values of species in a stand, for conspicuous plants such as sun- 

 flowers or goldenrods may appear more widespread and abun- 

 dant than they actually are in comparison to other species. An- 

 other good example is seen in Iowa prairies where the blazing star 

 and prairie clover, with their extremely slender stems and leaves, 

 have little cover value although the appearance of the prairie at 

 certain seasons is caused by their high frequencies.'' 



Many species having low cover or population density also rate 

 low in frequency, but some may have high frequency because of 

 their uniform distribution. Usually, however, if the cover and 

 population density are high the frequency will be high. In nat- 

 ural communities the individual plants or animals tend to aggre- 

 gate because the offspring are more numerous near the parents 

 or in the more favorable habitats than elsewhere; in other 

 words, they are clumped (over-dispersed, showing contagious 

 distribution) (Figures 1-27 and 2-4). Organisms under some 

 conditions are regularly spaced (under-dispersed) such as grasses 

 set out on a sand dune or plants in a corn field (Figure 1-3). When 

 the plants or animals occur entirely by chance, as may happen 

 in a small, uniform area, the distribution is normal or at 

 random.^^^ 



Raunkiaer^^^ was the first to use frequency extensively. He 

 classified the occurrence of species in an area into five classes of 

 frequency: A, 1 to 20 per cent; 5, 21 to 40; C, 41 to 60; D, 61 to 

 80; and £, 81 to 100. On the basis of about 1350 frequency de- 

 terminations in various types of communities in Denmark, the 

 following distribution was found: A, 65 per cent of the total; B, 

 W; C, 1; D, 6; and E, l\. The classification of 8078 determina- 

 tions by several investigators in various countries gave the 

 following weighted average percentages: A, 53; B, 14; C, 9; D, 

 8; and E, 16. The normal distribution of the frequency percent- 

 ages, derived from such classifications, is expressed as A ^ B ^ C 

 = D <CE, and has been named Raunkiaer's "Law of Frequence." 

 The ratio is the result largely of the effects of the dominant species 

 which, by their superior competitive capacity, prevent others 

 from equalling them in frequency; but they cannot prevent many 

 species from invading some of the spaces. 



Analytic CHaracteristics of the Comnznunity • 113 



