Local Conditions and Environmental Gradients Shape the Abundance and Size Structure of a Non-Native Intertidal Species in Atlantic North America

Variation in abundance and size along environmental gradients provides insights into the capacity of species to adapt to changing environmental conditions and predict range expansions, this being particularly relevant for non-native species. Here, we evaluate how population density, shell height, and size structure vary in relation to environmental factors across a broad latitudinal gradient in the non-native Littorina littorea. We sampled ten locations, covering nearly the entire range of L. littorea in North America and measured snail density, shell height, substrate rugosity and algal biomass. We extracted other environmental factors from satellite data and used them to assess the influence of local characteristics and environmental gradients on mean snail density, mean shell height and size structure. We found evidence of a weak positive relationship between snail density and rugosity. Mean shell height showed a strong positive relationship with growing season length and a negative relationship with air temperature. We also found that high and variable temperatures during summer, length of the growing season, and high rugosity, negatively impacted the frequency of small individuals. Instead, high water temperature during the spawning period, low temperature variability during summer, and low substrate rugosity are positively associated with a higher frequency of small individuals. Our results indicated that substrate rugosity positively influenced the abundance of this non-native littorinid independent of the climatic conditions experienced. Instead, variation in shell height largely reflected the environmental gradient found throughout its range. In particular, we argue that the lower abundance of small snails in sites with high and variable summer temperatures reflects increased juvenile mortality, while the higher frequency of small snails in sites with warmer spawning periods and more stable summers indicates enhanced recruitment and population growth. These findings support the potential for L. littorea to expand its range poleward with the progression of climate warming.