SUMMARY

CONCEPT 4.1 Each species has a range of environmental tolerances that determines its potential geographic distribution.

4.1.1 Explain why the physical environment is the ultimate determinant of the geographic distribution of a species.

The physical environment affects an organism’s ability to obtain energy and resources, thereby determining its growth and reproduction and, more immediately, its ability to survive the extremes of that environment. The physical environment is therefore the ultimate constraint on the geographic distribution of a species.

4.1.2 Differentiate between adaptation and acclimatization by explaining how both individual organisms and populations respond, but differently, to changes in the environment.

Individual organisms can respond to environmental change through acclimatization, a short-term adjustment of the organism’s physiology, morphology, or behavior that lessens the effect of the change and minimizes the associated stress.

A population may respond to unique environmental conditions through natural selection for physiological, morphological, and behavioral traits, known as adaptations, that enhance individuals’ survival, growth, and reproduction under those conditions.

4.1.3 Illustrate how adaptation and acclimatization may result in trade-offs with other functions.

Acclimatization and adaptation are not “free”; they require an investment of energy and resources by the organism. They represent possible trade-offs with other functions of the organism that may also affect its survival and reproduction.

CONCEPT 4.2 The temperature of an organism is determined by exchanges of energy with the external environment.

4.2.1 Describe how the body temperature of an organism influences its functioning.

Temperature controls physiological processes through its effects on enzymes and membranes.

4.2.2 Use information about the gains and losses of energy to determine whether an organism’s temperature is rising or dropping.

Gains of energy from and losses of energy to the external environment determine an organism’s temperature. Modifying this exchange of energy with the environment allows an organism to control its temperature.

4.2.3 Identify the heat exchange mechanisms used by plants and animals to regulate their body temperatures.

Terrestrial plants may modify their energy balance by controlling transpiration, increasing or decreasing absorption of solar radiation, or adjusting the effectiveness of convective heat loss.

Animals modify their energy balance mainly through behavior and morphology to adjust heat losses and gains and, in the case of endothermic animals, metabolic heat generation and insulation to lower heat loss.

4.2.4 Contrast ectothermy and endothermy, and explain how each influences the geographic distributions of organisms, along with organisms’ sensitivities to changes in body temperature.

Ectothermy is the regulation of body temperature through energy exchange with the external environment, while endothermy is the regulation of body temperature through internal heat generation.

Generally, ectotherms have a greater tolerance for variation in their body temperature than endotherms.

CONCEPT 4.3 The water balance of an organism is determined by exchanges of water and solutes with the external environment.

4.3.1 List the three factors that influence the movement of water from a high-energy state to a low-energy state (i.e., with reference to water potential gradients) in biological systems.

Water flows along energy gradients determined by solute concentration (osmotic potential), pressure or tension (pressure potential), and the attractive force of surfaces (matric potential).

4.3.2 Explain how organisms can control water gains and losses by adjusting resistance to water movement, and describe how high resistance may involve trade-offs with other functions.

Plants and microorganisms can influence water potential by adjusting the solute concentration in their cells (osmotic

adjustment).

Terrestrial organisms can control their gains or losses of water by adjusting their resistance to water movement, as by the opening or closing of stomates in plants or adaptations of the skin in animals.

4.3.3 Describe how salt and water balances can become challenges for organisms exposed to hyperosmotic and hypoosmotic environments.

Aquatic animals that are hypoosmotic to the surrounding water must expend energy to excrete salts against an osmotic gradient. On the other hand, aquatic animals that are hyperosmotic to their environment must take up solutes from the environment to compensate for solute losses to the surrounding water.

REVIEW QUESTIONS

1. Organisms exhibit different degrees of tolerance for environmental stresses. How does tolerance for variation in body temperature vary among plants, ectothermic animals, and endothermic animals? What factors influence the differences in tolerance among these groups? Can plants exhibit avoidance of temperature extremes?

2. Organismal adaptations to environmental conditions often affect multiple ecological functions, leading to associated trade-offs. The following are two different trade-offs to consider.

a. Plants transpire water through their stomates. What effects does transpiration have on temperature regulation in leaves? What is the trade-off with transpirational temperature regulation in terms of leaf physiological function?

b. Animals can more effectively warm their bodies by absorbing solar radiation if they are a dark color. Many animals, however, are not dark but instead have a coloration close to that of their habitat (camouflage, as in the case of the basking crocodile in Figure 4.15). What is the trade-off between animal coloration and heat exchange?

3. List several ways in which plants and animals in terrestrial environments influence their resistance to water loss to the atmosphere.

HONE YOUR PROBLEM-SOLVING SKILLS

Higher albedo leads to lower heat gain from solar radiation, and our earlier discussion of leaf pubescence in desert brittlebush (Encelia) species described the link between water availability, air temperature, and the amount of leaf pubescence.

1. Populations of E. farinosa occur in Superior and Oatman, Arizona, and at Death Valley, California. Annual precipitation at the three sites is 453 mm, 111 mm, and 52 mm, respectively. What sites would you expect to have the most, least, and intermediate amounts of leaf pubescence? What site should exhibit the most seasonal change in leaf pubescence?

2. Using a graph of time (x-axis) versus leaf absorption of solar radiation (ó-axis, using a relative scale from low to high), plot your predictions of the differences in the three populations as determined by variation in pubescence. Indicate any seasonal changes in pubescence you might expect as soils dry out.

3. The following data are from a controlled experiment run by Sandquist and Ehleringer (2003) to test whether there was ecotypic differentiation among the E. farinosa populations. Plants from each population were grown from seed in a common garden under a uniform environment for 2 months under well-watered conditions. The soils were then allowed to dry out for a month under warm temperatures. Leaf absorptance of solar radiation (as a percentage of the incoming light) was measured at three times: under well- watered conditions, halfway into the drydown, and under dry conditions after a month of no added water. Use the data to test the hypotheses you developed for Question 2 by plotting them on a graph with soil conditions on the x-axis and absorptance on the ó-axis. Do the data support or refute your hypotheses about differences in pubescence among the populations and seasonal changes associated with decreased water availability and increased temperature?

Source: Data from D.

LIST OF KEY TERMS

acclimatization adaptations avoidance boundary layer climate envelope dormancy ecotypes ectotherms endotherms gravitational potential hibernation lower critical temperature matric potential osmotic adjustment osmotic potential physiological ecology pressure (or turgor) potential pubescence

resistance stomates Stress thermoneutral zone tolerance torpor turgor pressure water potential