564 



THE COMMUNITY 



Besides these and other physical factors, 

 biotic influences are continuously at work. 

 These are intracommunity events, in which 

 the resident organisms bring about changes 

 within their community, the accumulative 

 eflFect of which is a potent factor in produc- 

 ing, retarding, accelerating, or altering the 

 rate and/or course of the serai changes. 



These biotic changes are embodied in 

 the special, technical sense of the term 

 development in use by many ecologists. For 

 example, Shelford (1931) employed the 

 term "development" as the growth taking 

 place within the community, where no suc- 

 cession occurs; Phillips (1934, 1935) used 

 "development" for the growth of com- 

 munities, both where no succession occurs 

 and where it is taking place. 



type of community gradually develops. Here 

 we should remember that the change from 

 one type of community to another usually is 

 gradual, over a long time. 



Both the development within the com- 

 munity and the serai sequence of com- 

 munities can be followed, or modified, 

 under the artificial conditions of the labora- 

 tory. This is best exemplified in the se- 

 quence that takes place in the culture jars 

 of protozoans, where the successive stages 

 are consummated in a relatively short span 

 of time, and the changes are strictly biotic 

 (developmental). From the standpoint of 

 protozoology, it is both pertinent and 

 feasible to study the interaction of ex- 

 cretion products and available food of pro- 

 tozoans with the species population, and 



Table 51. Protozoan Sequence in Hay Infusions (From Allee, 1932, after 



Woodruff, 1912) 



In the long run, it is obvious that, given 

 sufficient biotically induced change, the oc- 

 cupied habitats will become so altered that 

 the contemporary residents will languish 

 and finally disappear. Their activities serve 

 to change the environment on an ever- 

 widening front, until all parts of the com- 

 munity are afiFected. The falling foliage, 

 flower parts, fruits, seeds; excretions, feces, 

 dead and decomposing bodies of plants and 

 of animals; the aeration by burrowing; in- 

 crease in humic acids, building up of soils, 

 extraction of mineral substances; changes 

 in light, temperature, evaporation rate, and 

 relative humidity through plant growth— 

 these are but a few of the many events 

 that tend to alter the community. 



Eventually, the constituents are unable to 

 tolerate the changed conditions, or other 

 species can tolerate these conditions more 

 efficiently. Given a sufficiently long enough 

 time, many populations die or emigrate. 

 They either drive themselves out, or are 

 driven out by other organisms, and a new 



the effect of such items on different species 

 populations. The stimulating work of Wood- 

 ruff (1911, 1912, 1913) involved the study 

 of changes in the protozoan fauna of hay 

 infusions. There was a regular sequence 

 of species populations, each population 

 entering a period of growth, followed by a 

 period of decUne, until a relatively stable 

 end product was reached. A typical se- 

 quence involved (1) a pioneer stage, char- 

 acterized by a transparent medium, (2) 

 the medium becoming cloudy as a conse- 

 quence of the rapid multiplication of bac- 

 teria, (3) a monad flagellate stage, (4) 

 ciliate protozoans of the genus Colpoda. 

 (5) hypotrich cihates, (6) paramecia, and 

 (7) stalked ciliates of the genus Vorticella. 

 At times (8) amebae persisted, but often 

 these protozoans did not appear, whereas 

 the vorticellids generally appeared, and 

 would persist in some cultures as long as 

 counts were taken. 



Table 51 summarizes this sequence in 

 hay infusions. 



