Weeding the Biodiversity Patch—A Mesocosm Study of Predation by <i>Boeckella major</i> (Copepoda: Calanoida) on Billabong Zooplankton

ABSTRACT We carried out short‐term field experiments (duration 1–3 days) to study the impact of predation by natural densities of Boeckella major on zooplankton in Australian ephemeral billabongs (oxbow lakes). Predation reduced total zooplankton densities by 33.6%–91.9%. Rotifers, Daphnia lumholtzi , Bosmina meridionalis , Ilyocryptus spinifer and copepod nauplii were markedly reduced, but late‐stage calanoid copepodites, cyclopoid copepods and D. carinata were little affected. Clearance rates on total zooplankton were constant at about 100–110 mL.copepod −1 .day −1 above c. 25 zooplankton.L −1 but declined below this. Clearances on rotifers, small cladocerans, D. lumholtzi and nauplii (190–120 mL.copepod −1 .day −1 ) were higher than on cyclopoid copepods and calanoid CI–III (50–60 mL.copepod −1 .day −1 ). Clearance was zero on D. carinata and calanoid CIV–CV. Clearance rates were negatively related to prey length. Functional response curves for most prey were Types I or II, but may have been Type III for D. lumholtzi , small cladocerans and calanoid nauplii. Boeckella major showed switching behaviour; rotifers, calanoid nauplii and small cladocerans were positively selected when &gt; 10% of total zooplankton abundance. D. carinata and calanoid CIV–VI were avoided, but all other prey were selected in proportion to abundance. Selectivity was negatively related to prey size. D. lumholtzi was more vulnerable than expected from its size and was attacked mainly on the head and antennae. B. major mainly took prey &lt; 2 mm long, resulting in an increase in mean length of the zooplankton community. Zooplankton composition changed in favour of calanoid copepods and D. carinata , and species richness was reduced by up to 50%. The effects of B. major predation are predicted to decline below adult densities of 10.L −1 and to be minimal below 1.L −1 . The study suggests that B. major predation is a selective force structuring zooplankton communities of ephemeral billabongs. The effects occur only early in billabong development and are short‐lived, but a suite of other invertebrate predators may have similar effects at later stages.