84 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



one of compensation for the acidity of decaying plant material as 

 well as for the acid casts, especially in view of the presence of cal- 

 ciferous glands in these organisms. The large amount of carbon diox- 

 ide given off in respiration must increase the acid reaction, both with 

 respect to the solution of minerals necessary for plants and the neces- 

 sity for neutralization. Salisbury, who has studied these relations 

 (1924), finds that earthworms are most numerous in soils approxi- 

 mately neutral, decreasing in the direction of both acidity and alka- 

 linity; they bring materials up from considerable depths, and the 

 general effect is to modify the horizon and diminish its organic con- 

 tent by mixing coarser material with it. As with ants, coaction is 

 often combined with reaction, and this is especially true of harvester 

 ants (Fig. 17, see page 83). 



In distribution, earthworms embrace practically all rainy portions 

 of the globe, from Iceland on the north through temperate and tropi- 

 cal zones to Kerguelen in the south, and upward to the alpine climax 

 of lofty mountains. In size, earthworms range from less than a foot 

 in temperate regions to a maximum of six feet in tropical and austral 

 ones, with a girth to correspond. As to number and density, Darwin 

 (1881:158) quotes Hensen to the effect that counts in a measured 

 space indicate a total of more than 50,000 worms to the acre in garden 

 soil and about half this number in fields. Their numbers are, how- 

 ever, greatest in rich cultivated soil, artificial meadows, and rich flood- 

 plain silt. Their numbers in original communities are relatively small. 

 In moist tropical areas these characteristics of earthworms may also 

 be reinforced by that of size, some species attaining lengths of three 

 to six feet, with corresponding relations to soil profile and the amount 

 of earth moved. 



Surface Disturbances. A host of animal reactions are exerted only 

 on the surface of the soil, or at most in the upper few inches. Obvi- 

 ously, no exact line can be drawn between these and holes or burrows, 

 since digging in some form is regularly involved. However, reactions 

 of this group may be characterized as those in which the surface alone 

 is affected, or in which breadth is increased at the expense of depth, 

 the depth usually being insignificant by comparison wdth the size of 

 the animal concerned. Such effects, though often of much interest in 

 connection with behavior, are rarely of significance in the habitat, 

 apart from the coaction upon vegetation. This is likewise true of 

 trampling, which, as a disturbance, belongs under compacting, in the 

 next section (Fig. 18). 



The more important reactions upon the surface are caused by 

 rooting, pawing, and trampling, chiefly by mammals. Rooting is char- 



