ECOLOGY 



137 



a different fauna (see below). Accompanying Helohdella, but in 

 smaller numbers, one finds Erpohdella octoculata, Glossiphonia 

 complanata and G, heteroclita, Hemiclepsis marginata and Thero- 

 myzon tessulatum. The most abundant leech in soft waters is 

 normally Erpohdella octoculata, alone or accompanied by small 

 numbers of Helobdella, Glossiphonia complanata or Theromyzon. 

 The change from a leech fauna dominated by Helohdella to one 

 dominated by Erpohdella is illustrated in Fig. 86. 



Water 



Soft 



Intermediate 



Hard 



Productivity 



Dystrophic 

 r 



Oligotrophia 



Transitional 



Eutrophic 



\ 



w 



I \ 

 I \ 



\ Ponds 

 \ 

 \ 

 \ 

 \ 



\ / 



Lakes and tarn 





I 17 i 19 I 



3033 35 38i40! 2 45'47l49'52l53i 7 I 9l II I I3'l5ll71 I9I2I 

 32 34 36 39 43 44 3 48 50 6 54 8 10 12 14 16 18 20 



Station nunnbers 



Fig. 86. The relative numbers of Erpohdella octoculata and 

 Helobdella stagnalis in different types of standing water. Collec- 

 ting stations are arranged in order of total alkalinity. The 

 dotted line connects stations of a surface area less than 5000 

 square yards. From Mann, 1955. 



As was stated above small bodies of water tend to have a different 

 fauna from large ones having the same calcium content. One 

 feature in which they differ is in the amount of rooted vegetation 

 relative to the volume of water. Small ponds have a large proportion 

 of rooted plants and the products of their decay lead to a high 

 concentration of " humic acids " in the water (Tucker, 1958). The 

 effect of this accumulation, either directly or indirectly, is to 



