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

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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 378 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.

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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 H 2 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. somewhere between these extremes. Salinity, as we shall see, reflects the relative “aridity” of aquatic environments. These preliminary descriptions in chapters 2 and 3 do not 40 6 include the situations faced by individual organisms within their microclimates—microclimates such as those experi- enced by a desert animal that lives at an oasis, where it has 5 ) access to abundant moisture, or a rain forest plant that lives in 3 30 the forest canopy, where it is exposed to full tropical sun and 4 drying winds. As with temperature, to understand the water relations of an organism we must consider its microclimate, including the amount of water in the environment. 20 3 Water Content of Air 2 As we saw when we reviewed the hydrologic cycle in chapter 3, water vapor is continuously added to air as water evaporates 10 Saturation water vapor density (g/m vapor Saturation water from the surfaces of oceans, lakes, and rivers. On land, evapo- 1 pressure (kPa) vapor Saturation water ration also accounts for much of the water lost by organisms. The potential for such evaporative water loss depends upon the temperature and water content of the air around the organisms. 0 0 As the amount of water vapor in the surrounding air increases, 0 10 20 30 the water concentration gradient from organisms to the air Temperature (8C) is reduced and the rate at which organisms lose water to the atmosphere decreases. This is the reason that evaporative air At low temperatures, air is As temperature increases, the coolers work poorly in humid climates, where the water con- saturated by low quantities amount of water vapor in air at of water vapor and water saturation and saturation water tent of air is high. These mechanical systems work best in arid vapor pressure is low. vapor pressure increase. climates, where there is a steep gradient of water concentration from the evaporative cooler to the air. A steep water concentra- Figure 6.2 The relationship between air temperature and two tion gradient is conducive to a high rate of evaporation. measures of water vapor saturation of air.

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128 Section II Adaptations to the Environment

water relations in very different environments helps us unify environments, water flows from organisms to the atmosphere our understanding of this very important area of ecology. We at a rate influenced by the vapor pressure deficit of the air usually think in terms of total atmospheric pressure, t h e p r e s - surrounding the organism. Figure 6.3 shows the relative rates sure exerted by all the gases in air, but you can also calculate of water loss by an organism exposed to air with a low versus the partial pressures due to individual atmospheric gases such high vapor pressure deficit. Again, one of the most useful fea- as oxygen, nitrogen, or water vapor. We call this last quan- tures of vapor pressure deficit is that it is expressed in units of tity w a t e r v a p o r p r e s s u r e. At sea level, atmospheric pressure pressure, generally kilopascals. averages approximately 760 mm of mercury, the height of a column of mercury supported by the combined force (pres- Water Movement sure) of all the gas molecules in the atmosphere. The inter- national convention for representing water vapor pressure, in Aquatic Environments however, is in terms of the pascal (Pa), where 1 Pa is 1 newton In aquatic environments, water moves down its concentration of force per square meter. Using this convention, 760 mm of gradient. It may sound silly to speak of the amount of water in mercury, or one atmosphere of pressure, equals approximately an aquatic environment but, as we saw in chapter 3, all aquatic 101,300 Pa, 101.3 kilopascals (kPa), or 0.101 megapascals environments contain dissolved substances. These dissolved (MPa 5 10 6 Pa). substances, however slightly, dilute the water. While ocean- The pressure exerted by the water vapor in air that is satu- ographers and limnologists (those who study bodies of fresh- rated with water is called saturation water vapor pressure . water) generally focus on salt content, or salinity, we take the As the black curve in figure 6.2 shows, this pressure increases opposite point of view in order to build a consistent perspec- with temperature and closely parallels the increase in satura- tive for considering water relations in air, water, and soil. tion water vapor density shown by the red curve. From this perspective, water is more concentrated in fresh- We can also use water vapor pressure to represent the water environments than in the oceans. The oceans, in turn, relative saturation of air with water. You calculate this mea- contain more water per liter than do saline lakes such as the sure, called the vapor pressure deficit , as the difference Dead Sea or the Great Salt Lake. The relative concentration between the actual water vapor pressure and the saturation of water in each of these environments strongly influences the water vapor pressure at a particular temperature. In terrestrial biology of the organisms that live in them.

The vapor pressure deficit (vpd) indicates the gradient in water concentration from a terrestrial organism to the air. A higher vpd indicates a steeper concentration gradient.

A high vpd indicates that the A low vpd indicates that the water vapor content of air is water vapor content of air is well below saturation. near saturation.

Evaporation Evaporation

Where the vpd is high, the Where the vpd is low, the rate of evaporative water rate of evaporative water loss by organisms is higher. loss by organisms is lower.

Figure 6.3 The potential for evaporative water loss by terrestrial organisms increases as vapor pressure deficit increases.

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Chapter 6 Water Relations 129

The body fluids of all organisms contain water and sol- In an isosmotic aquatic utes, including inorganic ions and amino acids. We can think organism, internal of aquatic organisms and the environment that surrounds them concentrations of water as two aqueous solutions separated by a selectively permeable and salt equal their Salts and water diffuse concentrations in the at approximately equal membrane. If the internal environment of the organism and environment. rates into and out of an the external environment differ in concentrations of water and Salts Water isosmotic organism. salts, these substances will tend to move down their concen- tration gradients. This movement is the process of diffusion . We give the diffusion of water across a semipermeable mem- brane a special name, however: osmosis . In the aquatic environment, water moving down its con- Marine invertebrate centration gradient produces osmotic pressure. Osmotic pres- sure, like vapor pressure, can be expressed in pascals. The Isosmotic strength of the osmotic pressure across a semipermeable Compared to the environment, a membrane, such as the gills of a fish, depends upon the differ- hyperosmotic aquatic organism ence in water concentration across the membrane. Larger dif- has a lower internal concentration Salts diffuse out of a of water and a higher internal ferences, between organism and environment, generate higher hyperosmotic organism at a concentration of salts. osmotic pressures. higher rate, while water Aquatic organisms generally live in one of three environ- diffuses in at a higher rate. mental circumstances. Organisms with body fluids containing Salts Water the same concentration of water and solutes as the external environment are isosmotic . Organisms with body fluids with a higher concentration of water (lower solute concentration) than the external medium are hypoosmotic and tend to lose water to the environment. Those with body fluids with a lower concentration of water (higher solute concentration) than the external medium are hyperosmotic and are subject to water Freshwater fish Hyperosmotic flooding inward from the environment. In the face of these osmotic pressures, aquatic organisms must expend energy to Compared to the environment, a hypoosmotic aquatic organism has a maintain a proper internal environment. How much energy higher internal concentration of water Salts diffuse into a the organism must expend depends upon the magnitude of the and a lower internal concentration of hypoosmotic organism at a osmotic pressure between them and the environment and the salts. higher rate, while water permeability of their body surfaces. Figure 6.4 summarizes diffuses out at a higher rate. the movement of water and salts into and out of isosmotic, Salts Water hyperosmotic, and hypoosmotic organisms.

Water Movement between Soils and Plants On land, water flows from the organism to the atmosphere at a rate influenced by the vapor pressure deficit of the air surround- Marine fish Hypoosmotic ing the organism. In the aquatic environment, water may flow Figure 6.4 Water and salt movements between the environment either to or from the organism, depending on the relative con- and isosmotic, hyperosmotic, and hypoosmotic aquatic organisms. centrations of water and solutes in body fluids and the surround- ing medium. But here, too, water flows down its concentration gradient. Water moving from the soil through a plant and into the locations of higher water concentration ( hypoosmotic) to atmosphere flows down a gradient of w a t e r p o t e n t i a l. We can locations of lower water concentration ( hyperosmotic). The define water potential as the capacity of water to do work. Flow- measurable “osmotic pressure” generated by water flowing ing water has the capacity to do work, such as turning the water down these concentration gradients shows that water flow- wheel of an old-fashioned water mill or the turbines of a hydro- ing in response to osmotic gradients has the capacity to do electric plant. The capacity of water to do work depends upon work. We measure water potential, like vapor pressure defi- its free energy content. Water flows from positions of higher to cit and osmotic pressure, in pascals, usually megapascals lower free energy. Under the influence of gravity, water flows (MPa 5 Pa 3 10 6). By convention, water potential is repre- downhill from a position of higher free energy, at the top of the sented by the symbol c (psi), and the water potential of pure hill, to a position of lower free energy, at the bottom of the hill. water is set at 0. In nature, water potentials are generally nega- In the section “Water Movement in Aquatic Environments,” tive. Figure 6.5 shows that water is flowing down a gradient we saw that water flows down its concentration gradient, from of water potential that goes from a slightly negative water

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130 Section II Adaptations to the Environment

Water potential through the plant all the way down to its roots. This nega- –100 Dry air c = –100 tive pressure reduces the water potential of plant fluids still Dry air has the further. lowest water So, the water potential of plant fluids is affected by sol- potential. utes, matric forces, and the negative pressures exerted by evaporation. Consequently, we can represent the water poten- Plant water tial of plant fluids as: potentials (c) Water potential at cplant 5 csolutes 1 cmatric 1 cpressure –4 –4.0 the top of the Here again, c is the reduction in water potential due to plant is lower pressure (more negative). negative pressure created by water evaporating from leaves. Meanwhile, the solute content of soil water is often so low –2.2 that soil matric forces account for most of soil water potential: –3 csoil > cmatric

–1.2 Matric forces vary considerably from one soil to another, depending primarily upon soil texture and pore size. Coarser –2 soils, such as sands and loams, with larger pore sizes exert lower matric forces, while fine clay soils, with smaller pore sizes, exert higher matric forces. So, while clay soils can hold Plant water a higher quantity of water compared to sandy soils, the higher potentials (c) matric forces within clay soils bind that water more tightly. –1 –0.9 As long as the water potential of plant tissues is less than the Water potential water potential of the soil, cplant , csoil , water flows from the of the soil is –0.6 higher (less soil to the plant. negative). Soil water The higher water potential of soil water compared to the potential (C) water potential of roots induces water to flow from the soil 0 Water potential of into plant roots. As water enters roots from the surrounding pure water soil, it joins a column of water that extends from the roots through the water-conducting cells, or xylem, of the stem Figure 6.5 Water potentials decrease (become more negative) from to the leaves. Hydrogen bonds between adjacent water mol- soil to plant to air (data from Wiebe et al. 1970). ecules bind the water molecules in this water column together. Consequently, as water molecules at the upper end of this col- umn evaporate into the air at the surfaces of leaves, they exert potential in the soil through the moderately negative water tension, or negative pressure, on the entire water column. This potentials of the plant to the highly negative water potential negative pressure helps power uptake of water by terrestrial of dry air. plants. Figure 6.6 summarizes the mechanisms underlying the Now let’s look at some of the mechanisms involved in flow of water from soil to plants. producing a gradient of water potential such as that shown in As plants draw water from the soil, they soon deplete the figure 6.5. We can express the water potential of a solution as: water held in the larger soil pore spaces, leaving only water c 5 csolutes held in the smaller pores. Within these smaller soil pores, matric forces are greater than in the larger pores. Conse- c is the reduction in water potential due to dissolved sub- solutes quently, as soil dries, soil water potential decreases and the stances, which is a negative number. remaining water becomes harder and harder to extract. Within small spaces, such as the interior of a plant cell or This section has given us a basis for considering the the pore spaces within soil, other forces, called matric forces , availability of water to organisms living in terrestrial and are also significant. Matric forces are a consequence of aquatic environments. Let’s use the foundation we have built water’s tendency to adhere to the walls of containers such as here to explore the water relations of organisms on land and cell walls or the soil particles lining a soil pore. Matric forces in water. In the face of variation in water availability, organ- lower water potential. The water potential for fluids within isms have been selected to regulate their internal water. plant cells is approximately:

cplant 5 csolutes 1 cmatric Concept 6.1 Review In this expression, cmatric is the reduction in water potential due to matric forces within plant cells. At the level of the 1. Why are the two curves shown in figure 6.2 so similar? whole plant, another force is generated as water evaporates 2. Which has a higher free energy content, pure water or from the air spaces within leaves into the atmosphere. Evapo- seawater? ration of water from leaves generates a negative pressure, or 3. Why are water potentials in nature generally negative? tension, on the column of water that extends from the leaf

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Chapter 6 Water Relations 131

As water evaporates from a This says simply that the internal water of an animal (W ia ) leaf, it moves from higher results from a balance between water acquisition and water water potential within the leaf loss. The major sources of water are: to much lower water potential of surrounding air. Wd 5 water taken by drinking H2O Wf 5 water taken in with food Evaporation of water from Wa 5 water absorbed from the air leaves reduces water potential of fluids in the leaf and creates The avenues of water loss are: negative pressure. We 5 water lost by evaporation Ws 5 water lost with various secretions and excretions including urine, mucus, and feces Because of hydrogen bonding between water H2O We can summarize water regulation by terrestrial plants in a molecules, the negative similar way: pressure created by water Wip 5 Wr 1 Wa 2 Wt 2 Ws evaporating from surfaces of leaves pulls water The internal water concentration of a plant (W ip ) results from through water-conducting a balance between gains and losses, where the major sources Plant fluids: cells (xylem) in the stem. of water for plants are: Cplant = Csolutes + Cmatric + Cpressure

Wr 5 water taken from soil by roots Wa 5 water absorbed from the air The major ways that plants lose water are: Soil water moves down a gradient of water potential Wt 5 water lost by transpiration from soil to roots. Ws 5 water lost with various secretions and reproductive structures, including nectar, fruit, and seeds H2O — Soil water: Csoil = Cmatric The main avenues of water gain and loss by terrestrial plants and animals are summarized in figure 6.7 . The figure pre- sents a generalized picture of the water relations of terrestrial Figure 6.6 Mechanisms of water movement from soil through organisms. However, organisms in different environments plants to the atmosphere. face different environmental challenges to which they have evolved a wide variety of responses. Let’s now look at the 6.2 Water Regulation on Land diverse ways in which terrestrial plants and animals regulate their internal water. LEARNING OUTCOMES After studying this section you should be able to do the following: Water Acquisition by Animals 6.6 List the major avenues for water gain and loss in Many small terrestrial animals can absorb water from the terrestrial plants and animals. air. Most terrestrial animals, however, satisfy their need for 6.7 Discuss the response of plant roots to differences in water either by drinking or by taking in water with food. In water availability. moist climates, there is generally plenty of water, and, if water 6.8 Compare water conservation by animals from becomes scarce, the mobility of most animals allows them to different environments. go to sources of water to drink. In deserts, animals that need 6.9 Explain how the Sonoran Desert cicada can remain abundant water must live near oases. Those that live out in the active when environmental temperatures exceed desert itself, away from oases, have evolved adaptations for their lethal maximum temperature. living in arid environments. Some desert animals acquire water in unusual ways. Terrestrial plants and animals regulate their internal water Coastal deserts such as the Namib Desert of southwest Africa by balancing water acquisition against water loss. When receive very little rain but are bathed in fog. This aerial mois- organisms moved into the terrestrial environment, they faced ture is the water source for some animals in the Namib. One two major environmental challenges: potentially massive losses of these, a beetle in the genus Lepidochora of the family Tene- of water to the environment through evaporation and reduced brionidae, takes an engineering approach to water acquisition. access to replacement water. Terrestrial organisms evolved by These beetles dig trenches on the face of sand dunes to con- natural selection to meet these challenges, eventually acquired dense and concentrate fog. The moisture collected by these the capacity to regulate their internal water content on land. We trenches runs down to the lower end, where the beetle waits can summarize water regulation by terrestrial animals as: for a drink. Another tenebrionid beetle, Onymacris unguicu- laris, collects moisture by orienting its abdomen upward Wia 5 Wd 1 Wf 1 Wa 2 We 2 Ws (Hamilton and Seely 1976). Fog condensing on this beetle’s

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132 Section II Adaptations to the Environment

Plants lose fuids with secretions such as nectar Plants lose water mainly through in fowers or extraforal transpiration (evaporation and nectaries. diffusion of water). Wt

Ws (secretions)

Wa (absorption)

The main avenue of water loss by animals In some environments is evaporation. plants absorb water from moist air.

The main avenue of water acquisition by most W e Wind terrestrial animals is with Wa (absorption) Wind increases food and drinking. Wind evaporative water loss.

Wd Wf The main avenue of water Ws (secretions and acquisition by plants is from excretion) the soil through their roots.

Wr

Wr

Figure 6.7 The water relations of terrestrial plants and animals can be summarized by analogous pathways for water gain and loss.

body flows to its mouth (fig. 6.8). Onymacris also takes in Fog-laden winds blow across dune crests. water with its food. Some of this water is absorbed within the Moisture in fog condenses tissues of the food. The remaining water is produced when on abdomen. the beetle metabolizes the carbohydrates, proteins, and fats contained in its food. We can see the source of this water if we look at an equation for oxidation of glucose: Beetles gather on dune crests, face into the fog-laden C H O 1 6 O 6 CO 1 6 H O Wind 6 12 6 2 2 2 wind, and tip their As you can see, cellular respiration liberates the water that abdomen upward. combined with carbon dioxide during the process of photo- synthesis (see chapter 5, p. 107). The water released during cellular respiration is called metabolic water . Paul Cooper (1982) estimated the water budget for free- ranging Onymacris from the Namib Desert near Gobabeb. He Grooves in the abdomen estimated the rate of water intake by this beetle at 49.9 mg of collect condensed water and direct it toward the head. H 2 O per gram of body weight per day. Of this total, 39.8 mg came from fog, 1.7 mg came from moisture contained within Beetles drink from the water droplet food, and 8.4 mg came from metabolic water. The rate of that collects around their mouths. water loss by these beetles, 41.3 mg of H2 O per gram per day, was slightly less than water intake. Of this total, 2.3 mg Figure 6.8 Some beetles of the Namib Desert can harvest sufficient were lost with feces and urine, and 39 mg by evaporation. moisture from fog to meet their needs for water. The water budget of the beetle studied by Cooper is shown in figure 6.9 . Kangaroo rats of the genus Dipodomys in the family Hetero- W h i l e Onymacris gets most of its water from fog, other myidae (see fig. 13.22a ) don’t have to drink at all and can small desert animals get most of their water from their food. survive entirely on metabolic water. Knut Schmidt-Nielsen

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Chapter 6 Water Relations 133

Food (Wf) contributes a moderate Most of water loss is through The kangaroo rat can go without amount to water gains. evaporation (We). drinking (no Wd) and obtain all the Most water loss is through water it needs from its food (W ). 100 f evaporation (We). 100

75 75

50

Percent 50

Little water is Percent 25 lost in feces and urine (W ). Moderate water is s 25 lost with feces and urine (Ws). 0 Water gains Water losses 0 Water fuxes Water gains Water losses Water fuxes The beetle obtains most Food moisture Evaporation of its water by drinking Oxidation of food Feces and urine Food moisture Evaporation (Wd) condensed fog. Fog Oxidation of food Feces and urine

Figure 6.9 Water budget of the desert beetle, Onymacris Figure 6.10 Water budget of Merriam’s kangaroo rat Dipodomys unguicularis (data from Cooper 1982). merriami (data from Schmidt-Nielsen 1964).

(1964) showed that the approximately 60 mL of water gained over 850 individual plants, digging over 3 m deep to trace from 100 g of barley makes up for the water a Merriam’s kan- some roots. They found that microclimate affects root devel- garoo rat, D. merriami, loses in feces, urine, and evaporation opment in many grassland species. For instance, the roots of while metabolizing the 100 g of grain. The 100 g of barley fringed sage, Artemesia frigida, penetrate over 120 cm into contains only 6 mL of absorbed water—that is, water that the soil on dry sites; on moist sites, its roots grow only to a can be driven off by drying. The remaining 54 mL of water depth of about 60 cm ( fig. 6.11 ). is released as the animal metabolizes the carbohydrates, fats, Deeper roots often help plants from dry environments and proteins in the grain. The importance of metabolic water extract water from deep within the soil profile. This gener- in the water budget of Merriam’s kangaroo rat is pictured in alization is supported by studies of two common grasses that f i g u r e 6.10 . grow in Japan, Digitaria adscendens and Eleusine indica. While animals generally obtain most of their water by The grasses overlap broadly in their distributions in Japan; drinking or with their food, these options are not available to plants. Though many plants can absorb some water from the air, most get the bulk of their water from the soil through On dry sites, the plant grows On moist sites, the plant their roots. a dense network of deeply grows a sparse network of penetrating roots. shallow roots.

Depth (cm) Water Acquisition by Plants Soil surface 0 0 The extent of root development by plants often reflects differ- ences in water availability. Studies of root systems in differ- ent climates show that plants in dry climates grow more roots 30 30 than do plants in moist climates. In dry climates, plant roots tend to grow deeper in the soil and to constitute a greater pro- portion of plant biomass. The taproots of some desert shrubs 60 60 can extend 9 or even 30 m down into the soil, giving them access to deep groundwater. Roots may account for up to 90% of total plant biomass in deserts and semiarid grasslands. In 90 90 coniferous forests, roots constitute only about 25% of total plant biomass. You don’t have to compare forests and deserts, however, to observe differences in root development. R. Coupland and 120 120 R. Johnson (1965) compared the rooting characteristics of plants growing in the temperate grasslands of western Can- Figure 6.11 Soil moisture influences the extent of root development ada. During their study, they carefully excavated the roots of by Artemesia frigida (data from Coupland and Johnson 1965).

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134 Section II Adaptations to the Environment

however, only Digitaria grows on coastal sand dunes, which Park’s results suggest that Digitaria can be successful in are among the most drought-prone habitats in Japan. the drier dune habitat because it grows longer roots, which Y.-M. Park (1990) was interested in understanding the exploit deeper soil moisture. With these deeper roots, Digi- mechanisms allowing Digitaria to grow on coastal dunes taria can keep the water potential of its tissues high even in where Eleusine could not. Because of the potential for relatively dry soils, where Eleusine suffers lowered water drought in coastal dunes, Park studied the responses of the potential. In other words, Digitaria maintains higher leaf two grasses to water stress. He grew both species from seeds water potentials because its greater root development main- collected at the Botanical Gardens at the University of Tokyo. tains a higher rate of water intake—higher Wr . Seeds were germinated in moist sand and the seedlings were The examples we’ve just reviewed concern rooting by later transplanted into 10 cm by 90 cm polyvinyl chloride individual plant species either in the field or under experi- (PVC) tubes filled with sand from a coastal dune. Park mental conditions. An important question that we might ask is planted two seedlings of Digitaria in each of 36 tubes and whether there have been enough root studies to make tentative two of Eleusine in 36 other tubes. He watered all 72 tubes generalizations about the rooting biology of plants. Jochen with a nutrient solution every 10 days for 40 days. At the end Schenk and Robert Jackson (2002) conducted an analysis of of the 40 days, Park divided the 36 tubes of each species into 475 root profile (see fig. 6.11 ) studies from 209 geographic two groups of 18. One group of each species was kept well localities from around the world. In over 90% of the 475 root watered for the next 19 days, while the other group remained profiles, at least 50% of roots were in the top 0.3 m of the unwatered. soil and at least 95% of roots were in the upper 2 m. How- Unwatered Digitaria and Eleusine responded differently. ever, there were pronounced geographic differences in rooting The root mass of Digitaria increased almost sevenfold over depth. Schenk and Jackson found that rooting depth increases the 19 days of no watering, while the root mass of Eleusine from 80 8 to 30 8 latitude—that is, from Arctic tundra to Medi- increased about threefold. In addition, the roots of Digitaria terranean woodlands and shrublands and deserts. However, were still growing at the end of the experiment, while those of there were no clear trends in rooting depth in the tropics. Eleusine stopped growing about 4 days before the end of the Consistent with our present discussion, deeper rooting depths experiment. Figure 6.12 summarizes these results. occur mainly in water-limited ecosystems. Park found that the differences in root growth were great- est in the deeper soil layers. Below 60 cm in the growing Water Conservation tubes, the unwatered group of Eleusine showed suppressed root growth, while Digitaria did not. With its greater mass of by Plants and Animals more deeply penetrating roots, Digitaria maintained high leaf Another way to balance a water budget is by reducing water water potential throughout the 19 days of no watering. Dur- losses. One of the most common adaptations to arid envi- ing this same period, Eleusine showed a substantial decline in ronments is waterproofing to reduce evaporative water loss. leaf water potential. The leaf water potentials of Digitaria and Many terrestrial plants and animals cover themselves with Eleusine over the 19 days are shown in figure 6.13 . a fairly waterproof “hide” impregnated with a variety of

400 Greater root mass allows Digitaria to maintain stable, high leaf water potential while growing Unwatered Digitaria on unwatered soil. 300 grows a larger root 0 mass than…

200 …Eleusine, a species restricted –1.0 to moist habitats. (MPa) Leaf water potential Root dry weight (mg/plant) 100 –2.00 0 10 20 Days The lower root mass 0 of Eleusine results in 0 10 20 decreasing leaf water Days potential.

Figure 6.12 A grass species from a dry habitat responded to a Figure 6.13 A grass species from a dry habitat maintained a simulated drought by greater root growth compared to a grass species higher water potential during a simulated drought compared to a grass from a moist habitat (data from Park 1990). species from a moist habitat (data from Park 1990).

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Chapter 6 Water Relations 135

waterproofing waxes. However, some organisms are more water two times as fast as C. obsoleta ( fig. 6.15 ). In other waterproof than others, and rates of evaporative water loss words, the species from the drier microclimate, C. obsoleta, vary greatly from one animal or plant species to another. appears to be more waterproofed. Why do the water loss rates of organisms differ? One rea- Waterproofing of terrestrial insect cuticles is usually pro- son is that species have evolved in environments that differ vided by hydrocarbons. Hydrocarbons include organic com- greatly in water availability. As a consequence, selection for pounds, such as lipids and waxes. Because of their influences water conservation has been more intense in some environments on waterproofing, Hadley and Schultz analyzed the cuticles than others. Species that evolved in warm deserts are generally of the two species of tiger beetles for their hydrocarbon con- much more resistant to desiccation than relatives that evolved tent. They found that the concentration of hydrocarbons in in moist tropical or temperate habitats. In general, populations the cuticle of C. obsoleta is 50% higher than in the cuticle of that evolved in drier environments lose water at a slower rate. C. oregona (fig. 6.16). In addition, the two species differ in For instance, turtles from wet and moist habitats lose water at a much higher rate than do desert tortoises (fig. 6.14). As the Cicindela oregona, which lives following example shows, however, the water loss rates of even in moist habitats, loses water at closely related species can differ substantially. a higher rate compared to… Neil Hadley and Thomas Schultz (1987) studied two spe- 60 cies of tiger beetles in Arizona that occupy different micro- climates. Cicindela oregona lives along the moist shoreline 50 of streams and is active in fall and spring. In contrast, Cicin- …C. obsoleta, which dela obsoleta lives in the semiarid grasslands of central and 40 lives in dry habitats. /hour)

southeastern Arizona and is active in summer. The researchers 2 suspected that these differences in microclimate select for dif- g/cm 30 ferences in waterproofing of the two tiger beetles. m Hadley and Schultz studied the waterproofing of the tiger beetles by comparing the amount of water each species lost 20

while held in an experimental chamber. They pumped dry air Water loss ( through the chamber at a constant rate and maintained its tem- 10 perature at 30 8 C. They weighed each beetle at the beginning of an experiment and then again after 3 hours in the chamber. 0 Streamside Desert grassland The difference between initial and final weights gave them an estimate of the water loss rate of each beetle. By deter- Habitats mining water loss for several individuals of each species, they Figure 6.15 A tiger beetle species from a moist habitat lost water estimated the average water loss rates for C. oregona and at a higher rate than one from a dry habitat (data from Hadley and C. obsoleta. Hadley and Schultz found that C. oregona loses Schultz 1987).

The cuticle of Cicindela …the cuticle of C. obsoleta, In a sequence of species from oregona contains less a species of drier habitats. wet to dry habitats, pond turtles hydrocarbons than… lose water at highest rate. 80 30 ) 2 60 Box turtles, which inhabit moist g/cm m terrestrial environments, lose

/hour) 20

2 water at intermediate rate. 40 g/cm m Desert tortoises lose water at lowest rate. 10 20 Water loss ( Cuticular hydrocarbons (

0 Streamside Desert grassland 0 Pond turtle Box turtle Desert tortoise Habitats Species Figure 6.16 The cuticles of tiger beetles from dry habitats tend Figure 6.14 Rates of water loss by two turtles and a tortoise indi- to contain a higher concentration of waterproofing hydrocarbons com- cate an inverse relationship between the dryness of the habitat and water pared to those of tiger beetles from moist habitats (data from Hadley loss rates (data from Schmidt-Nielsen 1969). and Schultz 1987).

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136 Section II Adaptations to the Environment

Investigating the Evidence 6

Sample Size

LEARNING OUTCOMES number of benthic mayfly, stonefly, caddisfly, and beetle spe- After studying this section you should be able to do the following: cies living in a short reach of a small, high-elevation stream in the Rocky Mountains. (We will revisit this study in Inves- 6.10 Define the term sample size. tigating the Evidence 16, p. 359, which concerns estimating 6.11 Design a study, including the number of samples to the number of species in a community.) In contrast, to make be taken, to document the impact of future floods generalizations about global patterns of rooting among plants, on the number of benthic insect species living in Schenk and Jackson (2002) reported on 475 root profiles at Tesuque Creek. 209 locations (see p. 134). The number of samples necessary depends on the amount of variability in the system under study The number of observations included in a sample, that is, sam- and the spatial and temporal scope of the study. However, ple size, has an important influence on the level of confidence whether the scope of a project is large or small, sample size is we place on conclusions based on that sample. Let’s examine one of the most important components of study design. a simple example of how sample size affects our estimate of C RITIQUING THE E VIDENCE 6 some ecological feature. Consider an ecologist interested in how disturbance by flash flooding may affect the number of 1. When designing an ecological study, it is important to take benthic insect species living in a stream. The stream is Tesuque a sufficient number of samples to test the hypothesis under Creek at about 3,000 m elevation in the mountains above Santa study. Why might a researcher try to collect a sufficient Fe, New Mexico. A flash flood, which completely disrupted number of samples to test the hypothesis but not more? one fork of Tesuque Creek, left a second, similar-sized fork 2. Judging from the data displayed in figure 1, how did dis- undisturbed. Nine months after the flood, samples were taken turbance by flash flooding affect the number of mayfly, to determine if there was a difference in the number of species stonefly, caddisfly, and beetle species living in the dis- of mayflies (Order Ephemeroptera), stoneflies (O. Plecoptera), turbed reach? caddisflies (O. Trichoptera), and beetles (O. C o l e o p t e r a ) l i v i n g in similar-sized reaches of the two forks. Samples of the ben- At disturbed and undisturbed thic community were taken at 5 m intervals with a Surber sites the cumulative number of 2 sampler, which has a 0.1 m metal frame, or quadrat, and an mayfly, stonefly, caddisfly, attached net. As a stream ecologist disturbs the bottom mate- and beetle species leveled off rial within the quadrat of a Surber sampler, the net trailing in at between 5 and 7 quadrats. the current catches benthic organisms that are dislodged. In the study of Tesuque Creek, the number of benthic insect species 20 captured in each 0.1 m2 sample ranged from one to six in the disturbed fork and from two to eight in the undisturbed fork. However, our question concerns the total number of species in 15 each fork and the number of benthic samples required to make a good estimate of that number of species. Figure 1 plots the data in a way that provides an answer to both questions. The Surber samples are plotted in the exact 10 order they were taken, beginning with the first that was taken at the downstream end of each study reach and ending with the twelfth sample taken 55 m upstream from the first. As shown 5 in figure 1 , each of the first few samples adds to the cumulative Cumulative number of species number of species collected at each site, which rises steeply Disturbed reach at first and then levels off at a maximum number of species in Undisturbed reach each study reach. The cumulative number of species stopped 0 0 2 4 6 8 10 12 increasing at a sample size of seven quadrats in the undisturbed 2 study reach and at five quadrats in the disturbed study reach. Number of 0.1 m quadrats How many samples should a researcher take? In the case Figure 1 The cumulative number of species increased with the of the benthic community just examined, seven replicate counts number of quadrats studied in both disturbed and undisturbed streams, from 0.1 m2 quadrats appears to be sufficient to estimate the eventually leveling off at a sample size of five to seven quadrats.

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Chapter 6 Water Relations 137

the percentages of cuticular hydrocarbons that are saturated Kangaroo rats from the driest with hydrogen. Fully saturated hydrocarbons are much more site lost water at lower rates. effective at waterproofing. One hundred percent of the hydro- carbons in the cuticle of C. obsoleta are saturated. In contrast, only 50% of the cuticular hydrocarbons of C. oregona are sat- 1 urated. These results support the hypothesis that C. obsoleta

loses water at a lower rate because its cuticle contains a higher O/g/h) concentration of waterproofing hydrocarbons. 2 Merriam’s kangaroo rats conserve water sufficiently that they can live entirely on the moisture contained within their food and on metabolic water (see fig. 6.10). This capacity is 0.5 assumed to be an adaptation to desert living. Over long periods of time as the American Southwest became increasingly arid, the ancestors of today’s Merriam’s kangaroo rats were subject to natural selection that favored a range of adaptations to dry environments, including water conservation. However, Merri- Evaporative water loss (mg H 0 am’s kangaroo rat is a widespread species that lives from 21 8 N Dry Intermediate Moist latitude in Mexico to 428 N latitude in northern Nevada. Over Study sites this large geographic range, Merriam’s kangaroo rat populations Figure 6.17 Water loss rates by Merriam’s kangaroo rats from are exposed to a very broad range of environmental conditions. across a moisture gradient suggest adaptation to local climate by each Intrigued by their large geographic range and exceptional of the populations (data from Tracy and Walsberg 2001). adaptation to desert living, Randall Tracy and Glenn Walsberg studied three populations of Merriam’s kangaroo rats across these studies supports the conclusion that these three popu- a climatic gradient. Their main objective was to determine if lations differ in their degree of adaptation to desert living. different populations of Merriam’s kangaroo rat vary in their Animals adapted to dry conditions have many other degree of adaptation to living in dry environments (Tracy and water conservation mechanisms besides waterproofing. These Walsberg 2000, 2001, 2002). The three populations studied mechanisms include producing concentrated urine or feces by Tracy and Walsberg live in southwest Arizona near Yuma, with low water content, condensing and reclaiming the water central Arizona, and north-central Arizona, at elevations of vapor in breath, and restricting activity to times and places 150 m, 400 m, and 1,200 m, respectively. Mean annual maxi- that decrease water loss. mum temperatures at the study sites are 31.5 8 , 29.1 8 , and Plants have also evolved a wide variety of means for con- 23.5 8C, while mean annual precipitation at the three sites is serving water. How much water a plant can conserve depends 106 mm, 336 mm, and 436 mm. Climatic differences at the in part on its leaf area relative to its root area or length. Plants three study sites are reflected in the vegetation. The habitat at with more leaf surface per length of root lose more water. the driest site consists of sand dunes with scattered shrubs; the Compared to plants from moist climates, arid land plants gen- intermediate site is a desert shrubland; and the vegetation at erally have less leaf area per unit area of root. Many plants the moist site consists of temperate, pinyon-juniper woodland. reduce leaf area over the short term by dropping leaves in One of the main questions asked by Tracy and Walsberg response to drought. Some desert plants produce leaves only was whether rates of evaporative water loss would differ in response to soaking rains and then shed them when the des- among the Merriam’s kangaroo rats at dry, intermediate, ert dries out again. These plants reduce leaf area to zero in and moist sites. The results of this study showed clear dif- times of drought. Figure 6.18 shows one of these plants, the ferences among the study populations. The mean rate of ocotillo of the Sonoran Desert of North America. evaporative water loss by kangaroo rats from the dry site Other plant adaptations that conserve water include was 0.69 mg of water per gram per hour, compared to 1 mg thick leaves, which have less transpiring leaf surface area H 2O / g / h a n d 1 . 0 8 mg H 2O/g/h by kangaroo rats from the per unit volume of photosynthesizing tissue than thin leaves intermediate and moist sites, respectively (fig. 6.17). Tracy do; few stomata on leaves rather than many; structures on and Walsberg expressed the rate of water loss by the kanga- the stomata that impede the movement of water; dormancy roo rats on a per gram basis because the kangaroo rats from during times when moisture is unavailable; and alterna- the three sites differ significantly in size. The average mass tive, water-conserving pathways for photosynthesis (C4 and of individuals from the moist site was approximately 33% CAM). (We discuss these alternative pathways for photosyn- greater than the mass of rats from the dry site. In additional thesis in chapter 7.) studies, Tracy and Walsberg found that acclimating animals We should remember that plants and animals in terrestrial to laboratory conditions did not eliminate the differences environments other than deserts also show evidence of selec- in water conservation among populations. In other words, tion for water conservation. For instance, Nona Chiariello even after being kept in the laboratory under controlled con- and her colleagues (1987) discovered an intriguing example ditions, Merriam’s kangaroo rats from the driest study site of adjusting leaf area in the moist tropics. Piper auritum, a continued to lose water at a lower rate. The evidence from large-leafed, umbrella-shaped plant, grows in clearings of

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138 Section II Adaptations to the Environment

the rain forest. Because it grows in clearings, the plant often faces drying conditions during midday. However, it reduces the leaf area it exposes to the midday sun by wilting. Wilt- ing at midday reduces leaf area exposed to direct solar radia- tion by about 55% and leaf temperature by up to 4 8 to 5 8 C. These reductions decrease the rate of transpiration by 30% to 50%, which is a substantial water savings. The behavior of this tropical rain forest plant reminds us that even the rain forest has its relatively dry microclimates, such as the forest clearings where P. auritum grows. The rapidity of the wilting response by P. auritum is shown in figure 6.19 . Organisms balance their water budgets in numerous ways. Some rely mainly on water conservation. Others depend upon water acquisition. However, every biologist who studies organisms in their natural environment knows that nature is marked by diversity and contrast. To sample nature’s variety, let’s review the variety of approaches to desert living.

Dissimilar Organisms with Similar Approaches to Desert Life On the surface, camels and saguaro cactus appear entirely dif- ferent. If you look deeper into their biology, however, you find that they take very similar approaches to balancing their water budgets. Both the camel and the saguaro cactus acquire mas- sive amounts of water when water is available, store water, (a) and conserve water. The camel can go for long periods in intense desert heat without drinking, up to 6 to 8 days in conditions that would kill a person within a day. During this time, the animal

In a shaded portion of a greenhouse, the leaves of the rain forest plant are unwilted and fully exposed to incoming light.

Minutes after being moved into the sun, the leaves begin to wilt.

2 minutes in sun

4 minutes in sun

6 minutes in sun

After 8 minutes, wilting reduces the surface area exposed to the sun by 55% and decreases rate (b) of transpiration by 30%–50%. Figure 6.18 Changing leaf area: ( a ) following rainfall, ocotillo plants of the Sonoran Desert develop leaves and flower; (b ) during dry Figure 6.19 Temporary wilting by this rain forest plant decreases periods, they lose their leaves and blossoms. rates of water loss (data from Chiariello, Field, and Mooney 1987).

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Chapter 6 Water Relations 139

survives on the water stored in its tissues and can withstand instead of drinking, the saguaro gets its water through its water losses of up to 20% of its body weight without harm. dense network of shallow roots. These roots extend out in a For humans, a loss of about 10% to 12% is near the fatal limit. roughly circular pattern to a distance approximately equal to When the camel has the opportunity, it can drink and store the height of the cactus. For a 15 m tall saguaro, this means a prodigious quantities of water, up to one-third of its body root coverage of over 700 m2 of soil. weight at a time. The saguaro also reduces its rate of evaporative water Between opportunities to drink, the camel is a master of loss in several ways. First, like other cactus, it keeps its sto- water conservation. One way it conserves body water is by mata closed during the day when transpiration losses would reducing its rate of heat gain. Like overheating tiger beetles be highest. In the absence of transpiration, in full sun, the (see chapter 5, p. 120), the camel faces into the sun, reducing internal temperature of the saguaro rises to over 50 8 C, which the body surface it exposes to direct sunlight. In addition, its is among the highest temperatures recorded in plants. How- thick hair insulates it from the intense desert sun, and rather ever, as we noted for the camel, higher body temperature can than sweating sufficiently to keep its body temperature down, be an advantage because it reduces the rate of additional heat- the camel allows its body temperature to rise by up to 7 8 C. ing. The saguaro’s rate of heating is also reduced by the shape This reduces the temperature difference between the camel and orientation of its trunk and arms. At midday, when the and the environment and so decreases the rate of additional potential for heating is greatest, the saguaro exposes mainly heating. Reduced heating translates into reduced water loss the tips of its arms and trunk to direct sunlight. In addition, by evaporation. the tips of the saguaro’s arms and trunk are insulated by a The saguaro cactus takes a similar approach. The trunk layer of plant hairs and a thick tangle of spines, which reflect and arms of the plant act as organs in which the cactus can sunlight and shade the growing tips of the cactus. store large quantities of water. During droughts, the saguaro The parallel approaches to desert living seen in saguaro draws on these stored reserves and, as a result, can endure cactus and camels are outlined in figure 6.20. Now let’s exam- long periods without water. When it rains, the saguaro, like ine two organisms that live in the same desert but have very a camel at an oasis, can ingest great quantities of water, but different water relations.

The saguaro reduces The trunk and branch heat gain by exposing tips are shaded and only tops of its trunk insulated with a high The camel stores fat and branches to the density of spines, which in its hump, a source midday sun. reduces heat gain. of metabolic water.

The camel reduces The camel is covered heat gain by facing with dense hair which into the sun. reduces heat gain.

Water is stored in the massive trunk and arms.

The saguaro reduces water loss The camel reduces by transpiration by keeping When water is available, both evaporative water loss by stomates closed and allowing the saguaro and camel take in not sweating and allowing its temperature to rise. massive quantities. body temperature to rise.

Figure 6.20 Dissimilar organisms with similar approaches to desert living. The improbable pairing of a dromedary camel, native to southwestern Asia, with saguaro cactus, from western North America, actually occurred during the mid-nineteenth century, when the U.S. Army imported camels for use as pack animals.

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140 Section II Adaptations to the Environment

Two Arthropods with Opposite Approaches to Desert Life

Although cicadas and scorpions are arthropods and may live While Sonoran Desert within a few meters of each other, they take sharply contrast- cicada sings from the ing approaches to living in the desert. The scorpion’s approach branches of a mesquite is to slow down, conserve, and stay out of the sun. Scorpions tree during a midsummer’s afternoon… are relatively large and long-lived arthropods with very low metabolic rates. A low rate of metabolism means that they can subsist on low rations of food and lose little water during respi- ration. In addition, scorpions conserve water by spending most of their time in their burrows, where the humidity is higher than at the surface. They come out to feed and find mates only at night, when it’s cooler. In addition, desert scorpions are well waterproofed; hydrocarbons in their cuticles seal in moisture. With this combination of water-conserving characteristics, …the scorpion spends scorpions can easily satisfy their need for water by consuming the day in its burrow near the base of the tree. the moisture contained in the bodies of their arthropod prey. F i g u r e 6.21 summarizes the habits of desert scorpions. In comparison to desert scorpions, the cicada’s approach to desert living may seem out of place. As we saw in the introduc- tion to this chapter, the Sonoran Desert cicada, Diceroprocta apache, is active on the hottest days, when air temperature is near its lethal limit. How can Diceroprocta do this and not die? A series of investigations and papers by Eric Toolson Scorpions emerge from their and Neil Hadley showed conclusively that cicadas, includ- burrows at night, when ing Diceroprocta, are capable of evaporative cooling. In one temperatures are lower. of these studies, Eric Toolson (1987) collected Diceroprocta from a mesquite tree and placed them in an environmental chamber. The chamber temperature was kept at 45.58 C; how- ever, Diceroprocta was able to maintain its body temperature at least 2.98 C lower. Since the cicadas within the chamber did not have access to any cool microclimates, Toolson concluded The low temperature that they must be evaporatively cooling. To verify this hypoth- and high humidity of esis, he placed cicadas in the environmental chamber and then the burrow reduces raised the relative humidity to 100%. At 100% relative humid- water loss. ity, the body temperatures of the cicadas quickly increased to the temperature of the environmental chamber. When Toolson Waterproofed cuticle A low metabolic rate reduced relative humidity to 0%, the cicadas cooled approxi- also reduces reduces respiration and mately 4 8C within minutes. The results of this experiment are evaporative water loss. further decreases water loss. outlined in figure 6.22 . How do the results of Toolson’s experiment support the Figure 6.21 These two desert arthropods, a scorpion and a cicada, hypothesis of evaporative cooling? Remember that air with a have evolved very different approaches to living in the desert. relative humidity of 100% contains all the water vapor it can hold (see p. 127). Consequently, by raising the humidity of chamber with a humidity sensor just above its cuticle. If the air surrounding the cicadas to 100%, Toolson shut off any Diceroprocta evaporatively cools, then this sensor would evaporative cooling that might be taking place. When he rein- detect higher humidity as the temperature of the environment troduced dry air, he created a gradient of water concentration was increased. This is exactly what occurred. As the tempera- from the cicada to the air and evaporative cooling resumed. ture was increased from 308 t o 4 3 8C, the rate of water move- This experiment by Toolson was analogous to Heinrich’s ment across the cicada’s cuticle increased in three steps. When tying off the circulatory system of a sphinx moth to deter- Toolson and Hadley increased the temperature from 378 to mine the role of the circulatory system in thermoregulation 39 8 C, water loss increased from 5.7 to 9.4 mg H2 O p e r s q u a r e (see chapter 5, pp. 118–119). centimeter per hour. At 418 C, water loss increased from 9.4 Toolson’s results are consistent with the hypothesis that to 36.1 mg H2 O per square centimeter per hour and at 43 8 C Diceroprocta evaporatively cools but does not demonstrate water loss increased from 36.1 to 61.4 mg H 2 O p e r s q u a r e that capacity directly. Consequently, Toolson and Hadley centimeter per hour. These results are graphed in figure 6.23. (1987) conducted observations to make a direct demonstra- The rate of water loss by Diceroprocta is among the high- tion. First, they placed a live Diceroprocta in an environmental est ever reported for a terrestrial insect. How does water cross

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Chapter 6 Water Relations 141

At 5% relative humidity, When returned to a chamber with low the body temperature of a live relative humidity, body temperature cicada stays several degrees falls below air temperature. Air temperature below air temperature. 44 Cicada temperature 40

C) 36 8 To stop evaporation, the cicada is placed in a chamber with 32 100% relative humidity; its body temperature rises rapidly.

Temperature ( Temperature 28

24

20 5 15 25 35 45 Time (minutes)

Figure 6.22 A laboratory experiment verified evaporative cooling by the cicada, Diceroprocta apache (data from Toolson 1987).

Between 258C and 398C, the rate of water loss across the cuticle of cicada increases High very little.

(a)

Then, between 398C Rate of water loss Rate of water and 438C, the rate of water loss increases by approximately 600%.

Initial to 378C Temperature temperature to 308C to 358C increases by 258C to 418C experimenter to 398C Low 1 2 3 Time (hours) (b)

Figure 6.23 High temperatures induce massive rates of water Figure 6.24 ( a ) Magnified view of Diceroprocte apache outlining loss by the cicada, Diceroprocta apache (data from Toolson and three areas with high densities of small pores; ( b ) dorsal pores under Hadley 1987). high magnification.

the cuticle of this cicada at such a high rate? Toolson and with large pores that might be involved in evaporative cool- Hadley searched the cuticle of Diceroprocta for avenues of ing ( fig. 6.24 ). When they plugged these pores, Diceroprocta water movement. They found three areas on the dorsal surface could no longer cool itself. In summary, Toolson and Hadley

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142 Section II Adaptations to the Environment

The cicada can remain active when Concept 6.2 Review environmental temperatures exceed its lethal maximum because it uses evaporative cooling 1. The tiger beetle Cicindela oregona (see figs. 6.15 and to reduce body temperature. 6.16 ) has a distribution that extends from Arizona through the temperate rain forests of Alaska. Why It compensates for high evaporative should the amounts of cuticular hydrocarbons vary geo- water loss (high We) by high rate of drinking (high Wd). graphically among populations of C. oregona? 2. During severe droughts, some of the branches of shrubs and trees die, while others survive. How might losing some branches increase the probability that an individ- ual plant will survive a drought? 3. How are water and temperature regulation related in many terrestrial organisms?

6.3 Water and Salt Balance in Aquatic Environments LEARNING OUTCOMES After studying this section you should be able to do the following: The insect gets the water it needs for evaporative cooling by tapping into 6.12 Define osmoregulation. water that its host plant draws from 6.13 Contrast osmoregulation by sharks versus marine deep below the surface of the ground. bony fish. 6.14 Compare osmoregulation by freshwater bony fish and freshwater mosquitoes.

Marine and freshwater organisms use complementary Figure 6.25 An ecological puzzle solved. mechanisms for water and salt regulation. Aquatic organisms, like their terrestrial kin, regulate internal water, W , by balancing water gain against water loss. We can rep- verified a previously unknown phenomenon, evaporative i resent water regulation in aquatic environments by modifying cooling by cicadas, and carefully demonstrated the underlying our equation for terrestrial water balance to: mechanisms. So, it turns out that these cicadas can sing in the hot- Wi 5 Wd 2 Ws 6 Wo test hours of the desert day because they sweat! Dicero- Drinking, W , is a ready source of water for aquatic organ- procta is able to maintain this seemingly impossible d isms. Secretion of water with urine, W , is an avenue of water lifestyle because it has tapped into a rich supply of water. s loss. By osmosis, W , an aquatic organism may either gain or Cicadas are members of the order Homoptera and distant o lose water, depending on the organism and the environment. relatives of the aphids. Like aphids, cicadas feed on plant fluids. So, though the cicada lives in the same macrocli- mate as the scorpion, it has tapped into a totally different Marine Fish and Invertebrates microclimate. The cicada’s scope for water acquisition is Most marine invertebrates maintain an internal concentra- extended up to 30 m deep into the soil by the taproots of its tion of solutes equivalent to that in the seawater around them. mesquite host plant, Prosopis juliflora. Diceroprocta can What does the animal gain by remaining isosmotic with the sustain high rates of water loss through evaporation, high external environment? The isosmotic animal does not have to We , because it is able to balance these losses with a high expend energy overcoming an osmotic gradient. This strategy rate of water acquisition, high W d . F i g u r e 6.25 illustrates is not without costs, however. Although the total concentra- how Diceroprocta uses mesquite trees to get access to deep tion of solutes is the same inside and outside the animal, there soil moisture. are still differences in the concentrations of some individual Sometimes, similar organisms employ radically differ- solutes. These concentration differentials can only be main- ent approaches to balancing their water budgets. Sometimes, tained by active transport, which consumes some energy. organisms of very different evolutionary lineages employ Sharks, skates, and rays generally elevate the concentra- functionally similar approaches. In short, the means by which tion of solutes in their blood to levels slightly hyperosmotic terrestrial organisms balance water acquisition against water to seawater. However, inorganic ions constitute only about loss are almost as varied as the organisms themselves. Similar one-third of the solute in shark’s blood; the remainder con- variation occurs among aquatic organisms. sists of the organic molecules urea and trimethylamine oxide,

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Chapter 6 Water Relations 143

Because the shark’s body Na1 and Cl2 diffuse fluid is slightly hyperosmotic into sharks from the to the surrounding seawater, surrounding seawater. water diffuses through its gills (1Wo).

Water Salts

Water Na1Cl2 Salt gland

Urine Urine Water Na1Cl2

Sharks excrete urine (Ws) Salts are concentrated by to compensate for water the salt gland and gained by osmosis. excreted with the urine.

Figure 6.26 Osmoregulation by sharks.

or TMAO. As a consequence of being slightly hyperosmotic, the problem of water loss, it imports another: large quantities sharks slowly gain water through osmosis; that is, Wo is of salts that must be eliminated. Saltwater mosquitoes secrete slightly positive. The water that diffuses into the shark, mainly these salts into the urine using specialized cells that line the across the gills, is pumped out by the kidneys and exits as posterior rectum. Here, saltwater mosquitoes do something urine. Sodium, because it is maintained at approximately two- that marine bony fish cannot. They excrete a urine that is thirds its concentration in seawater, diffuses into sharks from hyperosmotic to their body fluids, which reduces water loss seawater across the gill membranes and some sodium enters through the urine. The parallels in water and salt regulation with food. Sharks excrete excess sodium mainly through a by marine bony fish and saltwater mosquitoes are outlined in specialized gland associated with the rectum called the salt figure 6.27 . gland. The main point here is that sharks and their relatives reduce the costs of osmoregulation , regulation of internal salt Freshwater Fish and Invertebrates and water concentrations, by decreasing the osmotic gradient Freshwater bony fish face an environmental challenge oppo- between themselves and the external environment ( fig. 6.26 ). site to that faced by marine bony fish. Freshwater fish are In contrast to most marine invertebrates and sharks, hyperosmotic; they have body fluids that contain more salt marine bony fish have body fluids that are strongly hypoos- and less water than the surrounding medium. As a conse- motic to the surrounding medium. As a consequence, they quence, water floods inward and salts diffuse outward across lose water to the surrounding seawater, mostly across their their gills. Freshwater fish excrete excess internal water as gills. Marine bony fish make up these water losses by drink- large quantities of dilute urine. They replace the salts they ing seawater. However, drinking seawater increases salt gain. lose to the external environment in two ways. Chloride cells at The fish rid themselves of excess salts in two ways. Special- the base of the gill filaments absorb sodium and chloride from ized “chloride” cells at the base of their gills secrete sodium the water, while other salts are ingested with food. and chloride directly to the surrounding seawater, while the Like freshwater fish, freshwater invertebrates are kidneys excrete magnesium and sulfate. These ions exit with hyperosmotic to the surrounding environment. Freshwater the urine. The urine, because it is hypoosmotic to the body invertebrates must expend energy to pump out the water that fluids of the fish, represents a loss of water. However, the floods their tissues. They also expend energy by actively loss of water through the kidneys is low because the volume absorbing salts from the external environment. However, of urine is low. the concentration of solutes in the body fluids of freshwater The larvae of some mosquitoes in the genus Aedes live in invertebrates ranges from between about one-half and saltwater. These larvae meet the challenge of a high-salinity one-tenth that of their marine relatives. This lower internal environment in ways analogous to those used by marine bony concentration of solutes reduces the osmotic gradient fish. Like marine bony fish, saltwater mosquitoes are hypoos- between freshwater and the outside environment and so motic to the surrounding environment, to which they lose reduces the energy freshwater invertebrates must expend to water. Saltwater mosquitoes also make up this water loss by osmoregulate. drinking large amounts of seawater, up to 130% to 240% of Freshwater mosquito larvae are a good model for body volume per day! While this prodigious drinking solves osmoregulation by freshwater invertebrates. The larvae of

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144 Section II Adaptations to the Environment

Water diffuses from the Specialized cells in the gills gills of marine bony fish secrete Cl2 and Na1 follows. to the surrounding seawater (2Wo).

Water, Na1 Cl2 Drinking water 1 salt (Gills)

Marine bony fish and 21 22 saltwater mosquitoes Urine: Mg SO4 drink (Wd) to compensate for water 21 lost by osmosis (2Wo); Doubled-charged Mg they also take in salts 22 and SO4 are excreted with drinking water. with urine.

Drinking water 1 salt Urine: water 1 salt Water diffuses from Salts are excreted in saltwater mosquitoes to concentrated urine; the surrounding small amounts of water environment (2Wo). are lost with urine (Ws).

Figure 6.27 Main avenues of osmoregulation by hypoosmotic marine fish and saltwater mosquitoes.

approximately 95% of mosquito species live in freshwater, where they face osmotic challenges very similar to those pre- sented to freshwater fish. Like freshwater fish, mosquito lar- A p p l i c a t i o n s vae must solve the twin problems of water gain and ion loss. Using Stable Isotopes to Study In response, they drink very little water. They conserve ions taken with the diet by absorbing them with cells that line the Water Uptake by Plants midgut and rectum, and they secrete a dilute urine. Fresh- LEARNING OUTCOMES water mosquito larvae replace the ions lost with urine by After studying this section you should be able to do the following: actively absorbing Na 1 a n d C l 2 from the water with cells in their anal papillae. Freshwater mosquitoes and fish use totally 6.15 Describe stable isotope analysis. different structures to meet nearly identical environmental 6.16 List some major stable isotopes that have proved challenges (fig. 6.28). useful in ecological studies. In chapter 6, we have reviewed the water relations of indi- 6.17 Outline how stable isotopes have been used in vidual organisms. Studies of the relationship between individ- ecological studies. 1 ual organisms and the environment, a fundamental aspect of 6 . 1 8 Explain how the ratio of D:H can be employed to ecology, is now being advanced rapidly with the development assess the relative use by plants of moisture that of powerful analytical tools. falls as summer versus winter rain in southern Utah.

Concept 6.3 Review In order to fully understand the ecology of an individual plant or the dynamics of an entire landscape, ecologists 1. Why do isosmotic marine invertebrates expend less need information about what happens below the earth’s energy for osmoregulation compared to hypoosmotic surface, as well as about surface structure and processes. marine fish? However, ecologists have produced much more informa- 2. The body fluids of many freshwater invertebrate species tion about the surface realm than about the subsurface, the have very low internal salt concentrations. What is the domain of soil microbes, burrowing animals, and roots. benefit of such dilute internal fluids? While many ecologists have worked very hard to fill this gap in our knowledge, their work on belowground ecology

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Chapter 6 Water Relations 145

Specialized cells in the gills actively absorb Cl– Water diffuses into from the surrounding freshwater bony water; Na+ follows. fish through their gills (+Wo).

Water, Cl– Na+ Food + salt (Gills)

Urine: water + salt Both freshwater fish and freshwater mosquitoes take in Both freshwater fish and salts with their food. freshwater mosquitoes excrete water in large volumes of dilute urine (Ws); some salt is lost with urine.

Food + salt Na+Cl- Urine: water + salt

Water diffuses into The anal papillae of freshwater mosquitoes freshwater mosquitoes from the surrounding absorb Na+ and Cl– from environment (+Wo). the surrounding water.

Figure 6.28 Main avenues of osmoregulation by hyperosmotic freshwater fish and mosquitoes.

has been historically slow. Fortunately, progress has accel- of carbon include 13 C a n d 12C; stable isotopes of nitrogen erated in recent years. A major contributor to recent prog- include 15 N a n d 14 N; and stable isotopes of sulfur include ress in belowground ecology has been the development of 34 S a n d 32 S. The ratios of these stable isotopes can be used new tools. One of the most important of those tools is stable to study the flow of energy and materials through ecosys- isotope analysis , which involves the analysis of the relative tems because different parts of the ecosystem often contain proportions of stable isotopes, such as the stable isotopes of the light and heavy isotopes of these elements in different carbon 13 C a n d 12 C, in materials. Stable isotope analysis is proportions. increasingly used in ecology (Dawson et al. 2002). As we Different organisms contain different ratios of light and saw in chapter 1 (p. 5), stable isotope analysis is proving heavy stable isotopes because they use different sources of useful for tracking habitat use by migratory birds. It is also these elements, because they preferentially use (fraction- a very powerful tool in studies of water uptake by plants. To ate) different stable isotopes, or because they use different understand the application of this analytical tool, we need to sources and fractionate. For instance, the lighter isotope of know a little about the isotopes themselves and about their nitrogen, 14N, is preferentially excreted by organisms during behavior in ecosystems. protein synthesis. As a consequence of this preferential excretion of 14 N, an organism becomes relatively enriched in 15 N compared to its food. Therefore, as materials pass from Stable Isotope Analysis one trophic level to the next, tissues become richer in 15 N. Most chemical elements include several stable isotopes, Consequently, the highest trophic levels within an ecosys- which occur in different ratios in different environments or tem contain the highest relative concentrations of 15 N, while differ in their ratios from one organism to another. Stable iso- the lowest trophic levels contain the lowest concentrations. topes of hydrogen include 1 H a n d 2 H, which is generally des- Stable isotope analysis can also measure the relative con- ignated as D, an abbreviation of deuterium. Stable isotopes tribution of C3 a n d C 4 plants (see chapter 7, pp. 152–153)

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146 Section II Adaptations to the Environment

to a species’ diet (see chapter 1, p. 5). This is possible because In summer a succulent In contrast, herbaceous and 13 C4 plants are relatively richer in C. Other processes affect plant shifted from soil woody perennials continued the ratios of sulfur stable isotopes. Because different sources water that fell as winter to use at least some deep of water often have different ratios of D to 1 H , f o r e x a m p l e , rain to exclusive use of soil water that had fallen as summer rainwater. winter rain. shallow soil moisture versus deep soil moisture, hydrogen isotope analyses have been valuable aids to identifying where plants acquire their water. 0 The concentrations of stable isotopes are generally expressed as differences in the concentration of the heavier isotope relative to some standard. The units of measurement –25

0 D, ‰ are differences (6 ) in parts per thousand (6 /00) . These dif- ␦ ferences are calculated as:

Rsample –50 dX 5 ______2 1 3 103 [ ( Rstandard ) ] where: d 5 6 –75

X 5 the relative concentration of the heavier isotope, Hydrogen isotope ratio 13 15 34 0 for example, D, C, N, or S in /00

R sample 5 the isotopic ratio in the sample, for example, –100 D:1 H, 13 C: 12 C, or 15 N: 14 N

Rstandard 5 the isotopic ratio in the standard, for example, 1 13 12 15 14 Succulent Winter rain D: H, C: C, or N: N Summer rain The reference materials used as standards in the isotopic anal- Woody perennial yses of hydrogen, nitrogen, carbon, and sulfur are the D: 1 H Herbaceous perennial ratio in Standard Mean Ocean Water, the 15 N: 14 N ratio in Water sources atmospheric nitrogen, the 13 C:12 C ratio in PeeDee limestone, Xylem fluid, summer Xylem fluid, spring and the 34 S: 32 S in the Canyon Diablo meteorite. The ecologist measures the ratio of stable isotopes in a Figure 6.29 Stable isotope analysis identified the water sources sample and then expresses that ratio as a difference relative used by three groups of desert plants during spring and summer (data to some standard. If d X 5 0, then the ratios of the isotopes from Ehleringer et al. 1991). 0 in the sample and the standard are the same; if d X 5 2 X /00, the concentration of the heavier isotope is lower (e.g., 15 N) in since summer rains are relatively enriched with D and win- the sample compared to the standard, and if d X 5 1 X 0/ , the 00 ter rains are relatively depleted of D. The d D of summer and concentration of the heavier isotope is higher in the sample winter rains in southern Utah at the time of Ehleringer’s study compared to the standard. The important point here is that were 225 0/ and 290 0/ respectively ( fig. 6.29 ). these isotopic ratios are generally different in different parts 00 00 Ehleringer measured dD in the xylem fluid of several of ecosystems. Therefore, ecologists can use isotopic ratios plant growth forms during spring, when soil moisture at all to study the structure and processes in ecosystems. Here is rooting depths would be predominantly from winter precipita- an example of how hydrogen isotope ratios have been used tion and summer, when summer precipitation would be pres- to study the uptake of water by plants in a natural ecosystem. ent as moisture in surface soils and winter precipitation would predominate at deeper soil layers. Ehleringer and his research Using Stable Isotopes to Identify team found that a succulent, several herbaceous perennials, and several woody perennials used winter moisture in the Plant Water Sources spring (see fig. 6.29). However, when summer rains fell, the The laboratory of James Ehleringer has taken a leadership succulent species shifted entirely to using soil moisture from role in the development of stable isotope analysis as a tool for summer rains that were stored mainly at shallow soil depths. assessing water relations among plants and within ecosystems Meanwhile, herbaceous and woody perennials continued to (e.g., Ehleringer, Roden, and Dawson 2000). In an early study use significant amounts of deeper soil moisture that fell the Ehleringer and several colleagues (Ehleringer et al. 1991) previous winter. So, stable isotope analysis opens a window used deuterium:hydrogen (D:1 H) ratios, or d D, to explore the to the water relations of plants that would not be accessible use of summer versus winter rainfall by various plant growth without this innovative tool. We will explore the use of stable forms in the deserts of southern Utah. They could use d D to isotope analysis further in chapter 18, where we discuss its determine the relative utilization of these two water sources use in studies of energy flow in ecosystems.

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Chapter 6 Water Relations 147 Summary

Concentration gradients influence the movement of water and Ws 5 secretions and reproductive structures. Some very between an organism and its environment. The most different terrestrial plants and animals, such as the camel and familiar relative measure of the water content of air is relative saguaro cactus, use similar mechanisms to survive in arid humidity, defined as water vapor density divided by saturation climates. Some organisms, such as scorpions and cicadas, use water vapor density multiplied by 100. On land, the tendency of radically different mechanisms. Comparisons such as these water to move from organisms to the atmosphere can be approx- suggest that natural selection is opportunistic. imated by the vapor pressure deficit of the air. Vapor pressure Marine and freshwater organisms use complementary deficit is calculated as the difference between the actual water mechanisms for water and salt regulation. Marine and vapor pressure and the saturation water vapor pressure. freshwater organisms face exactly opposite osmotic challenges. In the aquatic environment, water moves down its con- Water regulation in aquatic environments is summarized by: centration gradient, from solutions of higher water concen- Wi 5 W d 2 W s 6 W o , where W d 5 drinking, W s 5 secretions tration and lower salt content (hypoosmotic) to solutions of and excretions, W o 5 osmosis. An aquatic organism may lower water concentration and higher salt content (hyperos- either gain or lose water through osmosis, depending on the motic). This movement of water creates osmotic pressure. organism and the environment. Many marine invertebrates Larger osmotic differences, between organism and environ- reduce their water regulation problems by being isosmotic ment, generate higher osmotic pressures. with seawater. Some freshwater invertebrates also reduce the In the soil-plant system, water flows from areas of higher osmotic gradient between themselves and their environment. water potential to areas of lower water potential. The water Sharks, skates, and rays elevate the urea and TMAO content potential of pure water, which by convention is set at zero, is of their body fluids to the point where they are slightly hyper- reduced by adding solute and by matric forces, the tendency osmotic to seawater. Marine bony fish and saltwater mosquito of water to cling to soil particles and to plant cell walls. Typi- larvae are hypoosmotic relative to their environments, while cally, the water potential of plant fluids is determined by a freshwater bony fish and freshwater mosquito larvae are combination of solute concentrations and matric forces, while hyperosmotic. the water potential of soils is determined mainly by matric While the strength of environmental challenge varies forces. In saline soils, solutes may also influence soil water from one environment to another, and the details of water potential. Water potential, osmotic pressure, and vapor pres- regulation vary from one organism to another, all organisms sure deficit can all be measured in pascals (newtons/m 2 ), a in all environments expend energy to maintain their internal common currency for considering the water relations of pool of water and dissolved substances. diverse organisms in very different environments. Stable isotope analysis, an important tool in ecology, Terrestrial plants and animals regulate their inter- involves the analysis of the ratios of stable isotopes in materi- nal water by balancing water acquisition against water als. Examples of stable isotopes include the stable isotopes loss. Water regulation by terrestrial animals is sum- of hydrogen 2 H (which is usually symbolized by D, refer- 1 marized by W ia 5 W d 1 W f 1 W a 2 W e 2 W s , where ring to deuterium) and H, and the stable isotopes of carbon, 13 12 W d 5 drinking, Wf 5 taken in with food, W a 5 absorption C and C. Stable isotope analysis has proved a very pow- from the air, We 5 evaporation, and W s 5 secretions and erful tool in studies of water uptake by plants. For example excretions. Water regulation by terrestrial plants is summa- deuterium:hydrogen (D:1 H) ratios, or dD, have been used to rized by W ip 5 W r 1 W a 2 W t 2 W s , where W r 5 uptake quantify the relative use of summer versus winter rainfall by by roots, Wa 5 absorption from the air, Wt 5 transpiration, various plant growth forms in the deserts of southern Utah. Key Terms diffusion 129 matric forces 130 relative humidity 127 vapor pressure deficit 128 hyperosmotic 129 metabolic water 132 saturation water vapor water potential 129 hypoosmotic 129 osmoregulation 143 pressure 128 water vapor pressure 128 isosmotic 129 osmosis 129 stable isotope analysis 145

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148 Section II Adaptations to the Environment Review Questions 1. The body temperature of the seashore isopod Ligia oceanica is 6. In the Sonoran Desert, the only insects known to evaporatively 308 C under stones, where the relative humidity is 100%, but 268 C cool are cicadas. Explain how cicadas can employ evaporative on the surface, where it is exposed to full sun and the relative cooling while hundreds of other insect species in the same envi- humidity is 70%. Edney (1953) proposed that the isopods in the ronment cannot. open had lower body temperatures because they evaporatively 7. Many desert species are well waterproofed. Evolution cannot, cooled in the open air. Explain why evaporative cooling would be however, eliminate all evaporative water loss. Why not? (Hint: effective in the open air but nearly impossible under stones. Think of the kinds of exchanges that an organism must maintain 2. Distinguish among vapor pressure deficit, osmotic pressure, with its environment.) and water potential. How can all three phenomena be expressed 8. While we have concentrated in chapter 6 on regulation of in the same units of measure: pascals? water and salts, most marine invertebrates are isosmotic with 3. Leaf water potential is typically highest just before dawn and their external environment. What is a potential benefit of being then decreases progressively through midday. Should lower leaf isosmotic? water potentials at midday increase or decrease the rate of water 9. Review water and salt regulation by marine and freshwater movement from soil to a plant? Assume soil water potential is bony fish. Which of the two is hypoosmotic relative to its approximately the same in early morning and midday. Are the environment? Which of the two is hyperosmotic relative to its water needs of the plant greater in early morning or at midday? environment? Some sharks live in freshwater. How should the 4. Compare the water budgets of the tenebrionid beetle, Onymac- kidneys of marine and freshwater sharks function? ris, and the kangaroo rat, Dipodomys, shown in figures 6.9 and 10. Ronald Neilson and his colleagues (1992, 1995) used the envi- 6.10 . Which of these two species obtains most of its water from ronmental requirements of plants to predict the responses of metabolic water? Which relies most on condensation of fog as vegetation to climate change. In chapter 1, we briefly discussed a water source? In which species do you see greater losses of the studies of Margaret Davis (1983, 1989) that reconstructed water through the urine? the movement of vegetation across eastern North America. She 5. In this chapter, we discussed water relations of tenebrionid bee- made this reconstruction using the pollen preserved in lake sed- tles from the Namib Desert. However, members of this family iments. How might the results of paleoecological studies, such also occur in moist temperate environments. How should water as those of Davis, be used to refine models based on plant envi- loss rates vary among species of tenebrionids from different ronmental requirements? (Assume that you can also reasonably environments? On what assumptions do you base your predic- reconstruct the climate when historic changes in vegetation tion? How would you test your prediction? occurred.)

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