BASES OF COACTION 105 



definite line can be drawn between the two types of coaction, the lichen 

 being a case in point. In groupings of two or more species, such as 

 certain ant colonies, the line between symbiosis proper and parasitism 

 is e^'en more vague. 



Objective or Purpose. It is sufficiently obvious that the most uni- 

 versal of coactions are concerned with shelter and food, directly or 

 indirectly. It is impracticable to evaluate the various coactions as to 

 relative importance, especially in general terms. The food relations 

 of the bottom fauna such as occur in the Danish waters are not all 

 significant coactions in the community, for many species feed 

 largely on detritus of remote origin. However, when these same species 

 are studied in other localities, their food relations take on more of the 

 character of community food coactions because they secure more food 

 from the living and dying plankton organisms. 



In the propagation of game birds, four essential conditions have 

 usually been noted, depending upon the circumstances, namely: (a) 

 cover or covert defined as shelter, (6) food, (c) suitable nesting con- 

 ditions, and (d) suitable climate. The last, though referring princi- 

 pally to the absence of extremes unfavorable to the species in question, 

 operates with reference to food and shelter and should rarely be con- 

 sidered independently of these relations. The enumeration of needs 

 of game, especially of deer and ciuail (Leopold, 1933), makes clear 

 that certain relations ai'e direct responses to habitat factors, and not 

 coactions or reactions in the strict sense. The deer requires open 

 spaces to play, and the quail selects sparse cover on a well-drained 

 spot with bare ground near by, where the young may dry out after 

 a rain. 



It is difficult to evaluate food, shelter, nesting and breeding site, 

 social relations, etc., in comparative terms, because they operate in 

 accord with the general principle of Liebig's law of the minimum, 

 which is equally applicable to maxima, as extended and restated by 

 Shelf ord (1913, a) as the law of toleration. In addition to operating 

 in this manner, the amount and kind of food and shelter, especially 

 with reference to specific requirements, vary with weather, climate, and 

 the presence of other organisms. In all studies of relations of species 

 to community and environment, it is necessary to seek a proper bal- 

 ance among these factors and resist the general tendency to over- 

 emphasize one or two. Food coactions have been more often studied 

 than other relations, and there is a large volume of records as regards 

 organisms of economic importance from which stomach contents can 

 readily be obtained and preserved in fluid. This has naturally limited 

 the studies to medium and small animals, such as birds smaller than 



