348 



POPULATIONS 



this definition is a convenient statistical in- 

 dex, it leaves much to be desired from the 

 ecologists' point of view. For example, when 

 a density response is demonstrated, the 

 question arises: What actually brings about 

 this observed eflFect at the operational level? 

 Information pertinent to this question gets 

 at the core of density-dependent popula- 

 tion operations. There is a tendency in 

 the older literature to think of crowding in 

 "psychological" terms, to attribute causa- 

 tion to something unique in the number re- 

 lationships themselves. This has led at 

 times to a certain mysticism. Undoubtedly, 

 there are situations— as, for example, in hu- 

 man and other social groups— in which such 

 an explanation may be legitimate, but it 



action," or the influence of the population 

 on the habitat. A primary cycle could take 

 place between one organism and its habi- 

 tat. However, since grouping of organisms 

 is the rule in nature, the primary cycle for 

 any particular population would be con- 

 stituted by the summation of many actions 

 and reactions. 



There is a secondary cycle in addition to 

 the primary one. This is based on those 

 operations that come about as a result of 

 the grouping of the population members. 

 Clements and Shelford call these opera- 

 tions "coactions," by which they refer to 

 interorganismic relations along with such 

 reciprocal effects as these relations may 

 have with their habitat. 



^q-\^^2-^^^^At 



r 



COOPERATION 



COACTION -< DISOPERATION 



COMPETITION 



AGGREGATI0N = l + 2>3 + 4 



PROGRESSION IN SPACE -TIME 

 ^ 



(POPULATION LIFE HISTORY) 



Fig. 128. Schematic representation of the Clements-Shelford action-reaction-coaction operations 



within ecological assemblages. 



should not be invoked until other reason- 

 able possibilities have been excluded. 



There is some meaning in thinking of the 

 basic ecology of populations in the same 

 terms that Clements and Shelford (1939) 

 use for communities.* Their ideas in simph- 

 fied form are illustrated in Figure 128, 

 which should be referred to in the brief 

 discussion following. Habitat is defined 

 as the total, effective physical-chemical 

 environment. The concept is presented 

 that a primary cycle of cause and effect 

 results from an interplay between the 

 habitat and the population members in- 

 habiting it. This cycle consists of two recip- 

 rocal operations: "action," or the influence 

 of the habitat on the population, and "re- 



• Although we develop, define, and use in 

 part this system of classification, we are not 

 completely satisfied with its terminology, and 

 we recognize that it has limitations. 



Coactions influence the population (or 

 community) through (1) "cooperation," 

 which has survival value for at least a ma- 

 jority of the cooperating organisms; (2) 

 " disoperation," which has deleterious effects 

 on the coacting organisms, either through 

 influence on the habitat, on contiguous 

 group members, or both; and (3) "compe- 

 tition," which is an expression of the fact 

 that certain coactions are directed towards 

 exploiting an environment limited in its po- 

 tentialities. Competition may be either 

 favorable or unfavorable in terms of long- 

 range results. A fourth category, "tolera- 

 tion," is added and discussed in the section 

 on Evolution (p. 704).' 



This concept, when more closely related 

 to our present interest, suggests that while 

 density-independent phenomena involve es- 



* An "ecosystem" would be characterized by 

 interacting primary and secondary cycles. 



