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TROPICAL CLIMATE STABILITY AND IMPLICATIONS FOR THE DISTRIBUTION OF LIFE 108 6 Tropical Climate Stability and Implications for the Distribution of Life Eric J. Barron The Pennsylvania State University ABSTRACT The tropics are generally viewed as an environment in which the physicochemical factors are not undergoing major changes, and therefore the biotic composition and character are defined largely by biological competition. However, many equatorial species are also characterized by narrow environmental tolerances, which suggests that relatively small climate changes may result in a substantial biologic response. The climatic stability of the tropics is therefore a central issue in global change research. Evidence from the geologic record and from climate models suggests that a temperature variation of 3 to 5Â° C and a salinity variation of several parts per thousand from present values are plausible. The key question becomes the significance of changes of this magnitude for the distribution and character of tropical life. Data on the tolerances of tropical organisms and a case study for the mid-Cretaceous indicate that climate change may substantially influence tropical life. The changes in tropical organisms and their distribution through Earth history should be viewed as a rich, underutilized record that can provide new insights into the climate sensitivity of the tropics. This record provides the only major source of data on the biologic response to global change. INTRODUCTION The tropical biological environment is strongly associated with the notion of physical and chemical stability. However, there is also abundant evidence indicating that climate is a significant limiting factor in the distribution of life (e.g., Valentine, 1973; Stanley, 1984a,b). Tropical organisms may be sensitive to climate, in particular, because of their narrow environmental tolerances. Even small climate changes in the tropics can have a substantial impact on life. The question of tropical climate stability with respect to future global change therefore becomes a central issue of research (Crowley, 1991). Oxygen isotopic paleotemperatures (Douglas and Savin, 1975; Savin, 1977; Shackleton, 1984) can be interpreted as evidence for large variations in tropical temperatures. However, these interpretations have been questioned by Matthews and Poore (1980) and Horrell (1990). Some