All organisms exchange heat, water, and nutrients with their surrounding environment, and microclimate can have a significant influence on these rates of exchange. While the exchange of each of these factors is equally important, throughout the rest of this article, heat exchange will be used to explain the intricate relationship between organisms and their local microclimates.
In general, heat flux between an individual organism and the environment is divided into six categories: short-wave (visible) solar radiation, long-wave (infrared, IR) radiation to and from the sky and from the organism's surroundings, conduction to and from the ground, heat convected between the animal and the air, heat lost through the evaporation of water, and (for endotherms) heat generated through metabolism. Environmental variables such as habitat type, substrate orientation, as well as characteristics of the organism itself such as mass, morphology, and color, also drive rates of heat exchange. For example, organisms that have large proportions of their total surface area in contact with the underlying substratum (e.g., barnacles, lichens) may have body temperatures that are tightly coupled with ground temperature. In addition, some organisms use behavioral mechanisms to moderate the amount of heat flux. Snakes bask on hot rocks during the day and take refuge underneath rocks at night when surface conditions are too cold for them to survive. Due to the many variables involved in heat exchange, microclimate (air or surface) temperature and organism temperature are often dissimilar. Moreover, two organisms, even ones of the same species, can experience markedly different rates of heat transfer when exposed to identical microclimates, and can thus experience very different body temperatures.
Endotherms are able to maintain a relatively constant body temperature (i.e., are homeothermic) through the production of metabolic heat (during periods of increased heat loss) and cooling. However, the ability to produce heat is energetically costly for endothermic organisms, particularly when they are in microclimates with temperatures above or below their ideal environmental range. On cold winter nights, elk can reduce loss of radiant heat, and thus save metabolic energy that may be used for breeding in the spring, by seeking refuge beneath trees with needles. The elk can further reduce heat loss by laying down, but a prone position may make them easier prey for wolf predators. For an endotherm, finding the ideal physiological microclimates may be a trade-off between ecological costs such as avoiding predators and the physiological benefits such as gestating young.
In contrast, ectotherms, with negligible metabolic heat production, have body temperatures that change with the rate of heat transfer in and out of their bodies (i.e., are poikilothermic). As a result, most ectotherms have body temperatures that change rapidly as microclimatic conditions change; in some cases, these fluctuations can be rapid and quite large.
Therefore, when making measurements of microcli-matic factors such as air temperature, wind speed, and surface temperature, it is important to consider how these factors are translated into factors such as body temperature, as well as to consider both the direct and indirect effects of body temperature on organismal physiology and ecology.
In most cases, the temperature of plants and animals does not track any single microclimatic parameter (such as air temperature; Figure 2). Moreover, because organisms respond to the same environmental parameters in different ways because of their size and morphology, patterns in the same environmental parameter may not translate into the same pattern of organism stress. Importantly, it is the relative impacts of microclimate on different species that determine the importance of microclimate on ecological interactions.
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