Species in complex ecosystems, such as streams, are influenced by multiple top-down and bottom-up control factors. Food-resource and competition/predation mechanisms are among the most influential factors controlling population ecology. Balancing the effects of bottom-up control by food resources and top-down control by competition/predation can trigger compensatory responses leading to changes in species ecology within the limit of their plasticity. These effects have been well studied in simple controlled conditions, but predicting their outcome is difficult for natural populations in genuine ecosystems because of the heterogeneity of environmental conditions and the complexity of biotic interactions. We investigated the ecological response in terms of feeding habits and population parameters of the stream-dwelling predator, Siphonoperla torrentium, to modifications of its prey abundance and of competition/predation mechanisms induced by a gradient of acidification. We conducted a 1-y survey in a set of 3 streams showing very similar abiotic conditions except for their acidification status. Despite significantly lower potential prey availability and higher potential intraspecific competition in the most acidic stream compared to neutral streams, the proportion of prey in the diet of S. torrentium did not differ across streams. However, S. torrentium was less aggregated, and had higher secondary production, longer larval development, and smaller adult size in the most acidic stream than in the less acidic streams, results suggesting compensatory mechanisms. Our study design did not permit us to differentiate the importance of the biotic factors involved, but the study contributes to our understanding of the importance of conjugated biotic effects on population ecology.
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Vol. 31 • No. 1