Literaturdatenbank
Literaturdatenbank |
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Tracy, R. C., Nussear, K. E., Esque, T. C., Dean-Bradley, K., Tracy, C. R., & DeFalco, L. A., et al. (2006). The importance of physiological ecology in conservation biology. Integrative and Comparative Biology, 46(6), 1191–1205.
Added by: Admin (14 Aug 2008 20:34:36 UTC) |
| Resource type: Journal Article BibTeX citation key: Tracy2006a View all bibliographic details  |
Categories: General Keywords: Fortpflanzung = reproduction, Gopherus, Gopherus agassizii, Habitat = habitat, Nordamerika = North America, Physiologie = physiology, Schildkröten = turtles + tortoises, Testudinidae Creators: Barber, Castle, Dean-Bradley, DeFalco, Espinoza, Esque, Nussear, Tracy, Tracy, Zimmerman Collection: Integrative and Comparative Biology |
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| Abstract |
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Testudinidae Gopherus agassizii Many of the threats to the persistence of populations of sensitive species have physiological or pathological mechanisms, and those mechanisms are best understood through the inherently integrative discipline of physiological ecology. The desert tortoise was listed under the Endangered Species Act largely due to a newly recognized upper respiratory disease thought to cause mortality in individuals and severe declines in populations. Numerous hypotheses about the threats to the persistence of desert tortoise populations involve acquisition of nutrients, and its connection to stress and disease. The nutritional wisdom hypothesis posits that animals should forage not for particular food items, but instead, for particular nutrients such as calcium and phosphorus used in building bones. The optimal foraging hypothesis suggests that, in circumstances of resource abundance, tortoises should forage as dietary specialists as a means of maximizing intake of resources. The optimal digestion hypothesis suggests that tortoises should process ingesta in ways that regulate assimilation rate. Finally, the cost-of-switching hypothesis suggests that herbivores, like the desert tortoise, should avoid switching food types to avoid negatively affecting the microbe community responsible for fermenting plants into energy and nutrients. Combining hypotheses into a resource acquisition theory leads to novel predictions that are generally supported by data presented here. Testing hypotheses, and synthesizing test results into a theory, provides a robust scientific alternative to the popular use of untested hypotheses and unanalyzed data to assert the needs of species. The scientific approach should focus on hypotheses concerning anthropogenic modifications of the environment that impact physiological processes ultimately important to population phenomena. We show how measurements of such impacts as nutrient starvation, can cause physiological stress, and that the endocrine mechanisms involved with stress can result in disease. Finally, our new syntheses evince a new hypothesis. Free molecules of the stress hormone corticosterone can inhibit immunity, and the abundance of "free corticosterone" in the blood (thought to be the active form of the hormone) is regulated when the corticosterone molecules combine with binding globulins. The sex hormone, testosterone, combines with the same binding globulin. High levels of testosterone, naturally occurring in the breeding season, may be further enhanced in populations at high densities, and the resulting excess testosterone may compete with binding globulins, thereby releasing corticosterone and reducing immunity to disease. This sequence could result in physiological and pathological phenomena leading to population cycles with a period that would be essentially impossible to observe in desert tortoise. Such cycles could obscure population fluctuations of anthropogenic origin. The important glucocorticoid that responds to stress in desert tortoise is corticosterone, and the chemical structure of corticosterone is very similar to the sex hormone testosterone (Hammond 1995Go; Breuner and Orchinik 2002Go). Not only are these 2 hormones similar in structure, but the binding globulin for controlling plasma titers of corticosterone also bind with testosterone (Hammond 1995Go). Furthermore, the seasonal dynamics of corticosterone and testosterone in desert tortoise are virtually identical (Lance and others 2001Go). This similarity in form and function between corticosterone and testosterone creates a potentially dangerous outcome for populations of desert tortoises. In particular, if increased production of testosterone during vernal and autumnal agonistic and reproductive behaviors resulted in competition between testosterone and corticosterone for combining with binding globulin, then this could reduce the effectiveness of the binding-globulin system in sequestering corticosterone. Freeing corticosterone could reduce immune competence and result in outbreaks of disease and population crashes. If this description of vernal and autumnal events is accurate, then these mechanisms could result in population cycles with a period of decades. In particular, individuals in low-density populations would be more likely to have fewer agonistic encounters, and thus produce less testosterone. As population densities increase, however, higher titers of testosterone could increase the vulnerabilities of individuals to disease. If behavioral, physiological and ecological mechanisms occur according to this model, then population crashes and cycles could be normal processes in the biology and ecology of desert tortoises. Recognizing this is critically important as management and conservation plans are developed.
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