Ecologic Study of Living Organisms 631 



these desirable characteristics will influence the ecology of such animals. 

 Some types of soils are unfit for making burrows and nests so that rab- 

 bits, gophers, skunks, and similar forms may distribute themselves 

 where they may find habitats to their liking. Each plant requires an 

 adequate amount of soil of the proper quality (chemical composition) 

 to meet its specific requirements. In some instances the requirements 

 are quite definite and specific. In such cases plants will not be found in 

 soils which do not satisfy their peculiar needs. Cranberry plants will 

 grow only in acid soils and not in alkaline. Certain weeds can be elimi- 

 nated by merely altering the acid-base reaction of the soil. Dandelions 

 evidently are not so specific in their requirements, for we find them 

 growing in a great variety of soil environments. 



(b) Quantity and Quality of Water (Moisture): All living organ- 

 isms require water for various purposes, although the quantity which 

 is sufficient for one type may be excessive for another. Protective sub- 

 stances and structures may prevent excessive evaporation and thus per- 

 mit an organism to live in less than the normal requirement of moisture 

 after it has once secured its normal supply. The presence in water of 

 salts which are more or less ionized determines the acidity or alkalinity 

 of that water. Some organisms apparently are not aflfected by the 

 acid or base content (hydrogen-ion content), while others require ah 

 environment with a rather definite reaction. Hydrogen-ion concentra- 

 tion of 7 is known as neutral; that above 7, as alkaline; that below 7, as 

 acid. When animals which normally live in a certain hydrogen-ion 

 concentration are artificially transferred to an entirely different con- 

 centration, the animals may attempt to move out of the latter or they 

 may be killed. Certain animals are constructed with hard, nonporous 

 coverings, oils, or mucus in order to prevent excessive evaporation of 

 moisture. In arid regions one would expect to find such characteristics 

 in animals. 



. The transparency of the water permits the entrance of light which 

 not only affects the animal directly but also affects the growth of plants 

 upon which those animals depend for food, oxygen, and protection. 

 The depth of the water, its suspended materials, and its rapidity are 

 also factors in animal ecology. The pollution of waters with wastes and 

 obnoxious materials is a decided factor in the ecology of certain ani- 

 mals, while the same conditions apparently do not affect others. Cer- 

 tain industrial wastes are responsible for the elimination of fishes, snails, 

 clams, and Crustacea from certain streams. This waste not only influ- 

 ences these types of animals but also the many other organisms which 



