Date of Award
5-1-2013
Degree Name
Doctor of Philosophy
Department
Zoology
First Advisor
Nielsen, Clayton
Second Advisor
Schauber, Eric
Abstract
Ecological communities are most commonly structured by a mixture of bottom-up processes such as habitat or prey, competition within the same trophic level, and top-down forces from higher trophic levels. Carnivore guilds play a vital role in the broader ecological community by stabilizing or destabilizing food webs. Consequently, factors influencing the structure of carnivore guilds can be critical to patterns in ecosystems. Coyotes (Canis latrans), bobcats (Lynx rufus), gray foxes (Urocyon cinereoargenteus), raccoons (Procyon lotor), red foxes (Vulpes vulpes), and striped skunks (Mephitis mephitis) occur sympatrically throughout much of their geographic ranges in North America and overlap in resource use, indicating potential for interspecific interactions. Although much is known about space use, habitat relationships, and activity patterns of the individual species separately, little is known about factors that facilitate coexistence and how interactions within this guild influence distribution, activity, and survival of the smaller carnivores. For example, gray fox populations appear to have declined in Illinois since the early 1990s and it is unknown if the increase in bobcat and coyote populations during the same time period is the cause. I conducted a large-scale non-invasive carnivore survey using an occupancy modeling framework to quantify factors affecting the structure of this widely-occurring carnivore guild. I used baited remote cameras during 3-week surveys to detect carnivores at 1,118 camera-points in 357 2.6-km2 sections (clusters of 3-4 cameras/section) in the 16 southernmost counties of Illinois (16,058 km2) during January-April, 2008-2010. I collected microhabitat data at each camera-point and landscape-level habitat data for each camera-cluster. In a multi-stage approach, I used information-theoretic methods to develop and evaluate models for detection, species-specific habitat occupancy, multi-species co-occupancy, and multi-season (colonization and extinction) occupancy dynamics. I developed hypotheses for each species regarding the occupancy of areas based on anthropogenic features, prey availability, landscape complexity, and vegetative landcover. I used photographic data, Poisson regression, and mixed-model logistic regression to quantify temporal activity of carnivores in the study area and how interspecific factors influence temporal patterns of activity. Of the 102,711 photographs of endothermic animals I recorded photographs of bobcats (n = 412 photographs), coyotes (n = 1,397), gray foxes (n = 546), raccoons (n = 40,029), red foxes (n = 149) and striped skunks (n = 2,467). Bobcats were active primarily during crepuscular periods, and their activity was reduced with precipitation and higher temperatures. The probability of detecting bobcats at a camera point decreased after a bobcat photograph was recorded, suggesting avoidance of remote cameras. Across southern Illinois, bobcat occupancy at the camera-point and camera-cluster scale (point = 0.24 ± 0.04, cluster = 0.75 ± 0.06) was negatively influenced by anthropogenic features and infrastructure. Bobcats had high rates of colonization (0.86) and low rates of extinction (0.07) during the study, suggesting an expanding population, but agricultural land was less likely to be colonized. The number of coyote photographs decreased with increased temperature, but increased with previous coyote photographs, suggesting an attraction to bait in cold weather. Nearly all camera clusters were occupied by coyotes (cluster = 0.95 ± 0.03) during the entire study. At the camera-point scale, coyote occupancy (overall point = 0.58 ± 0.03) was higher in hardwood forest stands with open understories than in other habitats. Similar to coyotes, gray foxes were more likely to be photographed in cold weather and after a previous detection had occurred. However, gray fox occupancy was much lower (point = 0.13 ± 0.01, cluster = 0.29 ± 0.03) at all scales. At the camera-cluster scale, with a buffer-area size that represented 20% of the estimated home-range size of gray foxes, the species selected spatially-complex areas with high proportions of forest, and low proportions of grassland and agriculture land cover. Gray fox occupancy of camera clusters was positively related to anthropogenic features within 100% estimated home-range buffers. Collectively, the results suggest gray fox occupancy was greatest near, but not in, anthropogenic developments. Red foxes occupied a similar proportion of the study area as gray foxes (point = 0.12 ± 0.02, cluster = 0.26 ± 0.04), but were more closely associated with anthropogenic features. Indeed, at all three scales of red fox occupancy analysis, anthropogenic feature models received more support than other hypotheses. Camera-cluster extinction probabilities were higher for both gray foxes (0.57) and red foxes (0.35) than their colonization rates (gray fox = 0.16, red fox = 0.06), suggesting both species may be declining in southern Illinois. I recorded more striped skunk photographs in January and February (i.e., during the breeding period) than in March and April. Striped skunks occupied a large portion of the study area (point = 0.47 ± 0.01, cluster = 0.79 ± 0.03) and were associated primarily with anthropogenic features, especially if the features were surrounded by agricultural land and not forest. Raccoons were essentially ubiquitous within the study area, being photographed in 99% of camera clusters. In some instances, the presence of other carnivores appeared to be an important factor in the occupancy of the 4 smaller species, but in general, habitat models were more supported than co-occurrence models. Habitat had a stronger influence on the occupancy of gray foxes and red foxes than did the presence of bobcats. However, the level of red fox activity, represented by the number of photographs recorded in a camera cluster, was negatively correlated with bobcat activity. Gray fox occupancy and level of activity were reduced in camera-clusters occupied by coyotes, but were not related to bobcat occupancy. When not considering the presence of coyotes, gray foxes appeared to use camera points with fewer hardwood and more conifer trees, which was counter to previous findings. However, when adding the effect of coyote presence, gray fox point models indicated a positive relationship with hardwood stands. Therefore, gray foxes were more likely to occupy camera points in hardwood stands than conifer stands if coyotes were also present; suggesting that hardwood stands may enhance gray fox-coyote coexistence. The 2 fox species appeared to co-occur with each other at the camera-point scale more frequently than expected on the basis of their individual selection of habitat. Similarly, camera-point occupancy of red foxes was higher when coyotes were present. These apparent canid associations may be a response to locally-high prey abundance or an unmeasured habitat variable. Activity levels of raccoons, bobcats, and coyotes were all positively correlated. Collectively, my results suggest that although gray foxes and red foxes currently coexist with bobcats and coyotes, the foxes have reduced activity in the areas occupied by larger carnivores, especially when bobcats and coyotes are highly active. Further, hardwood stands may contain trees with structure that enhances tree-climbing by gray foxes, a behavior that probably facilitates coexistence with coyotes. Therefore, efforts to manage gray foxes should focus on maintaining and increasing the amount of mature oak-hickory forest, which presumably provides a suitable prey base and refugia from intraguild predation. Additionally, the varying results from different scales of analyses underscore the importance of considering multiple spatial scales in carnivore community studies.
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