Water Relations
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Final PDF to printer 6 Water Relations A black-backed jackal, Canis mesomelas, takes a drink at a water hole in Botswana. As a relatively small predator, the black-backed jackal Applications: Using Stable Isotopes to Study takes a risk each time it visits a water hole, where it may become prey for the larger predators, such as leopards, with which it shares the Water Uptake by Plants 144 southern African landscape. It is a risk that must be taken, however, Summary 147 since the jackal cannot live without water. Key Terms 147 Review Questions 148 CHAPTER CONCEPTS 6.1 Concentration gradients influence LEARNING OUTCOMES the movement of water between After studying this section you should be able to do the following: an organism and its environment. 127 6.1 Describe the environmental circumstances, especially Concept 6.1 Review 130 temperature conditions, in which the Sonoran Desert 6.2 Terrestrial plants and animals cicada, Diceroprocta apache, emits its buzzing call. regulate their internal water by 6.2 List the research questions that Eric Toolson con- balancing water acquisition sidered as he went about collecting Sonoran Desert against water loss. 131 cicadas. Investigating the Evidence 6: Sample Size 136 ater plays a central role in the lives of all organ- Concept 6.2 Review 142 isms. However, water acquisition and conservation Ware particularly critical for desert organisms. As 6.3 Marine and freshwater organisms a consequence, many ecologists studying water relations have use complementary mechanisms focused their attention on desert species. The steady buzzing for water and salt regulation. 142 of the Sonoran Desert cicada, Diceroprocta apache, s e e m e d t o Concept 6.3 Review 144 amplify the withering heat. Air temperature in the shade hovered around 46 8C, and the ground surface temperature was over 70 8 C. All other animals had taken refuge from the desert heat. Nothing 125 moL37282_ch06_125-148.indd 125 7/23/14 6:01 PM Final PDF to printer 126 Section II Adaptations to the Environment else called, and nothing moved, except a lone biologist with an insect net who stalked in the direction of the calling cicada. The biologist was Eric Toolson. Toolson was well acquainted with the calls of all the cicadas in the region and Falling to the ground, he knew their natural history. He knew when they were active, Air temperature of 468C with a temperature of where they fed, and their natural enemies. Toolson associated is higher than lethal 708C, would be certain the call of Diceroprocta with the hottest hours of the desert maximum for the cicada. death for the cicada. day, when air temperatures often exceeded the lethal limit for the species. His goal was to understand the ecology of this extraordinary cicada. If Toolson could capture the calling cicada, he would put it in an environmental chamber in his laboratory and measure its body temperature and water loss rates under a variety of conditions. Later, he would release it to resume its midday serenade. Questions raced through Toolson’s mind as he made his way through the shimmering desert air toward the cicada. Above all, how could this species be active in apparently lethal air temperatures? We might ask the same question of Toolson himself. How did he maintain a body temperature of approximately 37 8 C in this desert heat? Humans evaporatively cool by sweating. If we placed humidity sensors just above Toolson’s skin, they would show that he sweated profusely as he picked his way through the cactus. To keep from becom- ing dehydrated, he took frequent drinks from the water bottle at his side. This enabled him to maintain sufficient internal water and continue to evaporatively cool by sweating. During his pauses to drink, more questions came. Did the cicada keep cool by using small, shady microclimates in the mesquite tree from which it called? Did the cicada somehow manage to evaporatively cool? This seemed unlikely, since biologists had long assumed that insects were too small and vulnerable to water loss to do so. If Diceroprocta did evapora- How does the cicada remain tively cool, how did it avoid desiccating in the desert heat? It active when environmental did not, like Toolson, have a water bottle strapped to its waist. temperatures exceed its lethal maximum? As Toolson stalked the cicada, he pursued an even greater prize: an understanding of how Diceroprocta can regulate the temperature and water content of its body while living in such an extreme environment. This second pursuit would lead Toolson to discover an unsuspected physiological process in these desert insects. Like Bernd Heinrich, who had discov- ered the mechanisms by which sphinx moths thermoregulate Figure 6.1 An ecological puzzle: the cicada, Diceroprocta apache, (see chapter 5, pp. 116–117), Toolson and his students, Stacy is active when air temperatures appear to be lethal for the species. Kaser and Jon Hastings, would be the first to comprehend a bit of nature that had escaped the notice of all researchers before them. Few scientists make such a fundamental discovery. organisms must balance water losses to the environment with Those that do never forget the thrill. Figure 6.1 summarizes water intake. How organisms maintain this water balance is the extreme physical conditions under which Diceroprocta called their water relations, which is the subject of chapter 6. lives that inspired Toolson and his colleagues to study its ecol- In some environments, organisms face the problem of ogy (Toolson 1987; Toolson and Hadley 1987). water loss. Elsewhere, water streams in from the environment. Before we discuss the ecology of these desert cicadas, The problem of maintaining proper water balance is especially we need to introduce some background information. Water strong for those organisms, such as Diceroprocta, t h a t l i v e i n and life on earth are closely linked. The high water content arid terrestrial environments. A parallel challenge faces organ- of most organisms, which ranges from about 50% to 90%, isms that live in aquatic environments with a high salinity. In reflects life’s aquatic origins. Life on earth originated in salty these extreme environments, the water relations of organisms aquatic environments and is built around biochemistry within stand out in bold relief. However, most organisms must expend an aquatic medium. To survive and reproduce, organisms must energy to maintain their internal pool of water. In the study of maintain appropriate internal concentrations of water and dis- relationships between organisms and the environment, which solved substances. To maintain these internal concentrations, we call ecology, the study of water relations is fundamental. moL37282_ch06_125-148.indd 126 7/23/14 6:02 PM Final PDF to printer Chapter 6 Water Relations 127 6.1 Water Availability We know how temperature is measured, but how is the water content of air measured? The quantity of water vapor LEARNING OUTCOMES in air can be expressed in relative terms. Since air is rarely After studying this section you should be able to do the following: completely saturated with water vapor, we can use its degree of saturation as a relative measure of water content. The 6.3 Compare relative humidity, water vapor pressure, satu- most familiar measure of the water content of air is relative ration water vapor pressure, and vapor pressure deficit. humidity , defined as: 6.4 Diagram the movements of salts and water between the surrounding environment and aquatic organisms Water vapor density Relative humidity 5 _________________________ 3 100 that are isosmotic, hyperosmotic, and hypoosmotic. Saturation water vapor density 6.5 Explain, using gradients in water potential, the The actual amount of water in air is measured directly as the movement of water from the soil, through a plant, mass of water vapor per unit volume of air. This quantity, the and to the atmosphere. water vapor density, is the numerator in the relative humidity equation and is given either as milligrams of water per liter Concentration gradients influence the movement of water of air (mg H2 O/L) or as grams of water per cubic meter of air between an organism and its environment. The tendency 3 (g H2 O/m ). The maximum quantity of water vapor that air of water to move down concentration gradients and the mag- at a particular temperature can contain is its saturation water nitude of those gradients from an organism to its environ- vapor density, the denominator in the relative humidity equa- ment determine whether an organism tends to lose or gain tion. Saturation water vapor density increases with tempera- water from the environment. To understand the water rela- ture, as you can see from the red curve in figure 6.2 . tions of organisms, we first review the basic physical behav- One of the most useful ways of expressing the quantity of ior of water in terrestrial and aquatic environments. water in air is in terms of the pressure it exerts. If we express In chapter 2, we saw that water availability on land var- the water content of air in terms of pressure, we can use similar ies tremendously, from the tropical rain forest with abundant units to consider the water relations of organisms in air, soil, moisture throughout the year (see fig. 2.10) to hot deserts with and water. Using pressure as a common currency to represent year-round drought (see fig. 2.19). In chapter 3, we reviewed the considerable variation in salinity among aquatic environ- ments, ranging from the dilute waters of tropical rivers drain- ing highly weathered landscapes to hypersaline lakes. The Water vapor in air can be …or by the pressure measured either as grams of majority of aquatic environments, including the oceans, fall exerted by the water vapor per cubic meter of air… vapor in air.