174 WET GROUND COMMUNITIES 



covered with sedges that we find marked differences in the ponds of 

 different ages. 



b) Vegetation choked temporary pond association (Stations 41, 42, 43; 

 Table XXXVII). — Sedges soon take possession of the bottom of such a 

 pond as we have been discussing. Just how long a time is required is not 

 known, though the pond which we are about to discuss is probably 

 several hundred years old. Here we find nearly all the groups men- 

 tioned as occurring in the younger ponds, but also certain ecological 

 types which are characteristic of sedge-bottomed ponds. Most notable 

 is the small green, flat, cigar-shaped worm {Vortex viridis) which usually 

 occurs in numbers, and a small brown species of Mesostoma similar 

 in form but brown in color. With them are often small larvae of 

 dytiscid beetles (species unknown), caddis-worms (Phryganeidae) with 

 cases made from pieces of grass (their relation to those in permanent 

 ponds is not known), and the snail (Lymnaea modicella). 



As such a pond grows older the sedge becomes thicker and other 

 plants make their appearance. What is known as low prairie develops. 

 At such a stage the small ponds like those we have been describing 

 usually become partially filled and so never contain the best development 

 of the older temporary pond community. We accordingly turn to the 

 later history of the ponds discussed in the preceding chapters, which 

 represent the best development of the temporary pond communities. 



In a forest climate when ponds are filled and drained they are 

 occupied by forest. In the steppe climate they are occupied by steppe 

 or prairie. In the forest border area, where our studies have been 

 carried on, some ponds when filled are occupied by prairies, others by 

 forest. Dr. Cowles is of the opinion that ponds with gently sloping 

 sides and bottoms become covered with prairie, while those with steep 

 slopes become covered with forest (Fig. 123). 



As ponds, such as we have discussed in the preceding chapter, 

 become ecologically old, they dry in dry seasons. They usually become 

 occupied by cattails, equisetum, or other grasslike plants. The red- 

 winged blackbird (Fig. 124) occasionally nests in them. At such a 

 stage the isopods (Asellus communis and Mancasellus danielsi), amphi- 

 pods (Eucrangonyx) (Fig. 113), and snails (Lymnaea reflexa) (Fig. 125; 

 compare with Fig. 104, p. 149). are common. The fringe-legged mosquito 

 (145) and the common marsh mosquito (Fig. 126) breed in such situations 

 while the crayfishes and various of the old-pond species continue. 



When such a stage is reached, it is only a step to the typical tempo- 

 rary pond. If the ground- water level is lowered, as is the case in many 



