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Integrative and Comparative Biology Advance Access published online on October 20, 2006
Integrative and Comparative Biology, doi:10.1093/icb/icl054
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© 2006 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commerical License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commerical use, distribution, and reproduction in any medium, provided the original work is properly cited.

Ecophysiology and Conservation: The Contributions of Energetics

The importance of physiological ecology in conservation biology

C. Richard Tracy 1 *, K. E. Nussear 2, T. C. Esque 2, K. Dean-Bradley 3, C. R. Tracy 4, L. A. DeFalco 2, K. T. Castle 5, L. C. Zimmerman 4, R. E. Espinoza 6, and A. M. Barber 4
1 Department of Biology, Colorado State University, Fort Collins, CO; Ecology, Evolution, and Conservation Biology, University of Nevada, Reno, NV; Present address: School of Science, Charles Darwin University, Darwin, NT 0909, Australia
2 Department of Biology, Colorado State University, Fort Collins, CO; Ecology, Evolution, and Conservation Biology, University of Nevada, Reno, NV; Present address: United States Geological Survey, Biological Resources Division, Western Ecological Research Center, Las Vegas Field Office, NV 89074, USA
3 Department of Biology, Colorado State University, Fort Collins, CO; Ecology, Evolution, and Conservation Biology, University of Nevada, Reno, NV; Present address: 2016 Rosemont Drive, Landenberg, PA 19350, USA
4 Department of Biology, Colorado State University, Fort Collins, CO; Ecology, Evolution, and Conservation Biology, University of Nevada, Reno, NV
5 Department of Biology, Colorado State University, Fort Collins, CO; Ecology, Evolution, and Conservation Biology, University of Nevada, Reno, NV; Present address: 2395 Placid Way, Ann Arbor, MI 48105, USA
6 Department of Biology, Colorado State University, Fort Collins, CO; Ecology, Evolution, and Conservation Biology, University of Nevada, Reno, NV; Present address: Department of Biology, California State University Northridge, CA 91330, USA

* To whom correspondence should be addressed.
C. Richard Tracy, E-mail: dtracy{at}biodiversity.unr.edu


  Abstract

Synopsis 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.

From the symposium "Ecophysiology and Conservation: The Contributions of Energetics" presented at the annual meeting of the Society for Integrative and Comparative Biology, January 4-8, 2006, at Orlando, Florida.
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