Because the external environment is intrinsically variable, living organisms must maintain a separate and distinct internal environment. Physiology is, at its core, the study of the mechanisms organisms use to maintain the internal environment. The maintenance of a constant internal environment is termed homeostasis, and it is achieved through behavioral changes and internal regulation at the system, organ, cellular, and molecular level. The concept of homeostasis was first recognized by Claude Bernard (1813-78), who worked primarily in the field of human physiology. Humans demonstrate exceptionally stable internal environments, in contrast to many other animals and plants, where some aspects of the internal environment are maintained, while others are allowed to vary with changing conditions. When a given parameter of the internal environment is maintained at a constant level, or within a relatively small range, it is said to be regulated; an example of this is the internal body temperature of most mammals. However, other aspects of the internal environment may be allowed to vary with changes in the external environment, and are said to be conforming, such as the water content in certain intertidal algae. It is likely that organisms reach an evolutionary 'compromise' between the potential physiological stress incurred by allowing a given parameter to vary with changes in the environment and the metabolic expense incurred by regulation.
Ecophysiology, or ecological physiology, is an area of research where physiological parameters are used to quantify the interaction between the external environment (i.e., ecology) and internal environment of the organism (Figure 1). As such, this field is not really different from physiological ecology (see Physiological Ecology) although some may choose one term over the other to reflect a relatively greater emphasis on ecological or physiological processes as part oftheir research program. One of the earliest areas of research within the ecophysio-logical realm was the field of 'energetics', which was pioneered by H. T. Odum (1924-2002) and examines the energy flux between organisms and the environment. Energetics remains an important area of research, and the field of modern energetics includes studies of the energy required to perform a certain task (e.g., cost of transport) as well as of energy budgets - how energy is acquired and used by the organism (over various timescales) in performing all the tasks important to its survival, such as foraging, reproducing, growing, etc. Other current areas of research in ecophysiology include (but are not limited to): (1) energy acquisition in systems without light for primary
Figure 1 Schematic diagram indicating the relative roles of ecology and physiology in creating the field of ecophysiology.
production - how carbon is fixed to form the basis of food webs (e.g., hydrothermal vents); (2) modifications to the oxygen transport system - how animals respond to life in oxygen limited (e.g., elevation) or anaerobic environments (e.g., tidal marshes); (3) physiological responses to short-term, unpredictable, environmental stressors - how organisms respond to temporary, but life-threatening, changes in their environments, such a sudden, dramatic increase in ambient temperature (e.g., heat shock proteins). It is important to note that these research areas span a large range of biological organization: from whole-organism, or behavioral, responses to molecular responses. In addition, ecophysiologists often focus on environments that pose myriad and extreme physiological challenges to organisms that inhabit them. Two examples of environments of this type that have been well studied by ecophysiologists are the desert and the marine intertidal.
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