630 General and Applied Biology 



the most desirable method of dispersal is lacking, an alternative method 

 may be used. If all usable types of dispersal are lacking, that particu- 

 lar animal or plant may be limited in its distribution. If certain types 

 of vegetation depend upon inoisture for dispersal, they may not be dis- 

 tributed during periods of extreme dryness. Certain types of seeds (burs, 

 etc.) which depend upon animals for their distribution may have little 

 or no distribution if animals are absent. The seeds of certain plants 

 may be distributed widely through the feces of birds. If birds are absent, 

 these plants must depend on alternative methods or not be dispersed at 

 all. 



2. Chemical Factors. — 



(a) Quantity and Quality of the Soil: The quantity and quality of 

 the soil affect not only animals living in the soil but also the aquatic 

 forms living in the water which necessarily comes in contact with the 

 soil. Certain soils, because of their high acidity, are not ideal for cer- 

 tain organisms, while alkaline or even neutral soils may be. Un- 

 doubtedly, the hydrogen-ion concentration (pH) of the soil is a vital 

 factor in the ecology of many animals. The hydrogen-ion concentration 

 of waters also is influential in determining their ecologic reactions. Cer- 

 tain chemical elements may either permit certain organisms to live in 

 that particular soil or cause them to select other habitats, depending 

 upon the quantity and quality of the chemical present. The quality and 

 quantity of the soil also determine the kind of vegetation growing on it. 

 Because certain types of vegetation are required as food, protection, and 

 concealment by certain forms of animals, the characteristics of the soil 

 may indirectly influence the ecology of the animals in that area. In ad- 

 dition to this, some plants supply moisture, oxygen, and homes for ani- 

 mals, and in this manner influence animal distribution. 



Earthworms do not abound in sandy soils because such soils contain 

 very little dead plant material to be used as food. The sand also irritates 

 the worm as it burrows through it and tends to roll into the tunnels made 

 by such worms, thus interfering with their movements and necessary 

 oxygen supply. The moisture content of sandy soils also may be a fac- 

 tor in earthworm distribution in them. Other types of soils which possess 

 a supply of available food, which are less irritable, and which retain the 

 tunnels efficiently are more frequently used as habitats by earthworms. 



Insects, turtles, and snakes deposit their eggs in soils of certain tex- 

 tures, temperatures, and moisture content. Decided variations from 



