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Carnivore Competition: Spatial and Dietary Implications of Gray Wolf Recolonization for Cougars in Northeast Oregon Public Deposited

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  • After a 40-year absence from Oregon’s landscape, expanding gray wolf (Canis lupus) populations are reestablishing elements of interspecific competition with sympatric large carnivores, like cougars (Puma concolor). This presents new challenges for management of large carnivores and their ungulate prey populations (e.g., elk, Cervus canadensis nelsoni; mule deer, Odocoileus hemionus) in these re-established multi-carnivore systems. Wolf range expansion and interactions with populations of sympatric cougars could alter predation on deer and elk populations in the Pacific Northwest. Competition could also affect the spatial distribution, demography, and population dynamics of cougars, the assumed subordinate predator in wolf-cougar interactions. However, the strength of competitive interactions dictate the trajectory of top-down effects and can be system specific. Coupled with a paucity of empirical data on cougar diets and space use across landscapes with and without wolves, prediction of subsequent effects to prey populations is challenging. Furthermore, the common assumption of additive predation effects when a missing predator is added back to an ecosystem may not be well-founded because elk populations have increased in some parts of the Pacific Northwest. My primary research objectives were to 1) estimate diet composition and kill rates for wolves in northeast Oregon, 2) estimate diet composition and kill rates for cougars and evaluate changes in cougar predation patterns across time periods with and without wolves, 3) evaluate cougar home range and kill site distribution for changes relative to pre-wolf patterns, and 4) investigate the influence of wolf presence on cougar movement patterns and habitat use in northeast Oregon. In addition to addressing key ecological questions about carnivore interactions, results from my research provide information on implications of expanded predator systems for elk and mule deer populations, and will be useful to other states and Canadian provinces in western North America facing similar changes in predator guild composition and community dynamics as wolves continue to expand their range. I implemented a 3-year study in northeast Oregon to investigate diet, kill rates, and space use of wolves and cougars in a recently re-established multi-predator, multi-prey system to better understand wolf-cougar interactions and the mechanisms by which competition may influence cougar populations. A study of cougar populations in northeast Oregon prior to wolf recolonization provided 3 years of pre-wolf cougar information (2009-2012) for comparison with similar information on cougars after wolf recolonization (2014-2016). During my research, 34 wolves and 15 adult cougars were captured and fit with Global Positioning System (GPS) collars to compare predation and space use patterns with 25 GPS-collared cougars prior to wolf recolonization. I documented predation patterns (diet composition, kill rate) for 11 wolves sympatric to GPS-collared cougars in northeast Oregon. I identified 159 predation events over winter (44 wolf-weeks) and summer (8 wolf-weeks) periods for wolves. Annually, wolf packs killed an average of 2.23 ungulates/wk (90% CI = 1.62 – 2.85). There was no evidence kill rates varied by pack (P = 0.92), but rates did vary by season (P = 0.005), with summer kill rates (x ̅= 3.46 ungulates/wk, SE = 0.52) 2.26 times higher than winter kill rates (x ̅ = 1.53 ungulates/wk, SE = 0.26), consistent with increased availability of smaller neonate prey on the landscape and summer diets primarily consisting of elk calves. Biomass intake rates did not vary by pack (P = 0.80) or season (summer = 243 kg/wk, SE = 56; winter = 182 kg/wk, P = 0.24). My results quantified kill rates for wolves in northeast Oregon, and I documented that wolf predation patterns were consistent with other elk dominant systems in North America. I compared cougar predation patterns (diet composition, kill rate, search and handling time) across periods before (2009-2012) and after (2014-2016) wolf recolonization in northeast Oregon and identified 542 predation events for cougars in the post-wolf period to compare with 1,213 cougar predation events documented before wolf recolonization. Cougar predation was different between study periods, with changes most evident in predation patterns of female cougars. Female cougars had lower proportions of mule deer in their summer diets (12%), lower summer kill rates (17% – 50%), lower biomass intake rates (14% – 61%), and longer search times (30% – 138%) than respective female cougar reproductive groups in the previous cougar study. My results quantified the dietary costs of interspecific competition for cougars in northeast Oregon. I used locations for wolves and cougars from GPS collar data to evaluate spatial overlap and shifts in home range (HR) and kill site distribution based on kernel density estimates (KDE) generated for 1,213 cougar predation sites before (2009-2012) and 481 sites after (2014-2016) wolves recolonized northeast Oregon. I compared seasonal post-wolf cougar 50% and 95% predation ranges for overlap with density estimates from 107 wolf predation sites (2014 – 2015). I found that the distribution of cougar predation sites differed between time periods with and without wolves and relative to areas wolves frequently made kills. Based on total predation site distributions, 19% of core summer range where cougars made kills overlapped core wolf prey use areas. Over annual periods, the amount of overlap between post-wolf cougars and wolves decreased 9% at the 50% predation range scale and increased 6% at the 95% predation range scale. Cougars were responding to the presence of wolves and our results suggest the spatial scale of that response occurs within a cougars’ home range. Relative to pre-wolf cougar predation sites, post-wolf cougar predation sites were higher in elevation and closer to water. Coupled with little change in cougar diet composition, distributional shifts in areas cougar frequently kill prey could signal relatively unchanged effects to prey populations in this multi-predator system. My results quantified spatial costs of interspecific competition (with wolves) for cougar populations in northeast Oregon. I evaluated activity patterns (distance traveled, rates of travel, time of day) for cougars before (2009-2012) and for wolves and cougars after (2014-2018) wolves recolonized northeastern Oregon. I compared movement rates (km/hr) based on 42,892 and 48,723 GPS relocations of cougars from pre- and post-wolf periods, respectively. I also compared diel patterns of cougars with patterns generated from 79,437 GPS relocations of northeast Oregon wolves. Cougar movement and diel activity differed between time periods. Cougars moved shorter distances per 3-hr time step with wolves on the landscape (x ̅pre = 0.60 km, 90%CI 0.49 – 0.70; x ̅post = 0.43 km 90%CI 0.38 – 0.47, %P = 0.34). I observed a change in the pattern of cougar activity over the diel cycle from peak movement rates in the evening followed by night > day > morning, peak rates of activity in the evening > morning > night > day. I used step-selection functions (SSFs) and a two-stage approach to incorporate movement and evaluate individual and population level selection for three variables associated with spatial risk of wolf interaction (openness, wolf kill density, wolf intensity of use). I found that female cougars were selecting for less open habitats in winter after wolf recolonization (%P = 0). By examining individual selection I was able to demonstrate different competition mitigation strategies between male and female cougars. My results demonstrated cougars changed aspects of their activity and exemplify an optimal foraging strategy to balance fitness costs of competition. Wolf recolonization simultaneously introduces elements of interspecific competition between wolves and other predators, including cougars. My results demonstrated the capacity for interspecific interactions to counter-intuitively alter top-down effects by reducing ungulate kill rates, and alter movement rates, daily activity patterns, and habitat selection of cougars. Overlooking interspecific competition between predators could result in underestimating costs to subordinate competitor species and overestimating effects of multiple predators on prey populations.
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  • Orning, E.K. 2019. Carnivore Competition: Spatial and Dietary Implications of Gray Wolf Recolonization for Cougars in Northeast Oregon. Dissertation, Oregon State University, Corvallis, OR
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