Predation by Shorebirds, Fish, and Crabs on Invertebrates in Intertidal Mudflats: An Experimental Test

An experimental tests, using exclosures, of the effects of different guilds of predators (shallow—feeding shorebirds, benthic—feeding fish, and a crab) on the invertebrate populations of three mudflats differing in sand concentration is reported. The effects of predation at these three sites on similar species of prey depended on the seasonality of the important predator species, which changed with sediment composition. Predation did not appear to affect the species composition of the infaunal community. The experimental treatments were designed to separate out effect of different groups of predators and to try to assure that observed differences were not caused by modifications in the physical environment inside the exclosures. Effects of fish were separated from those of shorebirds by comparing changes in infaunal densities in (1) exclosures with sides that floated up during high tide, which excluded only birds, and (2) in exclosures with rigid sides, which excluded both fish and birds. There were no detectable artifactual effects of the experimental manipulations. Fish appeared to be of minor importance. The effects of shorebird predation were very different among the three mudflats and depended upon subtle differences in sediment composition among sites. Shorebirds were most abundant at the muddiest site, and infaunal densities at this site were lowest in the winter when birds were present. The exclosure experiments demonstrated that the birds significantly reduced infaunal densities during that period. In contrast, shorebirds were least abundant at the sandiest mudflat, and infaunal densities were highest in the winter when birds were present. Excluding birds had no demonstrable effect on infaunal densities. However, the lined shorecrab, Pachygrapsus crassipes, was present only at the sandiest mudflat and only in the summer. Exclusion of crabs at this site demonstrated that the crabs reduced infaunal densities during the summer when they are active on the mudflats.