TABLE 8-1 Evaporation and condensation on a free water sur- 

 face in relation to temperature, relative humidity, saturation 

 deficit, and vapor pressure gradient (after Thornthwaite 1940). 



Water Water 



^nctoT evaporates condenses 



habitats or to activity only in times of high humidity. 

 Adult insects and other arthropods, reptiles, birds, 

 and mammals have evolved body surfaces of chitin 

 and waxes, scales, or cornification of the surface 

 layers of the skin that largely prevent uncontrolled 

 loss of moisture. Moisture loss through the respiratory 

 surfaces in these forms remains considerable, how- 

 ever. Water is also lost with the feces, although in 

 some species much water is reabsorbed by the large 

 intestine before the feces are ejected. The amount of 

 water removed from the body by excretory organs, 

 particularly the kidneys, varies directly with the 

 amount of water intake and inversely with the amount 

 lost through other devices. The kidneys are critical 

 to maintenance of proper concentration of salts in the 

 blood and body fluids. It is important that water in- 

 take balance water loss. Organisms are very sensitive 

 to disturbances in body water balance, and this factor 

 is very significant in determining the type of niche 

 which a species comes to occupy. 



In order that animals could exist in terrestrial 

 habitats, they had to acquire the ability to carry on 

 reproductive activities in the absence of water. The 

 chief reproductive adaptations involve the following 

 (Pearse 1930) : internal fertilization; a shell covering 

 the egg to conserve moisture and salts: food pro- 

 vision to the embryo and young, by yolk in the egg 

 cell, placenta in the uterus of the mother, or direct 

 feeding by the adults ; reduction in number of young 

 with more efficient parental care ; reduction or elimi- 

 nation of free-swimming larval stages; greater seg- 

 regation of species into different niches to avoid 

 interspecific disturbances. 



Temperature 



Aquatic animals are not ordinarily subjected to 

 temperatures below freezing and are in a relatively 

 stable temperature environment, but terrestrial spe- 

 cies are exposed to highly variable temperatures that 



may reach extremes of about — 68°C and +55°C. 

 No single species is required to withstand such a 

 wide range of temperatures, however. Optimum and 

 tolerance limits vary from one species to another, 

 inasmuch as each inherits a specific degree of acclimati- 

 zation. 



No aquatic species has evolved control over its 

 body temperature. Aquatic warm-blooded mammals 

 and birds are derived from terrestrial forms. An abil- 

 ity to maintain a constant body temperature has sur- 

 vival value in terrestrial habitats, however, and 

 consequently physiological mechanisms for homoio- 

 fhcrmisin developed independently in birds and mam- 

 mals. All other land organisms are poikilothermal; 

 that is, they have no physiological mechanism for 

 maintaining a constant body temperature. Some 

 poikilotherms, such as bees, have developed special 

 behavior patterns, that enable them to maintain fairly 

 constant conditions in the hive by cooperative 

 efforts ; some lizards, snakes, and turtles are able to 

 exert some control over their body temperatures by 

 moving into and out of sunlit areas. 



Rates of activity, food consumption, metabolism, 

 growth, and other physiological functions increase, 

 to a certain limit, with rise of body temperature. 

 Homoiotherms maintain a continuous high rate of 

 functioning because of their constant high body tem- 

 peratures, but the rate at which poikilotherms func- 

 tion varies with the temperature of their habitats. 



Latitudinal distribution of both poikilotherms and 

 homoiotherms is often limited northward and south- 

 ward by the extremes of temperature that they can 

 tolerate. The rate of energy exchange in poikilo- 

 therms is so directly dependent on the amount of 

 heat in the habitat that the total growth and repro- 

 duction of a species may be determined by the extent 

 to which it can accumulate developmental heat tmits 

 during the year (Shelford 1929: Chap. 7) ; the prin- 

 ciple is similar to the principle of heat budgets in 

 lakes. The closer a region lies towards either Pole, 

 the shorter the growth season is, and distribution may 

 be limited not by extreme low temperatures as such, 

 but by accumulation of heat energy insufficient to per- 

 mit completion of life cycles. 



The relation of energy balance in homoiotherms 

 to air temperature is even more complicated (Ken- 

 deigh 1949, Seibert 1949). In cold regions an ani- 

 mal may require all the energy its food provides it 

 simply to maintain its own existence, no surplus 

 available to meet the high demands of reproduction. 

 Under such conditions, a species cannot become per- 

 manently established in a region. A warm-blooded 

 animal requires a range of temperature that is com- 

 fortable and in which it can ingest and metabolize 

 food at a rate sufficient to maintain normal body tem- 

 perature, sustain physical existence, and carry on 

 reproductive activities, too. We can thus speak of 

 existence energy and productive energy, concepts es- 



98 



Habitats, communities, succession 



