760 ANIMALS AND THEIR ENVIRONMENT 



egg, embryo, or some other j^eculiarly sensitive stage in the life cycle. 

 Most species of organisms are not even found in all the regions of the 

 world where they could survive. The existence of barriers prevents 

 their further migration and enables us to distinguish the major bio- 

 geographic realms (p. 735), characterized by certain assemblages of 

 plants and animals. 



Biologists were aware more than a century ago that each kind of 

 animal requires certain materials for growth and reproduction, and is 

 unable to survive if the environment does not provide a certain mini- 

 mum of each of the materials required. V. E. Shelford pointed out in 

 1913 that too much of a certain factor would act as a limiting factor 

 just as effectively as too little of it. Thus, the distribution of each species 

 is determined by its range of tolerance to variations in each of the en- 

 vironmental factors. Much ecologic research has been done to define 

 the limits of tolerance, the limits within which species can exist, and 

 the results have been very helpful in understanding the pattern of 

 distribution of animals and plants. One stage in the life cycle— perhaps 

 the larvae or eggs— is usually more sensitive to some environmental 

 factor and is effective in limiting the distribution of the species. The 

 adult blue crab, for example, can survive in water of low salt content, 

 and can migrate for some distance up river from the sea, but the larvae 

 cannot survive low salinity and the species cannot become permanently 

 established there. 



Some organisms have very narrow ranges of tolerance to environ- 

 mental changes; others can survive within much broader limits. Any 

 particular species, of course, may have narrow limits of tolerance for one 

 factor and wide limits for another. Ecologists use the prefixes steno- 

 and eury- to refer to species with narrow and wide, respectively, ranges 

 of tolerance to a given factor. A stenothermic organism is one which will 

 tolerate only narrow variations in temperature. The housefly, in con- 

 trast, is eurythermic, tolerating temperatures ranging from 43 to 113° F. 



Temperature. Temperature is an important limiting factor, as the 

 relative sparseness of life in the desert and arctic testifies. Even birds 

 and mammals with temperatures kept relatively constant by physiologic 

 thermostats and body insulation may be limited by extremes of tempera- 

 ture. Extreme heat or cold may limit their food supplies or act in some 

 other indirect fashion to prevent their survival. Most of the animals 

 found in the desert have adapted to the rigors of the environment by 

 living in burrows during the day and foraging only at night. Many 

 animals escape the bitter cold of the northern winter by migrating south- 

 ward or by burrowing beneath the snow. Measurements made in Alaska 

 show that when the surface temperature is —68° F. the temperature 

 two feet under the snow, at the surface of the soil, is -|-20° F. Animals 

 such as deer and elk that spend the summer in the high mountains 

 migrate to lower levels during the winter. Certain bats, rodents and 

 shrews survive the winter in a state of markedly reduced metabolism, 

 known as hibernation (p. 448). The body temperature falls to just a 

 degree or two above that of the surrounding air, metabolism is greatly 

 decreased, and the heart beat and respiration become very slow. No 



