ELK HAVE ECOSYSTEM LEVEL EFFECTS
BY: KELLEY M. STEWART (Assistant Professor, University of Nevada Reno) and R. Terry Bowyer (Professional Member Boone and Crockett Club)
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. 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, we studied the effects of different sizes of elk populations on their habitats and foods available. We also examined the corresponding changes in body condition and pregnancy rates of those elk. We created 2 populations of elk, one at very low density, about 10 elk/mi2 (24 elk in a 1500-acre study area), and a another at very high density, about 60 elk/mi2 (200 elk in a 2100-acre study area) We looked at how competition among elk affected body condition, pregnancy, and recruitment of calves into the population. We documented 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. There were negative feedbacks associated with competition for food, and corresponding effects on the population of animals.
Our next question was related to the effects of elk on the plants and the ecosystem they inhabited. We measured seasonal productivity of plants in each of 4 major habitats and in each study area. We also built large exclosures in each habitat type so we could measure plant responses in areas with no foraging by elk, low levels of foraging, and high levels of forging.
CAPTION: Cross fence between population densities of elk. The right side of the fence is the high-density population of elk (60 elk/mi2) and the left side of the fence is low-density population of elk (10 elk/mi2).
We observed that 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. This process is called herbivore optimization; which occurs 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; 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. Conversely, if we mow too often then the lawn cannot grow fast enough to recover and we wind up with dirt. We observed the same process at high numbers of elk--plant production was low because elk removed a lot more plant material and the plants could not recover.
We investigated the effect of elk herbivory on the number of plant species. We noted that in areas where productivity was higher, diversity of plant species also was higher. We did not see a direct effect of elk removal on plants, but where elk stimulated productivity of plants, plant diversity was greatest.
- In areas where there were not a lot of elk competing for food, elk were in better physical condition (i.e. had more fat) and had higher rates of pregnancy and recruitment of young into the breeding population. Although not part of this study, higher body condition of males leads to larger body and antler sizes, as described in Kevin Monteith’s Trophy Points article.
- 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.
- Are research results from these study enclosures 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 we documented from high elk populations are likely to occur.
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.
Trophy Points: Big Game Research On Line is complied and edited by David G. Hewitt, a Professional Member of the Boone and Crockett Club and the Stuart W. Stedman Chair for White-tailed Deer Research at the Caesar Kleberg Wildlife Research Institute.