SYNTHETIC CHARACTERS OF THE COMMUNITY 



55 



The species-number: area curve is of more social significance. 

 As Kylin (1926) showed, it is influenced by three factors: the number of 

 species in the society, the size of the sample plots, and the density of the 

 individual species. Instead of the percentage of the number of species, 

 the absolute numbers of species are plotted as ordinates. Such a 

 curve shows not only the relation of the number of species to the size 

 of area but also the absolute number of species in the community 

 (Fig. 33). 



The curves given in the figure show that one of the two related 

 associations of the Caricion curvulae alliance has twice as many species 

 as the other and that the curve of the Festucetum halleri (which has 



eo-i 



<0 



SJ 50 



Festucetum halleri 



Size of sample plots ) sq. m. 



Fig. 33. — Curves showing the relation between the number of species and the 

 size of the sample plots in associations of the Caricion curvulae. {Braun-Blanquel and 

 Jenny.) 



more species) becomes horizontal later than the Curvuletum curve; 

 and thirdly they demonstrate what is the average number of species on 

 a given area of each association. Thus the species-number: area curve 

 becomes a most valuable sociological statement. 



In the Curvuletum the horizontal course of the curve begins with 

 areas of about 10 sq. m. and with about 30 species; but in the Festuce- 

 tum halleri the curve flattens with areas of 20 sq. m. and with about 52 

 species. In other words, the minimal area of the Curvuletum is 

 about 10 sq. m.; of Festucetum halleri, about 20 to 25 sq. m. 



Determination of Constancy. — If the working out of the essential 

 properties of communities were a purely mechanical process, it would 

 be necessary, for the determination of constancy, only to tabulate plots 

 of at least the minimal area and with a correspondingly high average 

 number of species. All smaller plots and those with fewer species 

 would be disposed of as fragmentary associations. 



