NO. 2226. 



DISTRIBUTION OF ENTOMOSTRACA—DODDS. 



79 



equally clear that one pair belongs primarily to the alpine and the 

 other to the montane zone. These two sets of species are nearly 

 mutually exclusive, the conditions necessary for the one being so 

 different from those demanded by the other that it amounts essen- 

 tially to mutual repulsion. This is especially true of the two species 

 of Diaptomus and to a marked degree also of Diaptomus shoshone 

 and DapJinia longispina. A third species of importance is Diaptomus 

 coloradensis (19 lakes), found also in the alpine zone, which seems 

 about equally well at home in either zone and to have about equal 

 relations to each of the two combinations. It is to be noted, how- 

 ever, that in neither zone is it so abimdant as the definitive Diap- 

 tomus of that zone. 



The significance of such combinations of species as the above is 

 that they may be used as a measure of ecological conditions. In 

 our area Diaptomus shoshone and Daphnia pulex are ecologically 

 similar, as are also the corresponding members of the montane pair, 

 and the two pairs are ecologically dissimilar, though the lack of 

 similarity is not the same in degree between all of these species. 

 Though we are unable to measure in physical and chemical imits 

 the complex of conditions required for any of the above species, the 

 frequency of their association together gives us an index for the 

 measurement of the similarity of the conditions demanded. Con- 

 ditions required by two species may be so similar that one is seldom 

 found without the other, or they may be so unlike that they are as 

 mutually exclusive as if one actually repelled the other. I have 

 reduced to percentages the frequency of association of the members 

 of these two pairs and also of Diaptomus coloradensis, which is a 

 frequent associate of both. In Table 5 are shown the association 

 percentages of each of these species in the alpuie and montane zones. 

 It is read as follows: Daphnia pulex is fomid in 45 lakes, in 27 per 

 cent of which Daphnia longispina is found, in 20 per cent Diaptomus 



leptotus, etc. 



Table 5. — Association percentages. 



Name. 



Daphnia pulex 



Daphnia longispina 



Diaptomus leptopus. . .. 



Diaptumus shoshone 



Diaptomus coloradensis . 



Daphnia 

 pulex. 



Daphnia 

 longi- 

 spina . 



27 



Diapto- 

 mus lep- 

 topus. 



Diapto- 

 mus sho- 

 shone. 



Diapto- 

 mus colo- 

 radensis. 



Number 

 of lakes. 



From the above table the high association percentages between 

 the two members of each pair are evident as well as the low correla- 

 tion between the two species of Diaptomus or between Diaptomus 

 shoshone and Daphnia longispina. Such figures as the above are 

 useful in giving other sorts of infonnation about the inter-relations 



