Master of Science
Bruce A. Kingsbury
Date of Award
Conservation management for any wildlife species relies on an understanding of habitat use and spatial patterns with an increasing need to identify the anthropogenic factors affecting populations (Sanderson et al., 2002). In order to understand those anthropogenic factors, it is generally necessary to first gain an understanding of the basic resource use patterns exhibited by the population(s) of interest. The Northern Copperhead (Agkistrodon contortrix mokasen) is a medium sized North American pitviper (Family Crotalidae) occurring throughout most of the eastern United States. Populations are thought to be stable throughout the majority of its range, but it holds protected status in three states and appears to be experiencing localized declines in some portions of southern Indiana. Copperheads are facing many of the same stressors causing global declines in other reptiles, including habitat loss and degradation, intentional killing and harvesting, as well as the introduction and proliferation of exotic invasive species (Gibbons et al., 2000). However, particular causes for declines in the Midwest are currently unknown given that the Northern Copperhead is a relatively understudied species. A limited number of studies pertaining to habitat use by copperheads of any subspecies exist (but see: Fitch, 1960, Reinhert, 1984, Smith et al., 2009), and no radiotelemetric studies have ever investigated habitat use by copperheads in the Midwest.
From June 2008 - November 2011, I radiotracked 22 copperheads at Clifty Falls State Park in Jefferson County, Indiana and nine copperheads at Clark State Forest at the border of Clark, Scott, and Washington County, Indiana. I present data herein on habitat use by each of these populations as well as some of the anthropogenic factors affecting the Clifty Falls State Park population, which appears to be experiencing rapid decline. I place an emphasis on the consequences of exotic plant invasions at Clifty Falls State Park and the implications and efficacy of mitigation techniques.
In Chapter One, I present data on general habitat use by copperheads at Clifty Falls State Park and Clark State Forest, which represent two geographically separated populations utilizing distinct habitat types. These habitats include oak-hickory ridges across the western portion of the Indiana copperhead range (e.g., Clark State Forest) and rocky gorges and canyons (e.g., Clifty Falls State Park) in the eastern portion where populations appear to be experiencing the greatest declines. Copperheads at Clark State Forest exhibited preference for forest macrohabitats while snakes at Clifty Falls State Park preferred a wider variety of habitats and appeared to exhibit avoidance of forest macrohabitats. A closer investigation into particular forest types at Clifty Falls State Park, however, revealed that each population exhibits preference for dry-upland forest and uses similar habitat when considering availability. Regression models also indicate few differences in microhabitat selection by each population, with snakes at both sites selecting habitats with lower canopy closure, higher leaf litter depth, and in closer proximity to native shrubs compared to random locations. Overall, the results of this study indicate the importance of multi-scale habitat use studies when attempting to determine important resource selection parameters for wildlife.
In Chapter Two, I demonstrate the influence of exotic plants on resource selection as well as the thermal limitations imposed by particular exotic plant species on copperheads at Clifty Falls State Park. Copperheads exhibited clear avoidance of most exotic plant species at multiple spatial scales, with exotic shrubs having the greatest influence on copperhead habitat selection. Avoidance of most exotic plants appears to be at least partially attributable to limited thermoregulatory opportunities within exotic-dominated habitats relative to native habitats, with exotic shrub habitats providing the lowest thermal quality as a group. Additional mechanisms underlying avoidance may include lack of suitable cover and/or decreased prey availability, but their significance is currently unclear. Careful planning and implementation of habitat restoration efforts in invaded habitats should benefit ectotherms in general. As exotic shrubs appear to exhibit the greatest influence on habitat use as well as the greatest impact on thermoregulation, management efforts for snakes and other ectotherms may receive the greatest return by targeting exotic shrubs when multiple species/types of invasive plants are present, although the current and potential risk that several non-shrub species present may be of equal or greater significance for different species or separate populations.
In Chapter Three, I address nonrandom use of artificial forest (recreational) gaps by copperheads at Clifty Falls State Park by simulating closure (to the public) of artificial gaps using ArcGIS. Results from this study demonstrate that by restricting human access to artificial forest gaps, encounters with Northern Copperheads could be reduced by 1.5 - 10 x the expected potential. I discuss results in terms of management implications and provide suggestions for land managers facing related concerns of human-wildlife encounters.
Finally, in Chapter Four I provide direct evidence of the impacts of property management and restoration activities copperheads in southern Indiana, showing that several managed habitats can and do attract copperheads and simultaneously place them at greater risk of injury and/or mortality. At the same time, however, management activities create or maintain forest gaps, providing thermoregulatory opportunities in an otherwise low quality landscape. I discuss my results in terms of the ecological trap concept and provide management recommendations that should be applicable to additional Northern Copperhead populations as well as to other forms of wildlife.
Overall, copperheads in southern Indiana will benefit from control of exotic invasive plants that overtake basking areas, gestation sites, foraging sites, and hibernacula. Large-scale mechanical means of exotic vegetation removal, while effective in eliciting positive responses by copperheads and providing thermally advantageous habitat, should be avoided during periods of time when copperheads and other forms of wildlife are likely to be present. Baseline habitat use data in conjunction with known anthropogenic influences suggests that smaller-scale, 'softer,' habitat restoration techniques will be most beneficial to copperheads when they occur in former glades and glade-like areas, grasslands, and thinner-canopy upland (oak-hickory) forest away from public access. These habitats provide important gestation sites and foraging habitats that appear to be at relatively high risk of exotic plant invasions and other anthropogenic impacts.
Evin T. Carter (2012).
Impacts of Invasive Plants on Resource Selection and Thermoregulation by the Northern Copperhead (Agkistrodon contortrix mokasen).
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