maliaii predator of the marsh ; it is an enemy of the 

 nniskrat. Foxes, raccoons, and coyotes may invade 

 ilie marsh wlien the water level is low. The otter 

 jireys on fish and crayfish of the marsh ; it has now 

 heen exterminated from most of its former range. 



The beaver makes its own marsh habitat by dam- 

 ming small streams, flooding the surrounding low- 

 land. Here it builds its large lodge and feeds on the 

 bark and twigs of adjacent aspen, willow, and Cot- 

 tonwood, and on the roots of aquatic plants. When 

 the supply of aspen and other food is exhausted, the 

 colony disappears, the dam decays, the water level 

 subsides, and marsh vegetation invades. After some 

 years the pond is converted into a heaver meadow. 



In the southern Atlantic seaboard and Gulf states, 

 the herbivorous rice rat is common to marsh vege- 

 tation. In northeastern Ohio, the meadow mouse at- 

 tains populations averaging 58 per hectare (23/acre) 

 in the marsh and persists in smaller numbers in the 

 swamp shrub and swamp forest (Aldrich 1943). 

 The smoky shrew averages 8 per hectare (3/acre) in 

 the marsh. The cinereous shrew and short-tailed 

 shrew are found in marsh vegetation and are common 

 in the swamp-slirub stage (30 and 52 per hectare, 12 

 per acre and 21 per acre, respectively). AH three of 

 the insectivorous shrews are also found in the swamp 

 forest. The white-footed mouse increases in numbers 

 from the marsh through the swamp-shrub into the 

 forest stages (2,5-22-32 per hectare, 1-9-13/acre). 

 Moles, chipmunks, and squirrels also occur in small 

 numbers where the ground is drier. 



FOOD CHAINS IN PONDS 



Many animals in ponds depend for food on 

 floating phytoplankton, bacteria, and bottom detritus. 

 Ponds, however, unlike lakes have additional pro- 

 ducers of organic matter in the rooted pondweeds. 

 Attached to the submerged pondweeds is a peri- 

 phyton composed of bacteria, diatoms, and green and 

 blue-green algae. This periphyton is important as 

 food to many small crustaceans, immature insects, 

 oligochaete worms, and snails (Frohne 1956). Pond- 

 weeds are consumed by insects, ducks, and herbivo- 

 rous mammals. Filamentous algae are also more abun- 

 dant in ponds than in lakes, and is a source of food 

 to various immature insects and frog tadpoles. Creep- 

 ing predators among the pondweeds and algae are 

 leeches, dragonfly and damselfly naiads, and water 

 mites. Small swimming predators are the dytiscid 

 beetles, most of the hemipterans, and the swimming 

 leeches Erpobdella and Macrohdclla. At the top of 

 the food chains, feeding on all these small animals, 

 and often on plants as well, are the fish and other 

 vertebrate groups (Lindeman 1941). 



.SEASONAL (:ii.\N(;ks 

 AND TEMJ'OHAKV PONDS 



Seasonal changes are greater in a pond 

 than in a lake, a conse(|uence of the smaller volume 

 of water. Because of decreased rainfall, increased 

 evaporation, and continuous seepage or drainage, 

 ])onds often become greatly diminished during the 

 summer, or tiie 0])en water may entirely disappear. 

 As the volume of water shrinks, water temperature 

 rises, and the oxygen content and pH decline. Ani- 

 mals must either adjust to these conditions of the 

 jxjnd or disappear altogether. 



Under the winter ice, active pond life is slight 

 because of the low o.xygen and pH, but all groups 

 increase in numbers as the temperature rises during 

 the spring. In one small Chara-cattail pond near the 

 south end of Lake Michigan, the snails Amnicola and 

 Hclisoma dcflectiis. the amphipod Hyalella azteca, 

 the isopod Lirccus danielsi, the back swimmer Plea 

 striata, and diving beetles Haliplidae attained maxi- 

 mum populations during April and May, but then 

 declined in numbers through August, increasing again 

 in the autumn. On the other hand, the water strider 

 Gerris. mayfly naiads Ephemeridae, damselfly naiads 

 Agrionidae, dragonfly naiads Libellulidae, and the 

 snails Physa and Helisoma parvus had highest popu- 

 lations during June, July, and August (Petersen 

 1926). Maximum populations of Protozoa are also 

 attained during the warm period of the year (Wang 

 1928). 



Fish sometimes suffer from lack of o.xygen in 

 the winter when the ice cover lasts a long time. Dur- 

 ing the summer thermal stratification often forces 

 them out of the deeper stagnant water. When photo- 

 synthesis is curtailed at night or in cloudy weather 

 mortality may become high. 



Bird nesting is ordinarily completed by the time 

 marshes dry up in late summer. At this time ducks 

 concentrate in the remaining deeper bodies of water, 

 and other marsh species start their southward migra- 

 tion. 



Periods of drought are times of stress and in- 

 creased mortality for muskrats, since the lack of 

 water interferes with their normal locomotion and 

 predation upon them by invading terrestrial species 

 increases. Intraspecific competition becomes intensi- 

 fied as animals become crowded together in the 

 shrinking habitat. Many individuals undertake over- 

 land journeys to new areas (Errington 1939). 



Many invertebrates have spores or eggs resistant 

 to the effects of desiccation, enabled thus to pass over 

 the period during which the pond is dried up. These 

 include representatives of the protozoans, sponges, 

 hydras, turbellarians, nematodes, annelids, bryozoans, 

 rotifers, mollusks, crustaceans, and insects (Mozley 



Ponds, marshes, swamps, and bogs 87 



