Cascading top-down effects on estuarine intertidal meiofaunal and algal assemblages

Nutrient and herbivore alterations cause uncoupled changes in producer diversity, biomass and ecosystem function, but not in overall multifunctionality

“…Overall, snail grazing caused 50 to 80% reductions in abundance and biomass, and around 10% reductions in diversity and richness. These values are comparable with previous experiments in our study system 39 , but higher than those reported in previous studies conducted on similar systems (i.e., 25–50% reductions 24 , 25 ).A likely explanation for these stronger grazing impacts could be the extremely high density of snails in our system 25 , 39 . Nutrient effects were not as strong in our study; however, these results coincide with similar studies in diatom-dominated systems similar to ours 24 , 25 .…”

“…The burrowing crab Neohelicegranulata is ubiquitous across the tidal flats and marshes, where it plays a number of important ecological roles 51 . The hydrobiid snail Heleobia australis (up to 7 mm in shell length) 52 is another prominent organism in the lagoon, reaching extremely high densities (>15,000 snails * m −2 ) in low mudflats, and exerting substantial top-down control on benthic microalgal abundance and diversity 39 . We performed our experiments in a 100 × 1 m strip in the low intertidal mudflat along the shoreline.…”

Environmental stressors induced strong small-scale phenotypic differentiation in a wide-dispersing marine snail

“…One possible explanation for the low density in marshes, relative to flats, is if this resulted from a negative interaction between H. australis and the grapsid crab Neohelice granulata , whose aggregation is also facilitated by marsh plants (Alvarez et al, 2013; Angeletti & Cervellini, 2015; Angeletti et al, 2014; Spivak et al, 1994) where it attains density peaks in late spring and early summer but it is absent throughout the year in unvegetated areas (flats or pans; Angeletti & Cervellini, 2015). It is therefore likely that the low density of H. australis in vegetated crab-rich habitats resulted from an increased bioturbation activity caused by N. granulata , particularly during that period of time (Alvarez et al, 2013).…”

“…One possible explanation for the low density in marshes, relative to flats, is if this resulted from a negative interaction between H. australis and the grapsid crab Neohelice granulata , whose aggregation is also facilitated by marsh plants (Alvarez et al, 2013; Angeletti & Cervellini, 2015; Angeletti et al, 2014; Spivak et al, 1994) where it attains density peaks in late spring and early summer but it is absent throughout the year in unvegetated areas (flats or pans; Angeletti & Cervellini, 2015). It is therefore likely that the low density of H. australis in vegetated crab-rich habitats resulted from an increased bioturbation activity caused by N. granulata , particularly during that period of time (Alvarez et al, 2013). Another mutually non-exclusive hypothesis is that this difference between marshes and flats could result from a higher predation pressure caused by this crab (Barutot et al, 2011; D’Incao et al, 1990), in which case it might have induced predatory shell defenses as shell thickening or narrower apertures (Boulding & Hay, 1993; Johannesson & Johannesson, 1996; Rolán-Alvarez, 2007; review by Bourdeau et al, 2015).…”

“…On the one hand, the grapsid crab Neohelice granulata is one of the most abundant macroinvertebrates of intertidal areas of the SW Atlantic estuaries where it commonly inhabits the upper vegetated area (marshes) but uses the entire intertidal zone (Alvarez et al, 2013; Angeletti & Cervellini, 2015; Angeletti, Lescano, & Cervellini, 2014; Spivak, Anger, Luppi, Bas, & Ismael, 1994). This burrowing crab could exert a strong pressure on Heleobia australis since it can drastically reduce snail density in vegetated areas due to an intense bioturbation activity (Alvarez et al, 2013; Angeletti & Cervellini, 2015; Angeletti et al, 2014; Spivak et al, 1994) and/or through snail predation (Barutot, D’Incao, & Fonseca, 2011; D’Incao, Silva, Ruffino, & Braga, 1990). On the other hand, parasite pressure also differs along the intertidal zone.…”