Ecological data on intact predator systems in Oregon is lacking. Gray wolf recolonization in the state raises questions regarding interspecific competition between wolves and cougar and the effects of multiple predators on ungulate populations (elk and mule deer). Wolf populations have expanded in distribution following reintroductions in central Idaho and Yellowstone National Park in 1995-1996, with consistent wolf sign observed in northeast Oregon since 2006, and the first breeding pair documented in 2009. Since near extirpation in the 1960’s, Oregon cougar populations have recovered to stable population levels widely dispersed throughout the state. With the presence of two top predators, the Blue Mountain area of northeast Oregon is ideally suited to examine interactions between wolves and cougar, changes in predation patterns, and multi-predator effects on ungulate populations as a second predator (wolves) recolonizes an area formerly occupied solely by cougars.
In recent years, many elk and mule deer populations throughout the western United States, including northeast Oregon, have declined. Specifically, the elk population in the Mt. Emily Wildlife Management Unit (WMU) had declined to approximately 50% of the management objective established by the Oregon Department of Fish and Wildlife (ODFW). Declining elk calf recruitment, the number of calves per 100 cow elk observed during end of winter population surveys, was thought to be the primary factor leading to overall elk population declines.
In response to declining calf recruitment, ODFW conducted a study in northeast Oregon investigating the role of nutrition and predation on elk calf survival. This study provided evidence that the body condition of cow elk was not a major determinant in the survival of elk calves. Recruitment levels were low even though cow elk pregnancy rates and body condition scores in northeast Oregon were some of the highest in the state. Thus, nutrition was likely not the driving factor behind the low elk calf recruitment documented in northeast Oregon. The elk calf survival study observed that elk calf recruitment was negatively correlated with an index of cougar density, that cougars were the major source of elk calf mortality, and demonstrated that cougars played an important role in determining elk calf survival.
Subsequently, ODFW initiated a cougar predation study in the Mt. Emily WMU to investigate the impacts of cougar predation on elk population dynamics. The results of this study strongly supported the hypothesis that cougar predation was influenced by season, gender, and cougar reproductive status. Cougars killed ungulates more frequently during summer than winter, in part because juvenile ungulates comprised most of the diet and were smaller than prey killed in winter. Female cougars selected elk calves in the summer and deer fawns in the winter, whereas male cougars selected for elk calves in both seasons as well as yearling elk during summer. The observed selection of juvenile classes of elk and deer suggested the possible mechanism by which cougars negatively impact ungulate population growth. To test this, a population model using vital rates for elk in northeast Oregon was developed to investigate the factors influencing elk population growth rates. Results provided a novel finding that cougars can be a strong limiting factor on elk populations in northeast Oregon. However, the ecosystem in northeast Oregon has now changed to include a second top predator, wolves.
At the time this project was initiated (2013), wolves were recolonizing northeast Oregon, with 3 wolf packs known to use areas in the Mt. Emily WMU and a minimum count of 64 wolves (8 packs) throughout northeast Oregon (ODFW 2014 Annual Wolf Report). Wolf-cougar interactions have been studied in Yellowstone, Montana, and parts of Canada. However, the existence of data on cougar populations and predation patterns prior to wolf recolonization provided a unique opportunity to conduct a natural experiment that would allow us to identify and compare changes in cougar ecology and population demographics as wolves increased their distribution in the state of Oregon. Because cougar predation on juvenile elk was identified as a primary limiting factor for elk recruitment, and high cougar densities can negatively impact elk populations in Oregon, wolf-cougar interactions have the potential to be important in the community dynamics for wolf, cougar, elk, and deer species in northeast Oregon. The recolonization of a second predator to this system could increase predation on the juvenile elk class, or, alternatively, change cougar behavior and population dynamics to shift predation to another elk age class or ungulate species (e.g., mule deer).
However, the effects of wolf-cougar interactions and the role of wolf predation is still not well understood in an ecosystem with multiple predator and multiple prey species and can vary greatly from system to system. Therefore, several questions need to be addressed including:
1. What are the predation rates of wolves in northeast Oregon?
2. Do wolves in northeast Oregon utilize prey in proportion to their availability or select particular prey species?
3. Has cougar abundance, density, prey selection and space use changed with the recolonization of wolves? If so, how have these dynamics changed and to what degree could changes affect the community dynamics in northeast Oregon?
To answer these questions, we need to obtain accurate and unbiased estimates of wolf predation rates, prey selection, and resource selection. Currently, the best way to obtain these estimates is through the use of GPS collars. GPS collars have made it possible to obtain large, continuous, and accurate location data on animals with much less expense and effort than conventional radio-telemetry studies. Recently, techniques have been developed to identify wolf predation rates and prey selection across all seasons through the use of GPS collars. We are placing GPS and VHF collars on a sample of wolves and cougars to estimate the predation rates, prey selection, and habitat use of both carnivores in the Mt. Emily WMU. These estimates will provide information on how expanding wolf populations affect cougar populations and how predation risk for elk and mule deer populations may be affected with the return of a second top predator to northeast Oregon.