12°C; thereafter the adult population dies. Snails 

 born in the spring may reach sexual maturity by 

 autumn but oviposition is normally delayed until 

 spring because of cold weather. The life span is 

 usually 12-13 months, but may be prolonged if de- 

 velopment is interrupted by aestivation resulting from 

 the drying up of the pond during the summer 

 (DeWitt 1955). 



Many warm-water pond fish, such as the black 

 bass and sunfish (Breder 1936), spawn in nests or 

 redds prepared in shallow water by removing all 

 debris and vegetation over circular areas of one-half 

 to one meter diameter. There is some preference for 

 gravel and sand bottoms when they are available. 

 The male remains to guard the several thousand eggs 

 during the few days required for their hatching, and 

 the fanning movements of his tail and fins doubtless 

 help to aerate them. He may also guard the young 

 until they can take care of themselves. Both bullhead 

 parents guard the egg masses and keep them con- 

 tinually agitated for aeration ; they may even suck 

 the eggs into their mouths and expel them forcibly. 

 The adults keep the young in compact groups by 

 swimming about them. The European carp may 

 spawn promiscuously a half-million or a million eggs 

 during the early spring. The eggs settle in the water 

 and adhere to the roots and stems of vegetation there. 

 The eggs are not guarded, and the young are left to 

 care for themselves. 



Salamanders commonly hibernate in humus, un- 

 der logs, Qr in other nooks or crevices on land. They 

 usually emerge during the first warm rains of early 

 spring and proceed to the nearest pond, there to lay 

 their eggs. The males deposit their spermatophores 

 on submerged leaves or twigs from whence the fe- 

 male picks them up for fertilizing the eggs. The eggs 

 are laid in jelly-like masses and require several days 

 to hatch if the temperature is low. The eggs of 

 Ambystoina maciilatitm (Gilbert 1944) and A. tex- 

 anum (Burger 1950) hatch more successfully and at 

 a faster rate if they contain unicellular green algae 

 within the capsule. These algae apparently create a 

 symbiotic relationship for oxygen and carbon dioxide. 

 Larval salamanders possess gills, but in all but a few 

 forms these are later absorbed and the adult returns 

 to land. A. tigrimim sometimes breeds while still re- 

 taining the larval gills, and never leaving the water. 



Frogs commonly hibernate in the mud at the bot- 

 tom of ponds, although some forms, including toads, 

 hibernate in the soil on land. In the spring the males 

 go to small bodies of water where their loud choruses 

 attract the females for mating purposes. The jelly- 

 like masses or strings of eggs require only a few days 

 to hatch, but the tadpole stage lasts longer. Meta- 

 morphosis in toads that lay their eggs in temporary 

 ponds takes place rapidly, but in other species, such 

 as the bullfrog, adults do not occur until two years 

 after the eggs are laid (Wright and Wright 1933). 



Practically all species of birds characteristic of 

 northern latitudes that nest in the marsh are migra- 

 tory, as the freezing of the water and drying of the 

 vegetation eliminate their food supply. Nests are lo- 

 cated in a variety of situations : on floating masses 

 of plant debris built above the water level, typical 

 of grebes, terns, gulls, black-necked stilt, and ducks ; 

 in plant material, placed in tufts of vegetation or 

 formed into platforms, or nests attached to cattails 

 and other emergent plants well above the water level, 

 typical of cranes, gallinules, rails, avocets, snipes, 

 bitterns, ibises, marsh wrens, swamp sparrows, and 

 blackbirds ; in swamp shrubs, typical of flycatchers 

 and yellow warblers ; in holes in trees, typical of tree 

 swallow, prothonotary warbler, and wood duck; in 

 the tops of trees in adjacent forests, typical of herons, 

 cormorants, egrets, and wood ibis. 



The muskrat is one of the most conspicuous and 

 important mammals of both salt and fresh-water 

 marshes as well as river banks. Along rivers, the 

 animal lives in burrows that it excavates well back 

 in the bank. In marshes, it constructs a dome-shaped 

 lodge, as high as one meter, by heaping up freshly- 

 cut marsh vegetation. The lodge is hollow and dry 

 within, the floor is placed well above the water level. 

 The lodge has several underwater entrances and ex- 

 its. In it the animal cares for its young and finds 

 protection from enemies and weather in both winter 

 and summer. In addition to lodges, the muskrat con- 

 structs shelters, where it may feed out of sight of 

 enemies, and breathing holes, called push-ups, 

 through the winter ice. 



BIOMASS AND PRODUCTIVITY 



In a pond in Iowa, the average summer 

 population of bottom invertebrates in water 0.5 m 

 deep averaged 3819 individuals, 1334 mg/m-; in 

 water 1.5 m deep, 1540 individuals, 1370 mg/m^. In 

 the shallow water the most important components of 

 the biomass were, in descending order: snails (shells 

 removed), midge fly larvae, annelid worms, and the 

 amphipod HyaleUa. In the deeper water the biomass 

 was mostly midge fly larvae (Tebo 1955). Produc- 

 tivity of the midge fly Tanytarsiis, one generation per 

 year, averaged 7.5 g/m^ in a Michigan lake (An- 

 derson and Hooper 1956). By mooring a floating 

 cage over open water throughout the season in an 

 English pond, a total of 8988 midge flies and other 

 insects per square meter were caught as they emerged 

 from the bottom mud. In shallow water, where the 

 vegetation was thicker, a total of 5979 individuals per 

 square meter were captured, a total which included 

 fewer midge flies and more dragonflies and caddis- 

 flies (Macan and Worthington 1951). 



Average standing crops of fish in backwaters and 

 oxbows may be almost 500 lbs/acre {S7 mg/m^), 



90 



Habitats, communities, succession 



