THE ORIGIN AND GROWTH OF PLANT COMMUNITIES 595 



ecological strategy of a species could well be defined in terms of its 

 phytosociological responses. On a later occasion, in a symposium on 

 "the impact of population studies on ecology" (A.I.B.S. meeting, 

 Bloomington, Indiana, August, 1958), I attempted to show dijfferent 

 degrees of ecosystematic fitness by using synecological units as a meas- 

 ure thereof. The present purpose calls for a restatement of these issues. 



Population-community adjustments are of several kinds and of 

 many degrees. I have suggested (1956b) that there are four principal 

 thresholds to be recognized: (1) survival; (2) local abundance; (3) 

 euryvalence; (4) dominance. The fourth step does not necessarily 

 require the third; indeed, there are many examples to the contrary. 



For instance, considering the species of the Aceretum saccharo- 

 phori laurentianum, the following gradation can be recognized: 



A. Dominant species are those that have achieved, in their layer, 

 an absolute or relative monopoly of space: Acer saccharophorum 

 (two upper layers); Erythroniiim americanum, CImjfonia coroliniana 

 (lowermost layer in the spring); Osmorhiza claytoni (second layer in 

 the summer); Viola pennsylvanica, .Solidago flexicaulis (lowermost 

 layer in the summer). A dominant has achieved the maximum eco- 

 logical success. 



B. Subdominant species are those that occupy less space than 

 the dominants and yet a good deal more than any of the remaining 

 species: Fagiis grandifolia (in the uppermost layer); Dicentra cuciil- 

 laria (in the lowermost, in the spring). A subdominant is held in 

 check by the dominant. 



C. Constant species are those which, however small their popu- 

 lation, are usually present: Tilio americana, Cornus alfernifolio, Sam- 

 hucus puhens, Viburnum lantanoides, Desmodium glutinosum, Smi- 

 lacina racemosa, Botrychium virginianum, Carex plantaginea, Carex 

 arctata, Sanguinaria canadensis. A constant seems assured of wide- 

 spread persistence. 



D. Companion species are not out of place, in that their usual 

 ( "normal"? ) habitat does not differ too markedly from the Aceretum 

 saccharophori laurentianum, but they are not constant, although they 

 can occasionally be very abundant: Eupatorium rugosum, Laportea 

 canadensis, Actaea pachypoda, Tiarella cordifolia, Dryopteris spinii- 

 losa, Trillium erectum, Streptopus roseus, Dentaria diphylla, Uvu- 

 laria grandiflora. A companion is almost always euryvalent and there- 

 fore not disturbed by shifting features in the environment. 



E. Accidental species are present only because of chance dis- 

 persal, local disturbance, or microtopic variation; they do not "be- 

 long," in fact are usually characteristic of some other association or 

 at least indicators of some other ecological condition (e.g., more 



