The functional response of a predatory plant preying on swarming zooplankton

In a laboratory study, we determined the functional response of the carnivorous aquatic plant Utricularia vulgaris feeding on Polyphemus pediculus, a cladoceran zooplankton that forms swarms. The number of prey eaten increased linearly with prey density up to a density of 35 prey per 125 ml and decreased slightly above this density. Independent estimates of handling time showed that the number eaten was not limited by handling. Thus, we hypothesized that the functional response levelled off because attack rate decreased with increasing density. Direct observations of the predation act at high and low prey densities showed that prey per capita mortality rate was markedly lower at high densities. An analysis of the components of the predation cycle showed that encounter rate and attack probability but not capture success decreased with increasing prey density. We, then, studied the degree of aggregation and the movement behaviour of Polyphemus . The tendency to form swarms increased with density and this was associated with reduced swimming speed and swimming along a more tortuous path. Presence of Utricularia leaves did not influence the spatial distribution and swimming behaviour of Polyphemus . We concluded that the unusual shape of the functional response was due to density dependent prey mortality rates that resulted from a density dependent tendency to form swarms. We, therefore, suggested a modification of Holling's type II functional response model that included density dependent attack rate and this model fitted data significantly better than the original model.