Seasonality dictates changes in the ecological interactions among spatial dominants

Abstract

Marine coasts are dynamic environments where spatial-physical gradients interact with seasonality. In the Patagonia in particular, high temperatures and strong winds lead to stressing physical conditions that shape intertidal communities. In a previous study, we found that the interaction between the sponge Hymeniacidon perlevis and the macroalga Ulva lactuca was context-dependent, switching from positive at the harsher intertidal to negative at the milder subtidal environment. Temporal variation in the physical variables, however, may also affect species performance and interaction strength. Since Patagonian shores are strongly seasonal environments, we evaluated how the interactions (competitive or cooperative) among these twospatially dominant species changed over a temporal-stress gradient throughout a year. We predicted that the macroalga would facilitate the sponge in seasons with more severe environmental conditions, whereas negative or neutral interactions would prevail when environmental conditions were milder. Our results demonstrated that the sign and intensity of the interaction between Hymeniacidon and Ulva changed over the seasons and coincided with a hump-shaped model, involving a neutral interaction in the milder seasons of autumn and winter, then switching to a positive one in the inclement springtime, and finally shifting to a negative relationship in the harshness of the summer. The amelioration of stressful conditions through shading did not satisfactorily explain the underlying mechanism for this varying interaction, an aspect that still remains unclear. Nevertheless, several mechanisms are discussed both within the context of the stress gradient hypothesis and the specific physiological constraints of intertidal organisms.