The Zoogeography of Specialized Cave Animals: A Bioclimatic Model

Howarth, Francis

The deep cave zone, to which the obligatory species are restricted, is perpetually dark, nearly isothermal, has a nearly constantly saturated atmosphere possibly above the limits of most terrestrial arthropods, and usually an apparent food-limited ecosystem. I postulate that in adapting to exploit the rigorous cave environment, terrestrial troglobites have had to cope with a water surplus and have become more like aquatic organisms in their water balance mechanisms. This hypothesis is corroborated by the general observation that the drying power of the cave environment is an important parameter in understanding the distribution of terrestrial troglobites. Three main physical factors all tend to create a drier and less stable cave environment in the tropics and reduce the proportion of colonizable area in tropical caves for terrestrial troglobites. These factors are (1) tropical caves are warmer and the rate of evaporation rises almost exponentially with temperature; (2) in the tropical regions the night-time temperature usually falls below the average annual temperature, so that for tropical caves nearly every night is a winter in terms of water vapor exchange; and (3) the higher solution and erosion rates in tropical caves create more entrances, larger passages, and raised relief, allowing more air communication with the surface. These factors do not preclude the existence of terrestrial troglobites in the tropics, but caves in the tropics that meet the requirements of an adequate moisture supply and a stable environment are more difficult to find and to survey In fact, the bioclimatic model predicts that many more troglobites will be discovered as more tropical caves are surveyed and also predicts that they will be found only in cave passages that have a stable saturated or nearly saturated atmosphere.

The Zoogeography of Specialized Cave Animals: A Bioclimatic Model

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