Detection of the impact of predation by migratory shorebirds: An experimental test in the Fraser River estuary, British Columbia (Canada)

Exclosure experiments are commonly used in marine soft sediments to examine changes in invertebrate density caused by shorebird predation. However, many of these exclosure experiments do not incorporate the spatial variability of the invertebrate prey in their design and/or analysis. This paper uses a short-term exclosure experiment and random sampling (Before/After predation) to assess reductions in invertebrate densities by western sandpiper Calidris mauri on the Fraser River estuary, British Columbia, Canada. During the spring >1 million western sandpiper stop over at the Fraser River estuary on their northward migration to breeding grounds in Alaska. Exclosure experiments and Before/After sampling were conducted at 3 sites on the Fraser River estuary that differed in their sediment characteristics and in their use for feeding by western sandpiper. An examination of cage effects showed that there were no alterations to sediment composition caused by the exclosure cage. However, there were changes in invertebrate numbers in the exclosure cage irrespective of shorebird predation. In particular, mobile invertebrates, such as gastropods and crustaceans, migrated into or out of the exclosure cages during the experiment. Overall, there was little evidence for reduction in invertebrate numbers caused by western sandpiper predation. In the exclosure experiment a significant decline was observed in the phyllodocid polychaete Eteone spp., but at a site with little evidence of shorebird predation. Before/After sampling showed some declines in invertebrate numbers, but this could reflect patchiness in invertebrate distributions rather than the direct action of predation. The use of statistical designs for detecting environmental impacts (e.g. BACI) is suggested for future studies of shorebird predation as they allow for the spatial and temporal variability of the invertebrate prey to be considered concomitant with the impact of the shorebird predator. General problems of sampling with sediment cores, high sample variance, and the reduced power of experimental tests need to be considered in future research on the interactions of shorebird predators and their marine invertebrate prey.