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What role do elk play in our ecosystem?

Big game animals have more complex interactions with their environments than many of us realize and large mammals, such as elk, have far reaching effects on ecosystems. Both quality and quantity of food are important for sustaining populations of game animals. One can imagine that as elk populations increase, the amount of forage removed also increases, which affects plant growth and diversity.

On the Starkey Experimental Forest and Range in northeastern Oregon, University of Nevada, Reno scientist Kelley Stewart and her team studied the effects of different sizes of elk populations on their habitats and foods available. They also examined the corresponding changes in body condition and pregnancy rates of those elk.

The team created two populations of elk, one at very low density, about 10 elk/miles2(24 elk in a 1500-acre study area), and another at very high density, about 60 elk/miles2(200 elk in a 2100-acre study area).

By looking at how competition among elk affected body condition, pregnancy, and recruitment of calves into the population. The team was able to document that the proportion of females that became pregnant and their body condition in the low- density population were much higher than in the high-density population. In other words, there were negative feedbacks associated with competition for food, and corresponding effects on the population of animals.

The team’s next question was how do elk effect plants and the ecosystem they inhabited. By building large exclosures in each habitat type, measurements were collected on how plants responded in areas with no foraging by elk, low levels of foraging, and high levels of forging.

As one might conclude, areas with low levels of foraging by elk had greater plant growth than areas where no foraging by elk occurred and greater growth than areas with high elk foraging. Dr. Stewart stated that this process is called “herbivore optimization”. A scientific term used to define when low levels of foraging stimulate plants to grow more than in areas where no foraging occurs or where high levels of forage are removed.

“This process is similar to mowing your lawn” said Dr. Kelley. “If you do not mow, the plants grow, create seedheads, and then often turn brown and die back”.

“If you mow every week or two the plants keep growing because they do not produce seedheads, and instead continue growing vegetative parts. In this way we keep our lawns green for a much longer period.”

Dr. Stewart also pointed out that “Conversely, if we mow too often then the lawn cannot grow fast enough to recover and we wind up with dirt.”

This exact same process was found at high numbers of elk--plant production was low because elk removed a lot more plant material and the plants could not recover.

The team’s investigation on the effect of elk herbivory on the number of plant species found that in areas where productivity was higher, diversity of plant species also was higher. Dr. Stewart said, “We did not see a direct effect of elk removal on plants, but where elk stimulated productivity of plants, plant diversity was greatest.”

So what does this mean?

  • In areas where there were not a lot of elk competing for food, elk were in better physical condition (that is, had more fat) and had higher rates of pregnancy and recruitment of young into the breeding population.
  • Low densities of elk made the plants more productive and produced a greater diversity of plants. Thus, properly managed elk populations can help land managers promote biodiversity and ecosystem functioning.
  • High densities of elk negatively affect the elk population itself, because lower plant productivity and quality of forage results in lower physical condition, lower pregnancy rates, and fewer calves entering the breeding population.   Those effects would result in fewer trophy-sized bulls in the population.

So one might ask, are these results representative of wild elk populations that move over greater areas?

With increasing human impacts on elk populations, with people expanding into wildlife habitat, and with roads and other changes on the landscape limiting elk movements, it is not difficult for elk populations to increase to the habitat’s carrying capacity, in which case the effects Dr. Stewart and her team documented from high elk populations are likely to occur.


This project was supported by the Institute of Arctic Biology and the Department of Biology and Wildlife at the University of Alaska Fairbanks, The Starkey Project, Pacific Northwest Research Station, USDA Forest Service, Department of Biological Sciences at Idaho State University, and the Department of Natural Resources and Environmental Science at University of Nevada Reno. Additional funding was provided by The Rob and Bessie Welder Wildlife Foundation and the Rocky Mountain Elk Foundation. We appreciate the assistance of John Kie, Brian Dick, Roger Ruess, and Starkey Project personnel, especially those that aided in data collection.

The study on which this edition of Trophy Points is based is described in:

K. M. Stewart, R. T. Bowyer, J. G. Kie, B. L. Dick, R. W. Ruess. 2009. Population density of North American elk: effects on plant diversity. Oecologia 161:303-312.

K. M. Stewart, R. T. Bowyer, R. W. Ruess, B. L. Dick, J. G. Kie. 2006. Herbivore optimization by North American Elk: consequences for theory and management. Wildlife Monographs 167: 1-24.

K. M. Stewart, R. T. Bowyer, B. L. Dick, B. K. Johnson, J. G. Kie. 2005. Density-dependent effects on physical condition and reproduction in North American elk: an experimental test. Oecologia 143:85-93.

Story by: Kelley Stewart, Assistant Professor, University of Nevada, Reno