The Tortoise and the Landscape: Examining the Habitat, Daily Movement, Home Range Size, and Thermal Ecology of the Texas Tortoise (Gopherus berlandieri)

1. Towards and exclusionary perspective on habitat as a more effective way to understand distribution of generalist species in nature. The association between an organism and its habitat is one of the most basic components of ecological knowledge, particularly as it relates to where we find the species in nature. Habitat research is typically conducted from an inclusive perspective which aims to delineate the conditions that are suitable for organismal persistence (i.e., the realized niche). However, this approach may not always be appropriate for generalist species that lack strict habitat requirements. For those species, a better approach to understanding the species distribution in nature may be to focus on and identify conditions where long-term persistence is not possible for the species (i.e., outside the bounds of the fundamental niche). We refer to this approach as the exclusionary perspective on habitat in that analysis of the species-habitat relationship and practical application to map potential species distribution utilizes environmental variables that directly exclude the species rather than variables that could be considered habitat requirements. We demonstrate conceptual and operational aspects of the exclusionary perspective by examining the habitat ecology of the Texas tortoise (Gopherus berlandieri). Within its limited geographic range in southern Texas and northern Mexico, G. berlandieri occurs across a variety of environments (i.e., vegetation types) and environmental gradients, but prior research has been unable to identify a set of precise habitat requirements. Despite this, there are clearly areas where G. berlandieri cannot persist (e.g., urbanized land and agricultural land). Basic autecological research identifies potential habitat of a species, but it can also reveal where a species cannot occur (i.e., species is excluded due to local environmental conditions). The application of an exclusionary perspective can produce maps of potential habitat and thus possible locations of populations. We advocate for the incorporation of the exclusionary perspective into research and practical conservation of generalist species. 2. An application of exclusionary habitat perspectives to model potential Texas tortoise (Gopherus berlandieri) habitat in protected natural areas across south Texas, USA. Identifying potential habitat can be a vital early step to further research and conservation of a species which could be difficult for generalist species when no set of environmental variables is known to overwhelmingly determine the suitability of an area. Instead, it could be easier to identify conditions that are known to preclude species presence. One poorly understood generalist is the Texas Tortoise (Gopherus berlandieri), although conditions that are broadly intolerable to the species have been identified. I modeled potential G. berlandieri habitat using an exclusionary perspective within south Texas, focusing on protected natural areas. I also examined the amount of anthropogenic development (as classed by the NLCD) near protected areas to assess the encroachment of human activity on potential tortoise habitat. At least one-third of all potential G. berlandieri habitat is already protected, with the vast majority controlled by six entities. On average, a quarter of surrounding land near protected natural areas is developed, and anthropogenic features such as roadways could threaten G. berlandieri populations. Identifying potential habitat and threats to populations contained within can be accomplished using relatively few environmental variables when approached from an exclusionary perspective. 3. Novel use of path analysis to investigate the effects of weather on movement: an examination of a slow-moving ectotherm’s movement patterns. Background: The daily movement behavior of ectotherms, and thus long-term space-use may presumably be influenced by prevailing weather conditions. Common movement metrics such as step lengths and turning angles are not necessarily independent and could be influenced by common causes (weather conditions). Our study applies path analysis to organismal movement data for the first time to test whether movement of a slow-moving ectotherm (Texas tortoise, Gopherus berlandieri) is affected by daily weather conditions. Methods: We obtained spatially explicit GPS data for G. berlandieri individuals from three different locations in south Texas. We estimated utilization distributions (UDs) using autocorrelated kernel density estimates. Using a combination of path analysis and separate linear regression models we tested whether several weather variables affected the distance and directionality of daily tortoise movement and whether the movement behavior (step lengths and turning angles) affected the size and shape of UDs. Results: Weather had no significant or consistent direct effect on tortoise movement. However, increasing photoperiod (number of daylight hours) lead to increasing total displacement which in turn positively affected net displacement. UD size, determined over the entire study period and during seasonal periods of prolonged heat and cold, varied significantly among tortoises and was significantly affected by mean daily net displacement. Conclusions: While we did not detect significant effects of weather alone upon movement, we demonstrate that path analysis is an appropriate method with which to examine movement behavior. Our study also revealed that UD size and shape can often be explained primarily by movement metrics such as displacement and turning angles. There were significant seasonal differences in UD size and shape that, while not directly affected by the weather, are likely due to less movement during periods of hot and cold weather. 4. Analysis of the thermal ecology of the Texas tortoise (Gopherus berlandieri) with regard to potential anthropogenic alteration of its thermal environment. Ectothermic organisms must behaviorally thermoregulate to maintain preferred body temperatures. The invasion of monodominant grasses and the ongoing climate crisis have the potential to disrupt and substantially worsen the thermal environment ectotherms. To understand the relationship between the Texas tortoise (Gopherus berlandieri) and its thermal environment, we recorded external body temperatures of tortoises at a protected natural area in south Texas to estimate key thermal metrics (e.g., the Tpref range). We placed operative temperature models in both invasive Guinea grass and native vegetation to determine if the former created a substantially worse thermal environment for G. berlandieri. The determined Tpref range has a lower bound that is 5 °C below previous estimates for the species, although the upper bound is similar to those observed both for the species and the genus. The thermal environment was not altered by the invasion of Guinea grass. Measures of thermoregulatory accuracy, the quality of the thermal environment, and thermoregulatory efficiency are highest in hot or heating seasons. Gopherus berlandieri, while uniquely adapted to cold among its congenerics, encounters the lowest-quality thermal environment during the coldest portion of the year. Gopherus berlandieri in south Texas live in a somewhat ideal thermal environment and might not be eminently threatened by rising temperatures. However, future extreme weather events and climate shifts may substantially alter the relationship between tortoises and the thermal environment to different extents across the species’ range. Further examination of these relationships and the threats they may pose could play a crucial role in future conservation for the tortoise.

Author dc:creator

• Guerra, Daniel

Contributors dc:contributor

• Veech, Joseph
• Esque, Todd
• Arellano-Castro, Ivan
• Davis, Drew
• Fitzgerald, Lee
• Gabor, Caitlin R.

• Texas Tortoise
• habitat ecology
• movement ecology
• thermal ecology

Language dc:language

en

Identifier

Guerra, D. (2024). The tortoise and the landscape: Examining the habitat, daily movement, home range size, and thermal ecology of the Texas tortoise (Gopherus berlandieri) [Doctoral dissertation, Texas State University].

OAI identifier oai:identifier

oai:tdl-ir.tdl.org:10877/19962