In this post, Dustin Ranglack describes his recent paper with co-authors Susan Durham and Johan du Toit “Competition on the range: science vs. perception in a bison–cattle conflict in the western USA”
In the western USA, few wildlife species are as controversial as American bison (Bison bison). Bison seem to be one of the few wildlife species that aren’t allowed to be wild. They are primarily confined to a few small areas that are often fenced and lacking natural predators. Additionally, most wild bison herds are very small, leading to concerns over the genetic viability of the species. Indeed, it has been suggested that bison are ecologically extinct (Sanderson et al. 2008). One of the reasons for this confinement is due to concerns from the livestock industry that focus around the potential for disease transmission and competition for forage. As a result, bison and cattle are generally kept separate from each other.
One of the few places where bison and cattle comingle is the Henry Mountains of southern Utah, which is primarily public land. While the bison in this area are disease free, there is a very real concern over competition between the two species due to their similarity in diet and body size. The local ranching community perceives the bison as a real threat to their livelihood and the ranching tradition of the area, resulting in an intense conflict between the local ranching community and the state and federal wildlife and land management agencies. This concern is understandable, as the bison are a very conspicuous presence on the landscape due to their large body size and herding behavior.
For part of my PhD research at Utah State University, I set out to determine the relative impacts of bison and cattle on the forage resources in the area by using a series of grazing exclosures that would exclude large herbivores, paired with a grazed reference plot. However, upon our first field visit, we noticed a large number of rabbit fecal pellets in the area that had been identified by the local ranching community, the state and federal agencies as the area of greatest concern. These rabbits are much less conspicuous than bison, but based on the number of pellets we felt they may be having a significant impact on forage availability. Thus we decided to add an additional exclosure at each site that would exclude large herbivores AND rabbits, allowing us to determine the relative impacts of all three players in this system – bison, cattle, and rabbits – over the course of one year (October 2011–October 2012). Additionally, we surveyed the local ranching community to determine their perceptions of the bison–cattle conflict, thereby combining the social and ecological to gain a more complete understanding of the system.
All of the vegetation from both exclosure types and a grazed reference plot at each of 20 sites was clipped and separated into grass, forb, shrub, and cactus and then dried and weighed for analysis. To determine the large herbivore impact, we simply looked at the difference between the ‘partial’ exclosure (bison + cattle out) to the grazed reference plot. We then broke down the large herbivore portion based on the amount of animal days (# of individuals * # of days in the area) for each species, based on the rancher-reported cattle numbers and the GPS collared bison. The rabbit impact was determined by the difference between the ‘full’ exclosure (bison + cattle + rabbits out) and the ‘partial’ exclosure. From this, after controlling for site specific differences, we determined that rabbits were having a significant impact on the amount of forage available. When broken down by species, cattle were responsible for a little more than 52% of the consumed grass, rabbits a little more than 34%, and bison just less than 14%, contrary to the perceptions measured by our survey. This was shocking to all involved, including the ranching community, who quickly stated that the long standing predator control efforts in the area may have contributed to an unnaturally large rabbit population.
The management implications of this work are clear. If the objective is to increase forage availability, this may be more readily accomplished by reducing the rabbit population than by direct management actions on the bison in the area. The large rabbit impacts we found may also be indicative of a trophic cascade, as the rabbits’ main predator, the coyote, has been subject to intensive predator removal effort by government and private entities for many years. Thus, reducing or eliminating expensive predator removal efforts may be more effective in increasing and forage availability (and possibly reducing the longstanding bison–cattle conflict) than by direct management of the bison population. This also highlights the need for science in making management decisions, as the perceptions of the local ranching community were shown to be overstated, indicating that even long term resource users may underestimate the complexities of trophic interactions.
Sanderson, E. W., et al. 2008. The ecological future of the North American bison: Conceiving long-term, large-scale conservation of wildlife. Conservation Biology 22:252-266.