Graduate Thesis Or Dissertation
 

Life History Trade-offs: The Effects of Habitat Selection on Columbian Black-tailed Deer Survival in Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/6d570431b

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  • The assessment of habitat quality for wild populations requires evaluation of vital rates associated with the use of that habitat. Factors associated with bottom-up (forage) or top-down (predation) regulation and the relative contribution of these processes on ungulate populations are difficult to quantify, especially for a cryptic, but widely distributed generalist species such as the Columbian black-tailed deer (Odocoileus hemionus columbianus; hereafter CBTD). In Oregon, CBTD populations have fluctuated over the past century, and survival rates were relatively low compared to estimates from other parts of this species range. Mechanisms driving these patterns remain understudied, but may be related to malnutrition, hunting mortality, or predator populations. The fall period in particular may impose unique challenges for CBTD due to increased mortality risk during hunting season, energetic demands related to breeding behavior and migration, and a seasonal decline in forage quality and/or quantity. However, an understanding of the relationship between CBTD survival and home range composition and habitat use patterns (i.e., habitat quality) during the fall is limited. To further our understanding of CBTD current survival rates and habitat quality in Oregon during the fall, I quantified the link between CBTD habitat use and survival using GPS radio-collar data collected by Oregon Department of Fish and Wildlife from adult female CBTD from 2012–2016 in the Coast and Cascade Ranges in western Oregon. To estimate fall habitat selection, I developed separate resource selection functions (RSFs) for female CBTD in the Coast Range (n = 78) and the Cascade Range (n = 71) at 2 spatial scales: home range selection within the landscape (second-order selection), and resource selection within home ranges (third-order selection). I used logistic regression to examine use of different land cover types (grouped by vegetation type and/or stand age), percent canopy cover, distance to road and forest edge, and land cover patch shape complexity relative to availability. I also estimated monthly, annual and seasonal survival rates for GPS radio-collared female CBTD in western Oregon (n = 165) from 2012 – 2016 using a known-fate framework as a function of mountain range, landownership, and environmental covariates. To measure fall home range habitat quality I used results from the RSF analysis to develop covariates I predicted would effect monthly survival for individual deer. Covariates included land cover diversity within home ranges, home range size, proportion of important land cover types identified from the RSFs within fall home ranges, and the average predicted relative probability of use of resources within individuals’ home ranges (Selectivity Index; SI). The SI is based on coefficients from third-order (SI3) and second-order (SI2) RSFs, and I predicted SI would be positively related to survival. The results of my RSFs were generally consistent with the generalist life history strategy of CBTD. Deer were more selective about home range establishment on the landscape, but behaved more opportunistically within home ranges. In addition, deer in both mountain ranges were less likely to use other land cover types relative to medium conifer at coarse and fine spatial scales. There were similar patterns in selection between the Coast and Cascade Ranges, and the strongest effects were selection against riparian areas and mature coniferous forest stands. At the landscape level, deer in both the Cascades and Coast Ranges selected land cover patches that were more “square-shaped” (i.e., low shape complexity), and areas closer to forest edges and roads. I also observed differences in home range size between mountain ranges, with the average fall home ranges in the Cascades (x ̅ = 1.99 km2) being roughly 4 times larger than in the Coast Range (x ̅ = 0. 57 km2). Adult female annual survival from 2012–2016 ranged from a low of 0.71 (SE = 0.07, 95% CI: 0.56 – 0.82) in 2014 to a high of 0.84 (SE = 0.08 = LCI= 0.61, UCI= 0.95) in 2012, which is generally higher than previous annual estimates (0.44 to 0.74) from the 1990s in southern Oregon, but similar to estimates from other regions. There was strong evidence indicating monthly survival was lower in the fall relative to spring, summer, and winter seasons. My results supported the effect of habitat selection operating at the home range scale, and included the negative effects of the amount of riparian cover, land cover diversity, and average predicted use of resources within the home range (SI3) on monthly survival of female deer during the fall. Thus, somewhat contrary to predictions, fall survival was higher for deer with home ranges with less riparian cover, lower land cover diversity, and with more resources that were predicted less likely to be used. These relationships could reflect instances where CBTD need to use certain habitats that may also subject them to higher mortality risk (i.e., land cover diversity, SI3). Such a situation could exist if CBTD are nutritionally limited and more willing to use habitat patches that are associated with both better forage and higher chances of encountering predators or hunters. Thus, the negative effects of land cover diversity, riparian habitats and SI3 may represent the challenges female CBTD face when making trade-offs between forage acquisition and mortality risk during the fall. This study represents the first to measure CBTD habitat quality using GPS satellite location data in Oregon by evaluating the effect of habitat selection on survival. My results support the hypothesis that fall represents a challenging part of the annual cycle for deer. However, future research should focus on understanding the mechanisms underlying fall habitat selection patterns and effects on apparent survival of CBTD, as well as evaluating habitat selection and survival for other periods of the annual cycle.
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  • Funding for the Black-tailed Deer in Western Oregon project, including my thesis, was provided by the United States Fish and Wildlife Service (USFWS) Pittman-Robertson Wildlife Restoration Act Grant F20AF11786. My Graduate Research Assistantship was funded by the Oregon Department of Fish and Wildlife (ODFW) and the Oregon Cooperative Wildlife Research Unit of Oregon State University (OSU).
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