TABLE 5-4 Food habits of immature stream insects in Yellow- 

 stone National Park, Wyoming (Muttkowski and Smith 1929). 



Number of Per cent food 



specimens types consumed 



examined Animal Plant Detritus 



contain more organisms, however, in spite of lower 

 densities per unit area, because they have a much 

 larger total bottom surface. The reason for this vari- 

 ation in density per unit area is not clear, but it may 

 be that per given population of sexually mature adult 

 insects in the surrounding region, small streams offer 

 less area than large streams, over which the females 

 can spread their egg-laying. 



The standing crop of fish in Indiana streams 

 varies from 5.2 to 106 g/m-' (46-939 lbs/acre) wet 

 weight for minnows, suckers, centrarchids, darters, 

 and bullheads (Gerking 1949) to 2.7-4.2 g/m^ (2^ 

 37 lbs/acre) for rock bass (Scott 1949). The fish 

 crop in warm water streams is generally higher than 

 in cool trout streams, a relation that also holds for 

 the biomass of invertebrates (Pennak and Van 

 Gerpen 1947). Fish are usually more abundant in 

 relatively deep streams than in shallower ones. Brook 

 trout and three other species in one stream in New 

 York State averaged 10.9 g/m^ (97.5 lbs/acre), a 

 ratio of 1 :2.1 to the invertebrate food supply (Moore 

 etal. 1934). 



Of a stream, the richness of a fauna and the size 

 of the biomass that develops depend largely on the 

 fertility and chemical composition of the water. 

 Hardwater streams, with an abundance of salts in 

 solution, tend to have a large and more varied fauna 

 than do softwater streams. Calcium salts, in particu- 

 lar, are required by mollusks for building their shells, 

 and by crayfish for the exoskeleton. The salts and 

 organic matter which are basic substances in all 

 aquatic food chains depend directly on the fertility 

 of the soil over which the water drains. Streams 

 draining areas of fertile soil usually have an abun- 

 dance of stream organisms ; biomasses of both in- 

 vertebrate organisms and fish in streams occurring in 

 areas of poor soil are low. 



The productivity of insects in Algonquin Pro- 

 vincial Park, Ontario, was periodically measured 

 during one summer by collecting, in cages a yard 

 square, all insects as they emerged from the water 

 and transformed into adults. The count varied over 

 different kinds of bottom between June 1 and August 

 31, 1940, as follows: rubble 6603, gravel 1636, sand 

 1079, mud 2618 individuals per sq m. Various moun- 

 tain streams in different parts of the country have 



TABLE 5-5 Relation between numbers per m' and biomass of 

 Insect groups in a riffles of a California coastal stream during 

 February and March (after Needham 1934). 



Total Wet weight 



number of 



Individuals Per cent Grams Per cent 



Insect 



43.9 

 28.0 



Caddisfly larvae 



and pupae 742 22.2 5.66 



Mayfly naiads 1,853 55.5 3.61 

 Fly larvae and 



pupae 343 10.3 1.02 7.9 



Stonefly naiads 260 7.8 1.58 12.2 



Miscellaneous 137 4.1 1.02 7.9 



Totals 



3,335 



12.89 



shown an annual productivity of trout taken by fish- 

 ermen of 2.2 to 3.9 g/m- (20 to 35 lbs/acre) wet 

 weight (Surber 1937). 



APPLIED ECOLOGY 



The chief problems in applied ecology of 

 streams are those of erosion and silting, pollution, 

 and maintenance of biotic productivity at the highest 

 possible level. 



Erosion and silting 



Stream erosion becomes considerable when up- 

 land vegetation is so reduced that there is little or no 

 retardation of runoff from heavy rains. Dredging and 

 stream straightening for drainage purposes usually 

 eliminates the riffles habitat. The bare, hard clay that 

 often emerges as the new stream bottom supports 

 very little animal life. 



Continuous erosion throws a heavy load of fine 

 silt into the stream. This is detrimental. It makes 

 the water opaque ; reduces or prevents photosyn- 

 thesis in algae, water moss, and other plant life ; 

 handicaps those fish and other animals that depend 

 on sight for finding and capturing food : and clogs 

 the filtering mechanism of various invertebrates. 

 Clams are ordinarily closed less than 50 per cent of 

 the time, but in silted waters they may stay closed 

 up to 95 per cent of the time. Clams secrete mucous 

 to keep the mantle cavity cleansed, but when silting 

 is heavy this may not be sufficient and mortality will 

 result (Ellis 1936). Deposition of silt on rock or 

 sand bottom may bring a considerable change in spe- 

 cies composition of animals present. During the last 

 several decades, greatly increased soil erosion in agri- 

 cultural areas has reduced pan and game fishes in our 

 streams, and rough fish, such as carp, have taken 

 their place. 



Chronically muddy streams may often be cleared 

 by reforesting the watershed, and by practicing mod- 



56 Habitats, communities, succession 



