Understanding context dependent species interactions is important for predicting the ecological effects of changing conditions. In the marine intertidal, epiphytes decrease seaweed growth and survival by inhibiting photosynthesis. Feeding by the marine snail Littorina sitkana reduces epiphytes on the blades of the intertidal rockweed Fucus distichus, which suggests that L. sitkana may indirectly facilitate F. distichus by removing epiphytes. However, L. sitkana also feeds on F. distichus tissue and can have a direct negative effect on the rockweed through herbivory.  To determine if there is a snail density threshold at which the interaction between F. distichus and L. sitkana switches from positive indirect facilitation to negative direct herbivory, I grew F. distichus in mesocosm tanks with a range of snail densities and measured growth and epiphyte load after two months.

Results/Conclusions

I found a decrease in epiphyte load and an increase in F. distichus consumption as snail densities increased. 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 the sign of this species interaction is not static and is context dependent.