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Kerans, B. L., Peckarsky, B. L. and Anderson, C. 1995. Estimates of mayfly mortality: is stonefly predation a significant source? - Oikos 74: 315-323. Field experiments and surveys were canied out in a Rocky Mountain alpine stream during the summers of 1990 and 1992 to estimate the proportion of natural losses of the mayfly Baetis bicaudatus resulting from the co-occurring, predatory stonefly, Kogotus modestus. Functional response experiments determined the number of prey consumed by male and female Kogotus by manipulating the densities of prey and the presence or absence of predators in stream-side chambers. Estimates of losses of Baetis and density of Kogotus were based on ten benthic samples collected weekly (except for last sampling date) from one study reach. Concurrently, drift density of Baetis was estimated upstream and downstream of the reach to determine gains or losses of Baetis resulting from migration. In the functional response experiments Kogotus consumed the same number of prey regardless of prey densities. Female predators tended to consume more prey (- 2 d-I) than males (- 1 d-I), although the result was only statistically significant in one out of three experiments. Per-capita mortality rates of Baetis declined from 0.01 to 0.001 d-' (predator m-?)-' with increasing prey density. In the study reach Baetis density declined 70% during the 4.5 wk and per-capita losses averaged 3.8% d-I. We estimated that predation by Kogotus could cause between 1.6 and 9.5% of the losses of Baetis from the study reach. This occurred because high losses of Baetis were combined with low consumption rates and densities (1.83 individuals m-') of Kogotus. Baetis drift density was higher at night than during the day. Drift densities of Baetis tended to be higher leaving than entering the reach in nighttime estimates, although the results were not statistically signficant. Few replicates resulted in low power to detect differences in upstream and downstream drift densities; therefore, it is possible that some losses could be the result of drift out of the study reach. Nonetheless our results suggest that Kogotus populations had little direct, lethal effect on Baetis populations in this study reach.
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