total number of species in each community were usually forbs. 

 The great variety of species in some stands, 86 in the Andropogon 

 gerardi prairie type, was possible because the resources of the 

 habitat were ample and because differences in life-form and sea- 

 son of maximum needs made association of diverse species 

 possible. When the number of species was very low, as 12 per 

 stand — averaging only 3.7 per square meter — it seems that more 

 kinds could not grow because their requirements could not be pro- 

 vided by the habitat. These results support a probable rule that 

 diversity of species and productivity are related to favorableness 

 of environmental conditions, and the greater the number and 

 diversity of species the greater will be the number and kinds of 

 interrelations (reactions and coactions). 



In alpine communities in the Arctic and Subarctic the same 

 species often occur in many different communities, but the pro- 

 portions of the various species change from one community 

 to another. In one type of community one species is dominant, 

 in a second type another is dominant, but the first species may 

 also be present. The vegetation is not at all a chaotic assemblage. 

 Similarly in grasslands in the Great Plains, blue gramagrass 

 {Bouteloua gracilis) and western wheatgrass {Agropyron smithii) may 

 both be present in two community types, but the former is dom- 

 inant in one, the latter in the other because of soil differences, 

 which may, however, cause only a slight change in the habitat as 

 a whole. Even with a low number of species per stand many 

 kinds of communities can be formed. For example, on the Cor- 

 nish serpentine, only ten species were found to play an important 

 part in determining the physiognomy and structure of the heaths 

 as a whole, although 1023 combinations of them are possible. 

 However, only four heaths are of cardinal ecological significance 

 in this region, namely, Festuca ovina-Calluna vulgaris (rock heath). 

 Erica vagans-Ulex europaeus (mixed heath). Erica vagans-Schoenus 

 nigricans (tall heath), and Agrostis setacea (short heath). ^i 



Six community types were recognized in the aspen groveland 

 mosaic in Glacier County, Montana, on the basis of frequency, 

 vitality, constancy, presence of species, and the age and height 

 of the trees. '3^ The endless number of variations of stands in the 



72 • Species and Populati 



