A multi-year experiment in which zooplankton species were manipulated in 12 ponds indicated that the presence of the carnivorous copepod Heterocope septentrionalis eliminated Daphnia pulex whenever H. septentrionalis became established. In contrast, the congener D. middendorffiana was able to persist only in ponds where H. septentrionalis became established. The fact that Daphnia middendorffiana could not successfully colonize ponds that did not contain H. septentrionalis indicates that this predator was necessary for D. middendorffiana to thrive...