Literaturdatenbank
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Nussear, K. E. (2004). Mechanistic investigation of the distributional limits of the desert tortoise gopherus agassizii. Unpublished thesis , University of Nevada, Reno.
Added by: Admin (06 Jan 2014 18:25:05 UTC) |
| Resource type: Thesis/Dissertation BibTeX citation key: anon2004n View all bibliographic details  |
Categories: General Keywords: Gopherus agassizii, Habitat - habitat, Nordamerika - North America, Schildkröten - turtles + tortoises, Testudinidae Creators: Nussear Publisher: University of Nevada (Reno) |
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| Abstract |
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Testudinidae Gopherus agassizii The severe climatic conditions and low resources associated with desert environments, result in selective pressures that potentially influence morphological, physiological, and behavioral adaptations in desert-dwelling animals, thus defining species distributions. Many factors act simultaneously to influence the natural distributions of tortoises. The factors limiting tortoise distribution may include: biogeographical and historical limitations to dispersal, the biophysical environment, local precipitation, and forage availability. Interactions among limiting factors will define the time and space constraints on habitat use. These constants define the realized niche, and help us to know the fraction of the available habitat that is actually useable. This understanding is critical to conservation of rare and endangered species. I investigated the mechanisms contributing to the distribution of desert tortoises by manipulating their distribution at the northeastern limits to the range, and measuring responses to different habitats and thermal environments. I measured critical temperatures during hibernation, and characteristics of the shade resources used during the active season. I also measured the production of eggs, and movements of animals displaced into atypical habitats to understand ecological constraints to distribution. Aspects of the thermal environment were addressed by experiments that sought to understand the mechanisms of heat exchange for tortoises, and by modeling tortoise activity as a function of the environmental variables. Tortoises at four study sites transecting the northeast Mojave desert had similar minimum body temperatures that were well above critical temperatures for this species. Tortoises that were translocated into Great Basin scrub/woodland habitats appeared to select shrubs for shade differently than shrub presence. Tortoises avoided the most abundant shrub (Artemisia spp.) as a resource for shade, and did not establish home ranges in Great basin scrub habitats. The use of shade in relation to summer thermal environment appears to be critically important to understanding the distribution of desert tortoises. Finally, I examined the mechanisms of heat exchange by manipulating carapace coloration and measuring responses of behavior, body temperatures, and heating rates. Shell coloration had little influence on the thermal biology of desert tortoises. That result that differed from the current paradigm typically used to predict the operative temperatures available to ectothermic vertebrates.
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