ABSTRACT

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Functional response curves of fourth instar larvae of Chaoborusflavicans preying on two size classes of Daphnia longispina were examined throughout three summer seasons in a small forest lake. Data for each size class were fitted to Holling's disc equation. The parameters a' (attack rate) and Th (handling time) were calculated for each prey size from these curves. Attack rate was greater and handling time was shorter for small (0.77 mm) than for large (1.82 mm) Daphnia. In 1:1 mixture of these prey size classes the predation rates of Chaoborus on small Daphnia at prey densities above 20 1-were greater than predicted from the single size-class experiments. The observed predation rates on large Daphnia were lower than predicted at all prey densities. Since both single size-class and two size-class experiments were run during the same period of time the difference in observed and predicted predation rates could not be attributed to seasonal changes in prey preference of Chaoborus larvae. In experiments with a concentrated mixture of lake zooplankton (dominated by D. longispina) Chaoborus preference for Daphnia decreased as prey body size increased. There was no obvious correlation between selectivity coefficients and size-frequency distributions of Daphnia. When medium-sized Daphnia were omitted from calculations the preference of small over large prey did not differ significantly from the predictions of the single size-class model.

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