Introduction

Changes in species composition are commonly associated with discontinuities or gradients in habitat temperature. It is common for one species of a genus to replace its congener as a function of change in latitude, altitude, depth or some other environmental variable which is associated with a shift in temperature (Graves & Somero, 1982; Dahlhoff & Somero, 1993a; Fields et al., 1993; Barry et al., 1995). Data accumulated by biogeographers studying marine, freshwater and terrestrial habitats suggest that even relatively minor differences in habitat temperature may be adequate to drive species replacements, especially in the case of ectotherms. With few exceptions, however, the physiological and biochemical adaptations that may be instrumental in establishing and maintaining these species replacement patterns are unknown (Graves & Somero, 1982; Dahlhoff & Somero, 1993a, b). Indeed, the complex interplay between physical and biological factors in establishing distribution patterns has led to uncertainty about the importance of fine-scale adaptation to temperature in setting biogeographical patterning. A shift in habitat temperature may lead to pervasive effects on community structure because some critical link in the food web, for example a plant which serves as the nutritional source for one or more dominant animal species, may be severely impacted. Perturbation of this key plant species could lead to major shifts in occurrence or abundance of animals which depend on the plant, even if the animals themselves are not directly impacted by the temperature change (cf. Lubchenco et al., 1993).