368 



POPULATIONS 



physical, biotic, or both, should be studied 

 comprehensively and excluded before ex- 

 tramundane influences can be accepted; 

 and that, even in the event that local en- 

 vironmental conditions can not be shown 

 to shape the cycles, there is no justifica- 

 tion in assuming a control by sunspot activ- 

 ity until that too has been most vigorously 

 analyzed in its own right. 



THE "mixed-species" pboblem: 



INTERSPECIES COMPETITION 



Students of laboratory populations stress 

 an aspect of interspecies population analy- 

 sis designated as the "mixed-species" 

 problem. This problem is primarily dis- 

 tinguished by the way it is viewed instead 

 of by the techniques employed or the in- 

 dividual principles considered. Such studies 

 are by no means confined to the laboratory, 

 however, as Elton (1946) and Lack 

 (1947) have pointed out. 



In the study of such interspecies 

 phenomena as, say, predator-prey or host- 

 parasite operations attention is characteris- 

 tically focussed primarily on a single, 

 major interaction {viz., predation or para- 

 sitization) as that interaction relates to the 

 growth form of the two interacting popu- 

 lations. In mixed-species studies attention 

 is focussed on the growth form of the two- 

 species unit, irrespective of a particular 

 coaction (or action-reaction) selected on 

 the basis of prior knowledge or interest, 

 but usually involving competition for food, 

 niche, or space. Thus the first, or major co- 

 action, approach starts with the premed- 

 itated view that a certain function is highly 

 important, and the analysis follows, while 

 the second or mixed-species approach 

 starts with the view that what results when 

 two species are brought together is signif- 

 icant, with further analysis suggested by 

 the findings. Actually, both approaches are 

 productive and are not necessarily mutu- 

 ally exclusive. Thus Cause (1934, and in a 

 series of individual papers) works from 

 both points of view. On the one hand, he 

 sets up several kinds of laboratory, mixed- 

 species cultures and plots the resulting 

 growth curves of the two populations. On 

 the other hand, Cause's choice of experi- 

 mental material stems from a desire to ex- 

 plore selected operations. By a wedding 

 of the mixed- species approach (which to 

 date is largely a laboratory exploration) 



with the other approach. Cause studies in 

 micro-organisms 'mechanism of competi- 

 tion in yeast cells," "competition for com- 

 mon food in protozoa," and "the destruc- 

 tion of one species by another." The last 

 phase, involving the predation of Didinium 

 upon Paramecium, is reviewed on page 

 372. 



Mixed-species studies are particularly 

 well adapted to laboratory analysis be- 

 cause relatively simple populations can be 

 estabUshed that can be controlled and 

 manipulated according to a preconceived 

 plan. A particularly meaningful appUcation 

 of this approach obtains when two species 

 that occupy identical, or nearly identical, 

 niches are brought together as competi- 

 tors, and the influences of the two popula- 

 tions upon each other's growth form are 

 assayed. Such investigations, conducted 

 under laboratory conditions in which ac- 

 curate census counts are feasible, yield de- 

 pendable and interpretable, though simpli- 

 fied, knowledge about competition and 

 selection. They also exhibit a minimum of 

 artificiality because the problem is directly 

 related to more complex (and harder 

 to analyze) situations that exist among 

 natural populations. Since control popula- 

 tions of single species constitute an inte- 

 gral part of such studies, it becomes pos- 

 sible to diflFerentiate intraspecies from 

 interspecies operations, and owing to the 

 way the experiments are designed, to form 

 some judgment also as to the role of the 

 physical habitat. 



Such analyses can be extended either by 

 further, careful dissection of the factors 

 responsible for a demonstrated two-species 

 interaction, or several species that occupy 

 more divergent niches can be brought to- 

 gether in the same microcosm in order to 

 measure quantitatively the ecological gen- 

 eralization that competitive pressure be- 

 tween two locally associated species varies 

 in direct relation with the similarity of the 

 niches they occupy.* 



* Several mixed-species population studies 

 have already been dealt with earlier in this 

 section from various points of view. In Chapter 

 21 Cause's investigation (1935) of oscillations 

 between yeast and paramecia cultures were 

 discussed (Fig. 121). In the same chapter the 

 findings of Park, Cregg, and Lutherman (1941) 

 about extinction of Gnathoceros and Trogo- 

 derma cultures were reviewed briefly (p. 329). 

 In this chapter a number of points are made. 



