Interactions among shorebird, crab, and their invertebrate prey populations

Ruiz, Gregory

In April 1985, hundreds of millions of Dungeness crab (Cancer magister) larvae settled and metamorphosed at Bodega Harbor, California representing by far the largest "outbreak" of Dungeness crabs at this site from 1969-1987. The abundance of crabs declined exponentially with time, and virtually no recruitment of crabs has occurred in the two subsequent years. The brief, overwhelming presence of crabs had devastating and long-lasting effects upon the populations of their tidal invertebrate prey. We documented the most severe impacts, approaching local extinctions, for populations that occurred near the sediment surface: the bivalves Transennella confusa and T. tantilla and the tanaid crustacean Leptochelia dubia. In contrast, other species were spared by various types of refuges from crab predation: 1) depth below the sediment surface (Macoma spp., Nephtys caecoides, Glycera robusta), 2) leaving the sediments at night when crabs were present (Cumella vulgaris, amphipods), and 3) chemical defenses (Phoronopsis viridis). For shorebird populations at Bodega Harbor, the impact of Dungeness crabs caused a dramatic reduction of previously important food resources. Bird abundance declined sharply following the crab settlement event, with more species exhibiting their lowest abundance for 1985-86 than would be expected by chance. This decline did not occur at nearby Bolinas Lagoon or Drake's-Limantour Estero, where similar shorebird communities exist and where crabs did not recruit, indicating that the change was a local and not regional event. After the crab settlement event, dunlin (Calidris alpina) and willets (Catoptrophorus semipalmatus) were in much poorer condition (as measured by body weight, lipid weight, pectoral muscle weight, liver weight, and progression of prenuptial molt) than 1) those at Bodega Harbor before the crabs' arrival and 2) those at other sites for the same years. Our data suggest that the high nutritional costs of remaining at Bodega Harbor most likely caused abandonment by birds (and low abundance) during 1985-86. The second year after crab settlement (1986-87), we observed relatively low return rates of banded adult dunlin and western sandpipers (Calidris mauri) to Bodega Bay, suggesting increased mortality for birds that did remain during 1985-86.