PREY SWITCHING BY ACARTIA-CLAUSI - EXPERIMENTAL-EVIDENCE AND IMPLICATIONS OF INTRAGUILD PREDATION ASSESSED BY A MODEL

Switching between algal (Thalassiosira weissflogii) and ciliate (Strob ilidium undinum) food by the marine copepod Acartia clausi was investi gated in the laboratory by short incubation experiments with C-14-labe led prey. A. clausi displayed a Holling type 3 functional response (wh ich differed significantly from a type 2 response, p < 0.05) for cilia tes when there was a constant abundance of algae present, and likewise for algae when there was a constant abundance of ciliates present. Th e results were implemented in a mathematical model to investigate the effect of different functional responses on a simple food web comprise d of nutrient, algae, ciliates and copepods. In the model, ciliates an d copepods competed for resources (algae) and ciliates were also prey for copepods. This blend of predation and competition among copepods a nd ciliates corresponds to intraguild predation as defined by Polis & Holt (1992; Trends Ecol Evol 7:151-154). Stable solutions with all sta te variables present were found over a range of nutrient concentration s when the copepods displayed type 3 functional responses. On the cont rary, when copepods displayed type 2 responses, such stable solutions were only found at very low input nutrient concentrations. Coexistence of ciliates and copepods further required that ciliates had a lower t hreshold prey concentration for positive net growth than copepods.