Prey switching by Acartia clausi: Experimental evidence and implications of intraguild predation assessed by a model

Switching between algal (Thalassiosira weissflogii) and ciliate (Strobilidium undinum) food by the marine copepod Acartia clausi was investigated in the laboratory by short incubation experiments with ¹⁴C-labeled prey. A. clausi displayed a Holling type 3 functional response (which differed significantly from a type 2 response, p < 0.05) for ciliates when there was a constant abundance of algae present, and likewise for algae when there was a constant abundance of ciliates present. The results were implemented in a mathematical model to investigate the effect of different functional responses on a simple food web comprised of nutrient, algae, ciliates and copepods. In the model, ciliates and copepods competed for resources (algae) and ciliates were also prey for copepods. This blend of predation and competition among copepods and ciliates corresponds to intraguild predation as defined by Polis and Holt (1992; Trends Ecol Evol 7:151-154). Stable solutions with all state variables present were found over a range of nutrient concentrations when the copepods displayed type 3 functional responses. On the contrary, 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 threshold prey concentration for positive net growth than copepods.