Influence of food supply and shore height on the survival and growth of the barnacle Balanus glandula (Darwin)

The contribution of energy limitation to patterns of intertidal zonation and vertical range limits is not well understood. Higher vertical positions on an intertidal shore are typically associated with greater temperature and desiccation stress during emersion; yet, organisms at higher shore heights also experience reduced feeding time and thus also face greater energy limitation than their lower shore counterparts. Disentangling the roles of abiotic stress and food limitation is critical to accurately predicting intertidal species' responses to climate change. This experiment manipulated food supply and shore height for the intertidal barnacle Balanus glandula (Darwin 1854). Adults were maintained in the field for 39 days at one of four shore heights spanning the species' upper vertical range limit, with or without supplemental feeding during high tides. Higher shore animals showed significantly lower growth and higher mortality relative to those lower on the shore. Across all shore heights, fed animals had significantly higher overall growth (20-27% greater final body mass), but not survival, than animals in the control treatments. No significant interactions between height and feeding treatment were detected, although the benefits of supplemental feeding tended to be smaller at higher shore heights. Our results suggest that food limitation occurs across a large portion of B. glandula's vertical range and that it can influence the success of barnacles at and above the current upper vertical limit. As such, energy limitation may play an important role in the vertical limits of many high intertidal species, and in distributional limits more broadly.