streams, rivers, and lakes. He also studied rangelands. Over decades he learned, “If you want small mammals, if you want large mammals, if you want birds, if you want pollinators, you better have healthy riparian areas.” When he came to visit Yellowstone in 1999, he was shocked to see how essential riparian areas—especially in the Lamar Valley—had been trashed and transformed by the overgrazing of elk.

He returned to Yellowstone in 2001 to analyze cottonwood growth and disappearance. He found cottonwoods in the Lamar Valley that were over 200 years old. And as he studied the ancient trees, he saw that those cottonwoods were healthy as far back as when Lewis and Clark crossed our continent. “Furthermore, coming toward the present, the data indicated that everything was doing fine until the early 1900s when wolves disappear.”  

The possibility that the killing off of wolves could have led to major ecosystem damage, sparked his scientific curiosity and led to more field work and data analysis. His research convinced him that the presence of wolves is the key to the health of Yellowstone’s ecosystem. 

The last stand that we bushwhack to consists of a single old aspen, leafless limbs on one side. It is surrounded by hundreds of recruits, some, ten feet tall. Beschta points to the ailing aspen and says that as the recruits grow, they will take more water and nutrients from the soil. Deprived of nutrition, the mother tree will die sooner than if there were no recruits struggling for life below her twisted old branches. I am touched by how similar that sounds to the way humans can sacrifice for their offspring.

As we pack up and start the three-mile return trip to the trailhead, Beschta stops, looks around, and says, “It’s nice to be on a positive ecological story. I’ve been on so many downers. This one keeps bringing me back.” 

He is excited by all the young aspen we have walked among today, especially those over the magic six-foot height. We have traipsed through a success story, he says, written by wolves. “Wolves,” he laughs, “have done more riparian restoration that I ever did in my career.”

All photos by Rick Lamplugh except where noted otherwise.

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In the Temple of Wolves

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  1. This is very encouraging news , the wolf aspens rebounding ecosystem , proves our wolves are vital to the biodiversity of our natural world ! Loved reading the blog ! Thank you !

  2. Thanks for your comment, Sandy. I have come to believe that not only Yellowstone but other national parks could benefit from reintroducing wolves.


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