ANIMALS AND THEIR ENVIRONMENT 



299 



worms, small bivalve mollusks, and the larvae of midges and of the 

 mosqiiitolike Corethra. Such a place would not seem favorable to much 

 life, yet Juday has found these animals make up a mass of over three 

 hundred pounds per acre on the bottom of a typical lake. 



The free-living population of a lake is subject to considerable fluctua- 

 tion in amount and distribution. There is a daily variation in distribu- 

 tion caused by the reactions of these animals to light. Since most of 

 them are positive to light, they accumulate at the surface during the 

 day and settle away from the surface at night. Reference has already 

 been made to this reaction in one of the parasitic Crustacea (page 285). 

 There is also great variation in the seasonal abundance of floating species. 

 The algae generally have one maximum each year, occurring in midsum- 

 mer, as have also certain protozoa. The diatoms, however, regularly 



Po/vD CommoNz if,^ Conditions 



I 



Submerged aquafK regefafion 

 Floating " •> 



I 



Fig. 264.- 



' Emergent 



-Section through pond and lake, showing contrast in extent of vegetation due 

 to difference in depth. (From Chapman, "Animal Ecology.^') 



have two maxima, in spring and fall, respectively. The animals which 

 feed upon these minute plants are naturally influenced by this seasonal 

 fluctuation. 



It will be observed that there is much overlapping in the general 

 kinds of organisms living in lakes and ponds, respectively. This is 

 largely due to the fact that the vegetated strip along the shore of a 

 lake is not very different from many ponds. The most characteristic 

 difference between lakes and ponds is in the swimming organisms. , Fishes 

 are common in lakes, but there are few ponds that contain them, and then 

 only certain species. Comparison of ponds of different ages shows that 

 the older the ponds the fewer the fish they harbor. In some regions the 

 amphipod crustacean Gammarus appears also to be a distinguishing 

 mark of lakes as compared with ponds. 



Streams. — Water in motion has characteristics, as the habitat of 

 animals, not possessed by standing water. The mechanical disturbance 

 which it offers is very considerable in young streams (that is, those 

 whose slope is steep) but much less in old streams. Soil may be carried 



