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W&L Biologist's Research Aims to Help Yellowstone Bison, Elk

Bill Hamilton with bull elk skeleton in Yellowstone.
Bill Hamilton with bull elk skeleton in Yellowstone.
W&L Audio:

Bill Hamilton discusses his research in Yellowstone.
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Jeffery G. Hanna
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What’s a bottle of Roundup Weed and Grass Killer doing in Yellowstone National Park?

It turns out that Roundup might be one of the most important tools in the battle invasive plant species that threaten one of America’s last native prairie grasslands, home to bison, elk and pronghorn antelope.

In 2005, Bill Hamilton, associate professor of biology at Washington and Lee University, took 11 W&L students to Yellowstone during the spring term as part of a pilot project to investigate the interactions of bison and elk with grasses and soil microbes. The results of that pilot project helped define an experiment conducted in the summer of 2005. The results were published in Soil Biology & Biochemistry with Paul Hinchey of the Class of 2006 listed as a co-author along with colleagues at Syracuse University.

In the spring of 2007, Hamilton led another team of students to Yellowstone where they investigated the effects of invasive mustard species. The results from these experiments were so intriguing that the National Park Service postponed a grasslands restoration project so Hamilton could gather more data. He is returning this spring with yet another student research team to work with NPS and USDA scientists on the start of a five-year grassland restoration project.

“One of the basic questions grassland ecologists ask is why do grasses re-grow?” said Hamilton. “For the longest time we thought it was a simple case of animal fertilizers — nitrogen. Animals run around; they pee and plants grow. You can see a link between a urine hit and plant growth. But that’s just part of the story, because it doesn’t explain why you can find lots of nitrogen in the soil in the absence of grazers. It turns out there’s something going on belowground that stimulates new growth as well.”

Bison, often called the ecosystem engineers of Yellowstone’s grasslands, play an important role in stimulating new growth. In the spring, like lawnmowers, they remove dead plant tissue from the previous summer, allowing fresh green grass to grow. Bison also trample dead tissue into smaller fragments that are broken down further down by soil micro-arthropods and microbes.

In previous studies, scientists have observed that grazing causes grasses to release carbon into the soil. “What happens is that when some species of grasses get eaten, it triggers a response mechanism that sends a flush of carbon into the soil,” explained Hamilton. “The higher concentration of carbon stimulates soil microorganisms that further break down organic matter, helping cycle nutrients through the ecosystem. As discussed in our Soil Biol & Biochem paper, we’ve documented this in the lab and have also seen this feedback system at work in some grass species in the field.”

Knowing that bison and prairie grasses depend on one another, Hamilton focused next on what happens when an invasive species — a mustard species, in this case — appears in a natural setting like Yellowstone. The question he asked was: “Does an invasive species break down that interaction between grazer, plant and microbe?” The answer seems to be yes, and the consequences for grazers such as bison are serious.

He and his students have been studying soil samples from the Gardiner Basin, located in the upper northwest corner of Yellowstone. “This is a prime winter range for bison, elk and pronghorn deer,” Hamilton said. “The problem is that these grasslands have been completely overgrown by mustard, and the animals, bison in particular, are wandering beyond the protection of Yellowstone’s borders in search of new pastures.” Rangers chase first-time wanderers back into Yellowstone. Second-time offenders are sent to slaughter. Last year approximately 3,000 bison were sent to slaughter.

What he and his students discovered is that mustard inhibits the growth of microbes in the soil. “You cannot find native grasses in the presence of mustard,” Hamilton noted. “There’s something going on that disrupts the chemistry between the native grasses and the microorganisms in the soil.”

The solution is to restore the grassland, first by killing the mustard with Roundup, planting barley to stabilize the soil and then planting a native species mix the following year.

If that process seems like a lot of work for a few thousand large herbivores, Hamilton argues it’s well worth the effort. “Other than Ted Turner’s ranch, the only genetically pure bison left in the U.S. are in Yellowstone and Wind Cave National Park. Herbivores impact how grasses grow, and without them, you change the entire ecosystem.”

So despite the tough conditions Hamilton and his students endured in Yellowstone — snow and 30-degree nights — he looks forward to returning. “I like working on this project, because I can do all this neat research with my students,” he said. “But it’s really great to be able to work on a project that has relevance. This is something the Park Service itself wants done because bison aren’t allowed to leave the Yellowstone, and the pronghorn population is in decline. So even though restoring these grasslands has merit from a purely ecological standpoint, it needed this political pressure to get the funding. Otherwise it wouldn’t get done at all.”