Every summer, thousands of elk and bison roam through the open hills and valleys of Yellowstone National Park, gorging themselves on lush grasses and wildflowers. While humans have long known the park’s wildlife herds depend on its grasslands, a new study has discovered that the grasses themselves also benefit from the relationship.
The 26-year study, among the longest that accounts for the amount of grass eaten by grazers, found that bison, elk and pronghorn grazing increases the annual growth rate of grass in the park by 20 percent. In the process, they unearthed evidence that the more heavily a patch of land was grazed, the more its grass grew. This stimulating effect herbivores have was especially pronounced during droughts, suggesting that Yellowstone’s ungulate herds might help its grasslands stay green even as climate change makes summers hotter and drier.
These findings contribute to our understanding of the relationship between large herbivores and their food. Previous research from the Mongolian Steppe, the Serengeti and other great grasslands has found that wild herbivores can enhance or hinder the growth of grasses depending on environmental conditions.
“If you graze it at the right time of year or add the right amount of grazing, that you’re actually going to have more productivity.”
Truman Young, restoration ecologist, UC-Davis
“When animals graze plants, there’s a whole suite of responses by the plants and the microbes in the soil,” said the study’s lead author Douglas Frank, a plant ecologist at Syracuse University in New York. This allows herbivores to increase not only the amount of available food, he added, but the quality of it as well.
Frank and his colleagues set up 25 moving barricades in the park to measure grass growth and grazing. They moved the barricades every 28 days and measured grass growth inside them to quantify plant growth in grazed areas. The group then took the same measurements just outside the barricades to see how much plant matter had been eaten by hungry herbivores. Measuring plant growth inside of 13 permanent fences, the researchers could see how grasses grew in the absence of grazing.
They then conducted a statistical analysis of the plant data that included soil nutrients, average summer temperature and average soil moisture to look for predictors of grass growth.
The intensity of grazing, measured through the difference in plant matter inside and outside of the moving barricades, emerged as the most important predictor of grass growth. Although grazing increased plant growth an average of 20 percent across years and study areas, its impact was even greater during drier years and under heavy grazing. Within the permanent fences, soil moisture enhanced plant growth, but only in warmer areas. Hungry bison, elk and pronghorn have a greater impact on plant growth in Yellowstone’s grasslands than yearly differences in climate, the researchers concluded.
“Grasses respond to being eaten by making more grass,” said Truman Young, restoration ecologist at the University of California-Davis. “It’s not necessarily surprising that if you graze it at the right time of year or add the right amount of grazing, that you’re actually going to have more productivity.”
The relationship between the amount of grazing and the increase in growth was linear — the more bison, elk and pronghorn ate, the more grass grew. This means that Yellowstone could probably hold more wildlife than it does today, according to Young, but it would come at the expense of other plants. “Hypothetically, we could just pack Yellowstone full of elk and bison,” he said. “They would just eat everything and what would be left would be the grass that’s very tolerant of grazing.”
In addition, a smaller-scale study published by Frank in 2016 found that as grazing pressure climbs, grass growth in Yellowstone tends to plateau or dip, although it still outpaces the growth seen in uneaten prairies. He suggested this reflects the true small‑scale relationship between herbivory and grass stimulation. The newer study produced a linear trend because it pooled data from many years and sites, he said.
The positive impact of grazing during droughts surprised Young, who has been researching the relationship between herbivores and grasses in Africa for decades. A 2013 study he worked on found that when cattle grazed alongside wild zebras in Kenyan grasslands, they gained weight faster during the wet season. During the dry season, cows that shared their pastures grew more slowly. Herbivores compete for limited food during dry times. When water is abundant, grazing stimulates fresh new growth, the authors of the 2013 study concluded.
In Yellowstone, wetter areas did not end up with less plant growth, Young said. Grazing just stimulated them less.
Herbivores might help plants save water because plants lose moisture through their leaves, Frank said. This makes grazing a buffer against droughts, which are growing more frequent as Yellowstone heats up, he added. But grass growth and its response to grazing decreased during the hottest years of the study, suggesting that warming itself could reduce the productivity of Yellowstone’s grasslands. Their response to future conditions, however, is difficult to predict, he said.
Many different variables can change how these ecosystems respond to their environment, Young concurred. “Contingency is the norm,” he said. “Every year is different, every site is different, every situation is different. And you’re looking for the generalities among all those peculiarities, which is one of the great challenges of ecology.”
