Mussel and dogwhelk distribution along the north-west Atlantic coast: testing predictions derived from the abundant-centre model

Abstract

Aim We performed the first test of predictions from the abundant-centre model using north-west Atlantic coastal organisms. We tested the hypotheses that the density of intertidal mussels (Mytilus edulis and M. trossulus) and dogwhelks (Nucella lapillus) and mussel age and size would peak at an intermediate location along their distribution range. We also assessed the latitudinal variation in critical aerial exposure time.Location North-west Atlantic coast between Newfoundland (Canada) and New York (USA), covering 1800 km of shoreline.Methods Using a nested design, we measured mussel density, age and size and dogwhelk density in 60 wave-exposed rocky intertidal sites spread evenly in six regions. Critical aerial exposure times were determined using online data.Results Mytilus edulis peaked in abundance in Maine and was much less abundant in the other regions. Mytilus trossulus peaked in abundance in southern Nova Scotia and Maine, was less abundant in the other regions to the north, and was absent in the southernmost region (New York). Both mussel species were least abundant in a northern region (Cape Breton), although not in the northernmost region (Newfoundland). Critical aerial exposure times were negatively correlated with overall mussel density. Mussel age and size were similar among regions. Dogwhelks peaked in abundance in Maine and were much less abundant in the other regions, being positively correlated with overall mussel density across regions.Main conclusions Density data for M. edulis and N. lapillus provide limited support for an abundant-centre pattern, while M. trossulus shows a clear rampedsouth distribution. Critical aerial exposure times suggest that physiological stress during summer and winter low tides may be lowest in Maine and southern Nova Scotia, which might partially explain mussel predominance in those regions. Winter ice scour in Cape Breton may explain the abundance trough observed there. Mussel size and age may be more limited by wave exposure at our sites (as they all face open waters) than by regional differences in environmental stress. Dogwhelks, which prey on mussels, seem to respond positively to prey density at the regional scale. Our study supports the notion that, while the abundant-centre model is a useful starting point for research, it often represents an oversimplification of reality.

Trait and abundance patterns in two marine molluscs: the influence of abiotic conditions operating across multiple spatial scales

“…Under the ACH, abundance of individual species should be highest in the middle of their geographical distribution and reduced towards the edges (Brown 1984, Hengeveld 1990). For marine invertebrates in coastal intertidal ecosystems, individual species can follow an abundant-centre distribution, although no patterns of increases or decreases in abundance with increasing latitude have concurrently been described (Rivadeneira et al 2010, Tam & Scrosati 2011. In our study, Batillaria australis showed evidence of an abundant-centre distribution (although location was not statistically significant in the nested ANOVA).…”

“…This contrasts with theory suggesting that the factors underlying species abundance distributions should also influence geographic variation in life-history traits (Rivadeneira et al 2010). For example, for 3 mussel species on the northwest Atlantic coast of the USA, there is a negative relationship between abundance and abiotic stress acting at large spatial scales (Tam & Scrosati 2011). In contrast, traits of the mussels such as body size and age are regulated more by local conditions.…”

“…Indeed, recent studies show that the traits and abundance of some species are more variable at local compared to broad spatial scales (e.g. Gilman 2005, Messier et al 2010, Tam & Scrosati 2011. Sagarin & Gaines (2002) suggest that a lack of generality in regard to species distribution patterns may be due to the dominant forces at local scales masking expected patterns at large scales.…”

“…Both species occur from the intertidal to shallow subtidal regions (max. depth ~4 m) (Edgar 1997).Anadara trapezia and Batillaria australis are ideal organisms with which to address our hypotheses for several reasons: (1) they co-occur and are broadly distributed over 1400 km of near-linear coastline (Edgar 1997), allowing us to easily test for geographic patterns in abundance and body size (Gilman 2005, Rivadeneira et al 2010, Tam & Scrosati 2011. (2) Body size and abundance differ between the 2 habitat types in which the species occur, i.e.…”

An environmental stress model correctly predicts unimodal trends in overall species richness and diversity along intertidal elevation gradients

“…For instance, the shores of northern Nova Scotia are inhabited by species from the NW Atlantic cold-temperate biogeographic region, which extends from Newfoundland, Canada, to Cape Hatteras, Unites States (Searles 1984;Adey and Hayek 2005). However, in northern Nova Scotia, cold stress during winter low tides is higher than on shores from the geographic center of this range, which in turn exhibit lower levels of heat stress than shores farther south within this range (Jones et al 2010;Tam and Scrosati 2011). In addition, disturbance of intertidal habitats by winter sea ice is common in northern Nova Scotia (annually on the Gulf of St. Lawrence coast and sporadically on the open Atlantic coast; Scrosati and Heaven 2006;Tam and Scrosati 2011).…”