Impact of shorebird predation on intertidal macroinvertebrates in a key North African Atlantic wintering site: an experimental approach

Shorebirds, as migratory aquatic birds and top predators in intertidal ecosystems, can be affected by global environmental changes and escalations in local impacts on coastal lagoons and estuarine trophic networks. Many shorebirds winter in North African Atlantic coastal sites, most likely because these locations provide constant and reliable food supplies with less energy costs in comparison with the wintering sites of northern Europe. Although more information is available for other important southern coastal sites (e.g. Saharan Atlantic coastal desert and Guinean mangroves coast), very little information is available for the North African Atlantic coast. Here, we focus on the impact of shorebird predation on benthic macroinvertebrates in a major wintering site in this area—Sidi Moussa coastal lagoon, Morocco—using an exclosure experiment. For most of the macroinvertebrate species there was no significant effect of the exclusion of shorebird predation. Overall, our results do not show evidence that predation by shorebirds influenced the overall standing biomass of the benthic community. This may indicate that the benthic productivity is high enough to provide constant and reliable food supplies for non-breeding shorebirds.     

Sex differences in body morphology and multitrophic interactions involving the foraging behaviour of the crab Carcinus maenas

Many animals are sexually dimorphic, but the underlying evolutionary causes and ecological consequences of sexual dimorphism are not fully understood. One predicted consequence for sexual dimorphism is that different sexes show niche differentiation. If sexual dimorphism is in feeding appendages, then differences may be manifested by different diets and thus contrasting behavioural responses to potential prey. Sexual dimorphisms in feeding appendages may also result in different handling times, which may then be correlated with differences in exposure, and, hence predation risk to the predator while feeding. In addition, the prey of the sexually dimorphic predator may respond differently to cues from each sex according to the predation risk each presents to the prey. We tested these predictions using a crab (Carcinus maenas) with sexual dimorphism in chelae dimensions, its predator the cuttlefish Sepia officinalis and prey with known differences in handling times; the gastropod molluscs Gibbula umbilicalis and Littorina littorea. We demonstrated that male C. maenas orientated more frequently to cues from L. littorea whereas females orientated more towards G. umbilicalis in contradiction of patterns predicted by handling times. Male crabs had a faster heart rate than females but this was not influenced by food‐based cues. We also showed no difference in foraging times with respect to changing levels of predator risk and also no differences in gastropod responses to odours from male or female crabs. Our results showed that predictions of handling time and sexual dimorphism are not associated. The experiments indicated the male and female crabs are probably ecological equivalents and thus niche differentiation is less likely.