Asymmetric interference competition between herbivorous gastropods, introduced Littorina littorea and indigenous L. obtusata

Competitive interactions may affect distribution and abundance of mobile organisms, such as the intertidal herbivorous gastropods Littorina littorea and L. obtusata, in areas of resource overlap. We examined intra- and interspecific competition between US New England populations by measuring effects of snail density and food availability on growth rates and abundances. We investigated mechanisms of interference, including resistance (phlorotannin) induction of fucoid algae, and impacts of snail presence, waterborne cues, and mucus on herbivory. L. obtusata density increased significantly within fucoid canopies following repeated (every 12 h for 3 d) removal of L. littorea at both study sites. In the laboratory, L. obtusata growth rate was lower in the presence of L. littorea, but not in the presence of conspecifics. In contrast, L. littorea abundances were not strongly affected by its congener, and growth rates were similar under varying L. obtusata densities and food supply. Instead, L. littorea demonstrated intraspecific effects under limited food and higher snail densities. While herbivory by L. littorea was not diminished on Fucus vesiculosus grazed by L. obtusata, fronds of F. vesiculosus and Ascophyllum nodosum grazed by L. littorea were less palatable to L. obtusata, indicative of asymmetric competition. Reduced herbivory by L. obtusata in the presence of L. littorea or its waterborne cues indicate that these forms of interference negatively influence L. obtusata. Coexistence of these species may be due to re new ability of resources and the ability to change location, allowing the competitively inferior L. obtusata to maintain modest abundances within perennial fucoid canopies.