Physical Characteristics

The ecological concept of microclimate depends largely on the question being addressed. To a landscape ecologist, a microclimate may comprise the side of a mountain, or a section of desert tens or perhaps even hundreds of kilometers in extent. To a reproductive ecologist interested in the life history of a mosquito breeding in a pitcher plant, the term microclimate refers to the inside of a plant only a few centimeters in diameter. Thus, from an ecological viewpoint, microclimate is very much defined by the organism or community in question. As a result, cohabi-tating species may have different responses to heterogeneity in their local microhabitat, and what constitutes ecologically and physiologically important variability for one species may comprise environmental noise for another. This complex interaction between each organism and its local environment is one of the foundations ofpopulation and community dynamics.

The dimensions of any particular microclimate are highly dependent on the mobility and dispersal capabilities of the organism in question. To an organism with limited movement and dispersal capabilities, the world may be restricted to a very small space; to an organism capable of foraging over large areas, such microclimates may appear as mere noise as it shuttles from place to place in search offood and shelter. Consider, for example, the section of rocky shore shown in Figure 1. To a mussel tightly adhered to the substratum, living on the north versus south face of a rock may make an enormous difference in terms of the amount of solar radiation that the animal receives, and thus the maximum body temperature that the animal experiences. Differences in body temperature of 10 °C or more are not uncommon between sessile animals separated by a few centimeters due to differences in solar radiation created by substratum angle. In contrast, a gull foraging in the same intertidal zone may not be able to distinguish this small-scale thermal variability from the larger

Figure 1 Rocky intertidal zone at Tatoosh Island, Washington State. The importance of microhabitat variability, and thus the concept of microhabitat, varies with movement and dispersal capability of the organism.

microclimate of the entire intertidal area. Furthermore, if microclimate conditions become intolerable in the intertidal area, the gull can spread its wings and fly to a more favorable location, perhaps a ledge on a nearby cliff.Measuring an organism's microclimatic conditions thus mandates that we first have an understanding of how far the organism is likely to 'sample' its local environment.

The ecological consequences of microclimate heterogeneity are directly dependent on the relative differences in scale between predators (e.g., gulls, crabs, seastars) and prey (mussels, barnacles), and between competitors. Moreover, this scale determines the scale that scientists must measure or model microclimatic parameters. Equally important, however, is the fact that organisms themselves affect the flux of materials between themselves and their local environments. In the next section, we discuss the role of organism morphology and size in driving the transfer of heat to and from the organism's body, and how this affects the means by which we measure microclimates in the field.

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