Food Habits of Stream Benthos at Sites of Differing Food Availability

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Abstract
The food habits of 17 spp. of macroinvertebrates were examined and compared with the potential foods present in a small stream in northwestern Colorado [USA]. The majority of common species were small-particle herbivore-detritivores, but Hesperophylax consimilis and Tipula commiscibilis were large-particle detritivores. Isoperla patricia was the most common invertebrate predator. Hydropsyche oslari was eclectic in food habits, but was primarily a large-particle herbivore-detritivore with carnivory exhibited only by late instars. Only Hydropsyche and Isoperla showed significant size-class differences in the composition of food ingested. No aquatic vascular plant material was identified in the gut contents of the species examined. Filamentous algae, although abundant at 1 location, were extensively utilized only as new growths or as decomposing fragments; epiphytic diatoms which extensively colonized algal filaments and vascular plant surfaces were an important food item. The use of t-tests following conversion of percentages to arcsin values showed that proportions of diatoms in the habitat and in the diets of macroinvertebrates were not significantly different. Nor did major dietary components differ significantly between sites despite considerable differences in food availability. Food ingested by herbivore-detritivores averaged 73% detritus, 22% diatoms and 5% filamentous algae. The similarity of food habits of herbivore-detritivores at different sites indicates that particle size, and possibly nutritive values, were the primary criteria determining food composition.

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