11 



Ecological Processes 



and Community 



Dynamics: 



Reactions, 



Soil Formation, 



and Chemical Cycles 



Tlie Iniildiiig up of s|)ecics ixjinilations in a new 

 habitat brings various reactions of the organisms with 

 tlie habitat wliich, together with coactions l)etween 

 different members of the community, l)ring about 

 succession of communities. We need now to examine 

 these processes. 



PLANT REACTIONS 



We have already considered, many of the 

 reactions of plants such as reduction of light and wind 

 intensities, mitigation of temperature extremes, inter- 

 ception of rainfall, and increase in relative humidity. 

 Plants also exert important effects on the formation, 

 structure, and characteristics of the soil or substratum 

 produced by accumulation of dead plant remains : they 

 further the weathering of rock through acid excretion 

 and the mechanical action of roots ; they offer obstruc- 

 tion to wind- and water-borne materials; they help 

 stabilize moving sand and talus slopes and help pre- 

 vent erosion generally ; they variously increase or de- 

 crease the water content of soil : they foster decom- 

 position of raw humus into usable nutrients, and so 

 forth. Water plants form marl. It is by these reac- 

 tions that plants exert dominance in terrestrial com- 

 munities, and establish the physical conditions of the 

 habitat which must be acceptable to all minor plants 

 and animals that dwell there. Succession of plant 

 stages eventually brings the interactions between 

 habitat and community into equilibrium upon the 

 establishment of the climax (Weaver and Clements 

 1938). 



SOIL FORMATION 



Texture, porosity, consistency, arrange- 

 ment of particles, chemical nature, and organic con- 

 tent of soils are determined by three sets of factors : 

 the parental rock material, the biota, and the climate. 

 Differences in topography modify the relative efifects 

 of these three factors, and plenty of time is required 

 before their full elTects are realized. 



The basic rock from which the mineral portion 

 of a soil is derived determines, to a large extent, not 

 only its chemical composition but also its structure. 

 For instance, soils derived from limestone are higHy 

 calcareous and more alkaline than soils derived from 

 sandstone. Clay soils are derived from feldspar ; sandy 

 soils, from quartzite. Clay forms a finely-textured, 

 compact, water-retaining soil. Sand is coarse-textured 



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