JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.Abstract. Invertebrates that graze periphyton growing on stones in coldwater streams in Michigan are at greatest risk of predation from a benthic-feeding fish (the mottled sculpin, Cottus bairdi) while on the top surface of substrates, where periphyton is most abundant, and during the day, when sculpin feed most actively. We conducted laboratory experiments with larvae of two strong exploitative competitors for periphyton (the mayfly Baetis tricaudatus and the caddisfly Glossosoma nigrior) to test if and how their foraging behavior was affected by presence of sculpin when sculpin were not allowed to attack prey. We incorporated four experimental factors in a factorial design: sculpin presence/absence, the abundance (low, high) of food that was patchily distributed on the top surface of substrates, grazer hunger level (fed, starved), and time of day (day, night).In feeding trials, Baetis was much more vulnerable than Glossosoma to sculpin predation. Similarly, only the foraging behavior of Baetis was strongly affected by the presence of sculpin. Baetis larvae responded to the presence of sculpin by: (1) significantly reducing the time spent on the top surface of substrates, and their movement rate while moving within and between food patches, and (2) significantly increasing the proportion of time spent in food patches, and the proportion of individuals that left the substrate by drifting downstream. Baetis larvae accepted greater risk of predation (by spending more time on top) when food abundance on top was high and when larvae had been starved for 27 h. Also, Baetis behavior was strongly affected by sculpin presence during the day but not at night. Collectively, these results suggest that Baetis larvae made adaptive compromises between feeding and avoiding sculpin. In contrast, Glossosoma behavior was not affected by the presence of sculpin. Glossosoma responded most strongly to alterations in food density and their own hunger level.Our results suggest that the presence of sculpin, even though the fish consumed no larvae, might indirectly affect the outcome of competitive interactions between Baetis and Glossosoma through the grazers' differential behavioral responses to the predator.

The mechanisms structuring benthic stream communities composed largely of highly mobile taxa are poorly known. I studied the role of competition in structuring the benthic community of hard substrates in a coldwater stream dominated by mobile insects that grazed upon periphyton and by relatively sessile filter-feeding insects. I expected competitive interactions among periphyton grazers to be significant and chronic because the community was not subject to frequent flow-related disturbances and grazer densities were high despite consistently low periphyton availability. Laboratory and field experiments were performed to determine: (1) the potential for, and relative strengths of, competition within and between two common, widely distributed grazers, the caddisfly Glossosoma nigrior and the mayfly Baetis tricaudatus, (2) the role of Glossosoma in affecting the organization of the community, and (3) the mechanisms of competition.Strong intraspecific effects on Baetis survivorship were observed in laboratory enclosures within the range of natural densities, but interspecific effects were not detected. Glossosoma survivorship was not affected by competition with itself or with Baetis. Individuals of both species experienced significant intra-and interspecific effects on growth, but the relative strengths of these effects differed between species. The effects of Baetis on Glossosoma growth were stronger than the effects of Glossosoma on itself. In contrast, Baetis growth was much less affected by the presence of Glossosoma than by conspecifics. Both species strongly depressed periphyton abundance, indicating that observed competitive effects resulted, in part, from exploitation.Exclusion of Glossosoma from habitats in the field resulted in a marked increase in periphyton biomass throughout a 1 0-mo study in which the movements of other taxa were not restricted. Most periphyton grazers responded to this increase in resource availability with increased densities in the Glossosoma exclusion treatment relative to the control. The size distributions of several chironomid grazers suggested that larval growth rates were greater in the absence of Glossosoma than with Glossosoma present. The larvae of two relatively small-bodied filter-feeders (Simulium, Rheotanytarsus) responded positively to the removal of Glossosoma, but larger bodied species (Hydropsyche, Brachycentrus) were not affected. There was not strong evidence for seasonal variation in the intensity of competition.Effects of Glossosoma on filter-feeders appeared to result from a form of interference competition, but there was no evidence for the operation of interference among grazers. Glossosoma had no effect on the colonization of habitats by grazers, and the feeding rates of Baetis and Glossosoma were not affected by the presence of conspecifics or heterospecifics.Contrasting patterns in periphyton abundance between the laboratory and field experiments suggest that, when their movements are unrestricted, Baetis departs periphyton patches at higher food densities...

1. Rapidly advancing geographical information systems (GIS) technologies are forcing a careful evaluation of the roles and biases of landscape and traditional site‐based perspectives on assessments of aquatic communities. Viewing the world at very different scales can lead to seeming contradictions about the nature of specific ecological systems. In the case of Michigan trout streams, landscape analyses suggest a predictable community shaped by large‐scale patterns in hydrology and geology. Most site‐based studies, however, suggest these communities are highly variable in structure over time, and are strongly shaped by site‐specific physical and biological dynamics. As the real world is comprised of processes operating both at local and landscape scales, an analytical framework for integrating these paradigms is desirable. 2. Decomposition of variances by factorial ANOVA into time, space and time–space interaction terms can provide a conceptual and analytical model for integrating processes operating at landscape and local scales. Using this approach, long‐term data sets were examined for three insects and two fishes common in Michigan trout streams. Each taxon had a unique variance structure, and the observed variance structure was highly dependent upon sample size. 3. Both spatially extensive designs with little sampling over time (typical of many GIS studies) and temporally extensive designs with little or no spatial sampling (typical of population and community studies), are biased in terms of their view of the relative importance of local and landscape factors. The necessary, but in many cases costly, solution is to develop and analyse data sets that are both spatially and temporally extensive.

Laboratory experiments were conducted with the periphyton—grazing mayfly Baetis tricaudatus to test the hypothesis that stream drift results from the passive removal of individuals from substrates. This hypothesis accounts for entry into the water column and diel drift periodicities, based on changes in the positioning of individuals on substrates and changes in their activity rates. A factorial design that incorporated food abundance (low or high), food distribution (low, intermediate, or high degree of patchiness), and forager hunger level (starved or well fed) was used to produce contrasting diel behavioral patterns necessary to test the hypothesis. Nymph behavior was monitored during the day and night using time—lapse cinematography and direct visual observations. Nymphs fed ad libitum had greater proportions of individuals on top surfaces at night that during the day. Starved nymphs showed little day—night variation in activity or positioning behavior, and their behavior during the day was similar to that of fed nymphs at night. Therefore, if drift—entry was passive, diurnal drift of starved nymphs should have been greater than or equal to nocturnal drift of fed nymphs in comparisons made within food levels. In all cases this prediction was not supported. These results refute the passive drift—entry hypothesis and support the alternative hypothesis of active entry into the water column. Drift patterns and foraging behaviors in these experiments and in an additional set of experiments were analyzed to identify proximate factors responsible for active drift—entry by Baetis. Drift was greatest at night and at the low food level. Periphyton patchiness and nymph hunger level did not significantly affect drift in the factorial experiments. In additional experiments, over a wide range of food abundance (2.5 x 105 to 107 diatom cells/cm2), the proportion drifting at night was significantly greater than zero when food was patchily distributed on substrates, but not when food was uniformly distributed. Based on these results and behavioral observations suggesting that food patches were less readily encountered with increasin degree of patchiness, I suggest that active drift is a component of Baetis food searching behavior and that individuals abandon habitats by enterin the water column when their estimate of habitat quality falls to some threshold level.