578 



INTERRELATIONSHIPS OF ORGANISMS 



numerous, and more rapidly reproducing parasites. Some of these rela- 

 tionships are just beginning to be understood, but instances of host- 

 parasite chains of three and four links are known. 



Succession. Another phenomenon that complicates the ecological 



relations of organisms is that known 

 as succession. This is the more or 

 less orderly progression of different 

 types and kinds of communities 

 that, in the course of time, come to 

 occupy any given local area. Gener- 

 ally speaking, succession is brought 

 about by two main sets of inter- 

 related causes: local geological pro- 

 cesses 1 and the changes in soil, 

 water level, light, humidity, etc., 

 that are produced by the activities 

 of the organisms themselves. 



When an area is first accessible 

 to terrestrial or to aquatic life, it 

 usually presents a rigorous and 

 difficult environment, with meager 

 food supplies and severe physical 

 resistance. Thus the sand dunes 

 that form on the leeward sides of 

 many sandy lake and ocean shores 

 are at first mere sand ridges, lack- 

 ing in many of the soil substances 

 required by most rooted plants, 

 subject to extreme fluctuations in 

 temperature, often desertlike in the 

 lack of available moisture, and still 

 liable to "blowouts" that expose 

 roots at one point and bury entire 

 plants at another. Very few organ- 

 isms can endure such violent fac- 

 tors, but certain drought, heat, and 

 wind-tolerant species are able to form a "pioneer community" here, 

 where they are free from the biotic resistance of less hardy competitors. 

 In time, however, as the pioneer community establishes itself, it begins 

 to effect changes in its environment. Organic material accumulates in 



1 Stream erosion with the production of new-made bars, banks, cliffs, and changes 

 in gradient; the shore deposits of lakes and oceans — bars, spits, dunes, and coastal 

 flats; the formation and the extinction of lakes, etc. 



Fig. 33.13. A longleaf pine fiatwoods in 

 northern Florida. This is a persistent sub- 

 climax community maintained by recur- 

 rent fires. It is characteristically developed 

 on Leon soils, in which there is a hardpan 

 at shallow depth; the soil above this layer 

 fluctuates from saturated, in rainy periods, 

 to very dry in times of drought. The 

 herbage and shrubs of the fiatwoods com- 

 prise only species able to survive in this 

 rigorous environment marked by extreme 

 variation in water supply. The pine roots, 

 however, penetrate the hardpan and reach 

 permanent moisture. If fire is kept out 

 these fiatwoods undergo succession toward 

 the regional climax, which is a broad-leaved 

 evergreen "hammock"; but fire is a normal 

 and frequent ecological factor in this type 

 of environment. (Photo taken in Dixie 

 County, Fla., by J. Speed Rogers.) 



