86 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



modified in structure, water and air content, and organic matter in 

 many cases, but to a much smaller or often imperceptible degree. 



Related to the surface as a source of material are the nest-building 

 activities of some birds, e.g., swallows and phoebes, and of a large 

 number of "mud daubers" and masons among Hymenoptera especially. 

 Here the amount of mud or pebbles removed and the consequent reac- 

 tion are inconsiderable, but when turrets are built, as by Anthophora 

 and others, the effect is often striking. 



Reaction by Compacting or Cementing Particles. While roots exert 

 a binding action upon coarse soils and hence compact them in some 

 degree, this consequence is much more characteristic of the activities of 

 animals. It is especially typical of the cursorial life habit, but is 

 also exhibited by fossorial species that make more or less definite 

 runways. The activity regularly concerned is trampling to such an 

 extent that the plant cover is largely or entirely destroyed and the 

 impact is then received directly by the soil surface, the resulting trail 

 often being converted into a gully by erosion. However, it is obvious 

 that compacting results in fine soils only in correspondence with the 

 amount of colloidal material present. In sand, the effect is the same 

 as that produced by trampling, owing to the lack of cohesion between 

 the grains. Other animal activities quite different in nature produce 

 much the same reaction as trampling, for example, rolling, bedding, 

 and wallowing, best illustrated by the "buffalo wallows" of the Great 

 Plains. However, all compacting reactions are narrowly circumscribed, 

 and they are much more striking than important. 



The property that humus possesses of acting as a weak cement and 

 thus binding together particles of soil has already been noted (Hall, 

 1908:47). It operates in practically all soils, but its consequences are 

 most pronounced at the two extremes, sand and clay. Humus is also 

 thought to be concerned in the formation of "ortstein" in heath sand, 

 its soluble fraction combining with mineral salts to produce an im- 

 pervious stratum. A somewhat similar result is to be found in grass- 

 land chmaxes, especially in semi-arid or arid regions, where the pene- 

 tration of water is restricted to the depth attained by the roots. The 

 immediate consequence is that the dry soil beneath becomes compacted 

 into a "hardpan," but this is usually secondary to the striking effect 

 produced by the accumulation of minerals at this level, leading to the 

 cementing of the particles into an impure limestone. In its turn, the 

 hardpan limits the downward growth of roots under normal rainfall, 

 but in times of excess it may be sufficiently softened and dissolved 

 to permit the passage of roots and the reformation of the hard layer 

 at a deeper level. 



