THE ENVIRONMENT 



307 



occurs in the summer (aestivation). Also, it need not 

 be limited to winter or summer or to a regular cycle 

 of annual repetition. Unfavorable conditions, re- 

 gardless of annual cycle, can cause dormancy; so if 

 ecological factors fluctuate from year to year, an 

 animal might aestivate and hibernate in one year, not 

 become dormant in another, or might even become 

 dormant without strict summer or winter associa- 

 tion in yet another year. 



Dormancy can be restricted to brief periods of a 

 single day. A daily cycle of dormancy and activity is 

 common in ectotherms of localities having daily 

 fluctuations of temperature. Even in birds and 

 mammals, especially bats, daily dormancy cycles are 

 known. In these endotherms, the intensity of 

 dormancy is associated directly with the amount of 

 temperature reduction. Such daily cycles are called 

 diurnation. 



Clarification of the term endolherm is now possible. 

 Strictly speaking, the term should be limited to those 

 organisms whose body temperature is controlled by 

 internal body functions. Automatically this limits 

 attention to the birds and mammals, because no 

 other living creatures contain the specific controls 

 that diagnose true endothermism. However, birds 

 and mammals that become dormant no longer con- 

 trol their body temperature. In many respects, when 

 torpid, these "endotherms" are not different from 

 ectotherms. For this reason, there is a trend to 

 provide special recognition to the shifting from one 

 mechanism to another by applying the name hetero- 

 therm to those endotherms that become truly dormant. 



Organisms usually become dormant in places hav- 

 ing fairly uniform environments. Apparently the 

 factors that stimulate dormancy also cause an animal 

 to seek shelter. Good shelters are environments in 

 which temperature and other ecological factors do not 

 reach extremes that would be fatal to the dormant 

 animals. This requirement allows for a good deal of 

 variety in the actual sites of successful dormancy. 

 Almost any possible underground or underwater re- 

 treat that can be reached is used. Even above-ground 

 places like log interiors and rock or debris piles are 

 inhabited. 



WATER 



Organisms need water. Meeting this requirement 

 can be a problem, even in aquatic environments if 

 they have fluctuating water levels, little incoming 



water, or a high salt content. Fluctuating waters 

 often have much alluvium and debris during the wet 

 season and a low oxygen and food supply. They may 

 even become dry during the dry season. This pre- 

 vents the survival of many creatures, but certain 

 taxa (especially algae and protozoans) are fairly 

 successful by being active during the wet season and 

 dormant during the dry season. However, even these 

 creatures must have incoming water to replenish such 

 necessities as food and oxygen and to prevent waters 

 from becoming salty. Salinity is an absolute barrier 

 to most life, whether the saltiness is in the ocean or in 

 saline or alkaline lakes on land, because the water is 

 not readily separated from the salt. 



WATER NEEDS AND TRANSPIRATION 



In any land habitat insufficient water can eliminate 

 certain creatures from the area. This is most readily 

 appreciated in reference to a vascular plant process 

 called transpiration, a function involving almost 

 constant movement of water through active plants 

 and consisting of water being absorbed by the roots, 

 transported via vessels into the leaves, and lost 

 (transpired) from the leaf surfaces. In general, 

 transpiration rate is directly related to temperature. 

 Most plants transpire large amounts of water, a rea- 

 son why plant cover tends to restrict temperature ex- 

 tremes. Among the plants having a high transpira- 

 tion rate are corn and alfalfa. Both transpire many 

 times their dry weight each day — corn about six to 

 nine times and alfalfa about thirty-six to fifty-six. 

 These rates are much greater than known rates of 

 desert plants which lose only 1/100,000 to 1/175,000 

 as much water. 



Transpiration appears to be an adaptation that 

 cools a plant. If this is true, transpiration is analo- 

 gous to animal perspiration, both functions acting 

 to reduce body temperature. 



AVAILABLE MOISTURE 



The moisture in a given habitat comes from many 

 sources, primarily the different forms of precipita- 

 tion. However, the actual amount of water that is 

 available to an organism also is dependent upon 

 humidity, temperature, wind, sunshine, and other 

 factors within an environment (Figure 17.4). 

 These factors are more specifically related to avail- 

 able moisture through their influence upon evapora- 

 tion and transpiration rates. Lower humidity and 



