Predation: a causal mechanism for variability in intertidal bivalve populations

Two caging experiments were conducted on an intertidal mudflat to assess the impact of predation on two species of bivalves; Macoma balthica and Cerastoderma edule. The first caging 2 experiment enclosed the shore crab, Carcinus maenas, carapace width 15–25 mm, in 1-m cages. The three treatments were zero Carcinus (control), five Carcinus (the highest natural densities found at this site) and 20 Carcinus (a raised density). The experiment ran for approximately 1 month in August 1996. An exclosure experiment was conducted at the same site the following year and ran for approximately 2 months during September and October. The three treatments were full cages, partial cages and an un-caged control. Results from the enclosure indicate that crab predation was responsible for a reduction of both species of bivalve even at high naturally occurring crab densities. Significant reductions were observed between all treatments for C. edule and between the control and 20 Carcinus treatment for M. balthica. Significant differences occurred in the exclosure experiment between all treatments for C. edule. Almost all of the C. edule were removed in the un-caged areas. Significant differences only occurred between the full cages and un-caged areas for M. balthica. In both experiments there was a preference for C. edule over M. balthica and for the larger size classes of both species. Previous work at the present study site showed Carcinus predation to be site-specific, with predation rates dependent on the sediment type. The density of Carcinus at this site is extremely variable; this together with sedimentspecific consumption of bivalves may be one of the causal mechanisms for both the spatial and temporal variability shown in populations of M. balthica and C. edule at this site.  1999 Elsevier Science B.V. All rights reserved.