Example 1 The Desert

Organisms that live in the desert must maintain the internal environment in the face of high temperatures, large daily temperature fluctuations, and a perpetual scarcity of water. Whole-organism, or behavioral, responses are often used to mitigate these temperature extremes. Because muscles operate more effectively when they are warmer, behavioral thermoregulation is often employed to warm muscles after a cool desert night. By sunning on a warm rock, desert reptiles (ectotherms that produce little metabolic heat) warm themselves in the early morning and quickly reach a temperature that facilitates locomotion and feeding behaviors. However, mid-day temperatures frequently reach levels that present a thermal challenge. At high temperatures, animals encounter problems of evaporative water loss, which affects osmotic balance (maintenance of blood and intracellular volume), ionic balance (maintenance of chemical gradients), and acid-base balance (maintenance of internal pH) of the organism. At still higher temperatures, protein damage, including the deterioration of metabolic enzymes, occurs.

Therefore, desert reptiles typically retreat to underground burrows or other shaded locations to avoid the heat of the day and are only active near dawn or dusk (crepuscular). Some mammals have taken this behavior to an extreme, and are active only during the cooler temperatures of the night (nocturnal). Here, mammals have an advantage because they are endothermic (i.e., produce substantial metabolic heat) and are able to maintain an internal temperature above that of the environment. Because muscles can be warmed by metabolic heat, nocturnal activity can be used to reduce water loss. Thus, evolutionary history may constrain the behavioral response of a given organism to a particular environmental stressor.

The stressors associated with the desert climate have also resulted in modifications, or adaptations, of the morphology and physiology of desert inhabitants. These adaptations often facilitate the retention of water. In plants, for example, leaves are greatly reduced or lost entirely thereby reducing evaporative water loss. Succulent for water retention stems are acquire. Interestingly, these modifications have occurred independently in several unrelated lineages, such as cacti in the Americas and euphorbs in Africa. In addition, some lineages of desert plants, notably CAM plants, have modified photosynthetic reactions with reduced CO2 requirements. These plants keep their stomata (pores used for gas transfer) closed during the day, thereby reducing evaporative water loss. Animals also demonstrate physiological modifications to retain water in the desert. Kangaroo rats have highly modified kidneys that allow them to recover most of the water in their urine, and thus reduce excretory water loss. In fact, kangaroo rats are able to survive almost entirely on 'metabolic water', that is, water produced as a by-product of the conversion of glucose into ATP. Consequently, they can survive without drinking water for extremely long periods of time.

The desert environment can also be used to illustrate research questions or ecological 'problems' that are of interest to ecophysiologists. For example, how long does it take a basking lizard (an ectotherm) to achieve its preferred body temperature to warm its muscles for effective foraging and escape behaviors? To answer this question, a researcher might construct a simple mathematical model that includes the potential sources of heat gain and loss: total heat (HT) = metabolic heat (Hm) ± conductive heat (He) ± convective heat (HV) ± radiant heat (HR) ± evaporative heat (HE) (Figure 2). The researcher might use this to make specific predictions, and then test this prediction by collecting empirical measurements from a model lizard (containing a thermocouple) placed on a basking rock. Alternately, measurements could be taken from a number of lizards of a given species in a certain size class

Figure 2 Heat is transferred between an organism and its environment by radiation (HR), convection (HV), conduction (HC), and evaporation (HE). Information about ambient temperature, wind speed, color of the organism, etc., can be used to generate a model to predict the rate of heat transfer between an organism and the environment to answer specific ecophysiological questions: e.g., how fast can a lizard achieve its preferred body temperature?

Figure 2 Heat is transferred between an organism and its environment by radiation (HR), convection (HV), conduction (HC), and evaporation (HE). Information about ambient temperature, wind speed, color of the organism, etc., can be used to generate a model to predict the rate of heat transfer between an organism and the environment to answer specific ecophysiological questions: e.g., how fast can a lizard achieve its preferred body temperature?

sunning themselves in a desert environment to construct an equation that quantifies the rate of heat gain in lizards basking on rocks under a given set of environmental conditions.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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