Ecophysiology of atmospheric moisture in the Namib Desert
Introduction
All organisms require water (Louw, 1993). Water is the source of hydrogen for plants to reduce CO2 and is also the source of oxygen that plants produce. As a good solvent of biochemicals and electrolytes it transports and facilitates chemical reactions, while acting as an important buffer system. Its thermal capacity and heat of vaporisation facilitate thermoregulation, while its poor compressibility enables hydraulic locomotion. The high surface tension of water facilitates drop formation and causes drops to run off hydrophobic bodies. All of these properties and more explain why life has evolved largely around the unique properties of water (Louw, 1983, Louw, 1993).
Obtaining and retaining water is important for life in deserts. In hyperarid areas such as the Namib Desert, where rainfall is extremely rare and unpredictable, the occurrence of fog, dew and atmospheric moisture plays an important role in the water economy of many organisms. In this paper we review some of the knowledge relating to how Namib desert animals and plants obtain atmospheric moisture in this desert and comment on the (potential) application of this knowledge (Nelson, 2003).
In particular we present examples of Gideon Louw's observations and perspectives of his Namib work, and outline how his knowledge inspired others. Extensive contributions of this doyen of desert ecophysiology range from water, energy, and salt balance, to thermal biology, interpreted in terms of physiology, behaviour and ecology. He inspired a generation of students and colleagues to elucidate these mechanisms and their consequences, and later reviewed some of these insights (Louw and Seely, 1982, Louw, 1990, Louw, 1993). With this paper we pay tribute to the late Professor Gideon Nel Louw who died on 22 March 2004.
Louw's first visit to the Gobabeb Training & Research Centre in the Namib Desert in 1966 followed the pioneering works by Walter (1936) and Koch (1961), who revealed that fog was important for life in the desert. Louw, 1971, Louw, 1972) soon recognised that ecophysiological mechanisms were a key towards understanding the relationship between atmospheric moisture and desert organisms. He and subsequent workers described processes and patterns of Namib rain, fog, dew, and water vapour in relation to other micro-climatic factors such as temperature so as to understand the source of water, this rare commodity. They followed this with detailed observations or tests that revealed how organisms obtain water from these sources in damp micro-habitats, or by drinking from wet surfaces, consuming moistened food, using their bodies to collect water, or absorbing water vapour. In this paper we describe these observations and finally evaluate the significance of some of the insights.
Section snippets
Rain
The Namib Desert annually receives less than 50mm of rainfall and in its western half generally between 0 and 12mm. Although rain water is rarely available on the surface (Fig. 1), the effects of rain are extremely important for many life forms (Seely and Louw, 1980). After rainfall, plant biomass increases by an order of magnitude and then gradually declines over the course of the following dry years. Louw (1972) emphasised that the ability to secure atmospheric moisture during the long
Damp micro-habitats
Moisture penetrates and lingers in sheltered micro-habitats under stones. Louw (1972) described how fog water deposition and dew condensation on stones trickles down the sides to below the stones where it supports the growth of Fensteralgen (Rumrich et al., 1989) and of a small community of invertebrates and micro-organisms. This micro-environment is even moist enough and has sufficient algal food to support snails Xerocerastus minutus (Hodgson et al., 1994).
Water uptake from wet surfaces
Louw and Holm (1971) noted the
Conclusions
Even in a hyperarid environment there are a number of ways for organisms to obtain atmospheric water, namely a) location in moist micro-environments, b) drinking from wet surfaces during dew or fog, c) consumption of moistened food, d) collecting fog water on the body, and e) absorbing water vapour. Both (d) and (e) involve some special adaptations, notably fog-basking, and formation of high osmotic pressure. These mechanisms do not scale up easily.
Applications for fog-and dew-water collection
Acknowledgements
Climatic data were recorded by the Desert Research Foundation of Namibia at Gobabeb. The Ministry of Environment and Tourism gave permission to conduct research in the Namib. We thank Yvette Evans, Bill Hamilton, Elsita Kiekebusch, John Pallett and two anonymous referees for comments.
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