Seasonal changes in mollusc abundance in a tropical intertidal ecosystem, Banc d'Arguin (Mauritania): Testing the ‘depletion by shorebirds’ hypothesis

• Published in: Estuarine, coastal and shelf science coastal and shelf science, 2014-01, Vol.136, p.26-34
• Main Authors: Ahmedou Salem, Mohamed Vall, van der Geest, Matthijs, Piersma, Theunis, Saoud, Younès, van Gils, Jan A.
• Format: Article
• Language: English
• Subjects: Abundance; Animal and plant ecology; Animal, plant and microbial ecology; Aves; Banc d'Arguin; Biological and medical sciences; Biomass; Bivalvia; Brackish water ecosystems; Calidris canutus; Construction; Density; Depletion; Dosinia; Fundamental and applied biological sciences. Psychology; Gastropoda; General aspects; Gibbula; Gibbula umbilicalis; Invertebrates; Knots; Loripes; Loripes lucinalis; Mathematical models; Mollusca; molluscs; optimal diet; seasonality; shorebirds; Summer; Synecology; Vertebrates: general zoology, morphology, phylogeny, systematics, cytogenetics, geographical distribution
• ISSN: 0272-7714; 1096-0015
• DOI: 10.1016/j.ecss.2013.11.009

At temperate latitudes densities and biomass of intertidal molluscs tend to be strongly seasonal. Here we provide a comparative study on seasonality of bivalves and gastropods in the tropical intertidal seagrass-covered soft sediment environment of Banc d'Arguin, Mauritania (20°N, 16°W). In this system, benthivorous shorebirds exert considerable predation pressure with strong seasonal variations. It has been proposed that during the period when (adult) shorebirds are absent (May–August) benthic biomass would be able to recover, but a first test was inconclusive. Over a full year (March 2011–February 2012), each month we sampled benthic invertebrates at sixteen permanent sites. The total of 3763 specimens comprised 20 species, representing eight orders and 19 families. Bivalves were much more common than gastropods. The bivalve Loripes lucinalis dominated the assemblage throughout the year (58% of total number), followed by Dosinia isocardia (10%), Senilia senilis (8%) and the gastropod Gibbula umbilicalis (6%). Average biomass amounted to 32 g AFDM/m2, of which the large West-African bloody cockle Senilia made up three-quarter, Loripes 16%, Gibbula 2% and Dosinia 1%. Across the 20 species, lowest densities were reached in late spring (May) and summer (Aug.), whereas highest densities occurred in autumn (Oct.). The lowest overall density of 676 specimens/m² in August more than doubled to a peak density of 1538 specimens/m² in October, most of the increase being due to strong recruitment in both Loripes (densities increasing from 322 specimens/m² in Sept. to 785 specimens/m² in Oct.) and Dosinia (densities increasing from 18 specimens/m² in Aug. to 265 specimens/m² in Sept.). Our results suggest that by the time the feathered molluscivore predators returned in high numbers to Banc d'Arguin (after their summer breeding season in the Arctic), benthic animals were at a peak. In order to quantitatively understand the seasonal changes in mollusc abundance, we build upon a recently published optimal diet model in which the most abundant molluscivore shorebird, the red knot (Calidris canutus), could choose between Loripes and Dosinia. Observed changes in densities of these two bivalves closely match depletion trajectories predicted by the model. We conclude that molluscivore shorebirds are able to deplete their food stocks in the course of their ‘winter’ in a tropical intertidal area.