Context dependent interactions between a seaweed and its snail herbivore are mediated by snail density and epiphyte load

Background/Question/Methods

A major challenge for predicting the outcome of species interactions is determining the relative strengths of different interaction pathways when they are context dependent. The marine snail, Littorina sitkana, consumes intertidal rockweed, Fucus distichus, directly but also feeds on epiphytes that could interfere with light capture and growth of F. distichus. The arrangement of species in this interaction web generates opportunities for direct and indirect interactions to have opposing effects. I predicted that the net effect of snails on rockweed would switch from positive indirect facilitation to negative direct herbivory across the range of snail densities commonly encountered in the field. I grew F. distichus in mesocosm tanks with 0-80 snails and measured growth, tissue loss due to consumption, and epiphyte load after one month.

Results/Conclusions

Epiphyte load decreased as snail density increased, and the loss of F. distichus tissue due to consumption increased with snail density. F. distichus growth was maximized at intermediate snail densities, where both epiphyte load and herbivory on F. distichus were low. The net effect of L. sitkana on F. distichus switched from positive to negative with increased snail density, and this was mediated by epiphyte load.  Results indicate that context dependence in species interactions can arise from consumer density when this changes the relative strengths of direct and indirect effects.