Latitudinal- and local-scale variations in a rocky intertidal interaction web

Natural assemblages are structured by a complex combination of positive and negative interactions, and the relative importance of each interaction can vary across spatial scales. By using a simple interaction web (barnacles-grazers-microphytobenthos) in a rocky intertidal system, we tested the hypothesis that the relative strength of positive and negative interactions would vary as a function of different environmental stress between 2 latitudinal levels and local environmental conditions. We manipulated the cover of barnacles and the presence of limpets at 2 sites in northern and southern Italy and non-destructively examined the response of microphytobenthos (MPB) (photosynthetic biomass, F-0, and maximum efficiency, F-v/F-m) and of its small-sized grazers (littorinids). At northern sites, a density-dependent facilitative effect of barnacles on F-0 was detected. The effect was likely due to the addition of favourable secondary habitat (i.e. barnacle shells) and amelioration of abiotic conditions to nearby rock. At southern sites, a positive effect of increasing barnacle cover was observed on littorinid density. The subsequent indirect trophic effect on F0, in addition to local nutrient input from anthropogenic activities, likely masked the positive effects of increasing cover of barnacles on F0 at this latitude. No factors influenced F-v/F-m, suggesting a lack of changes in the relative abundance of MPB species. There was no effect of excluding herbivores at either latitude. Results suggest that species interactions supported by foundation species might be undermined by the concomitant action of temperature-related changes and local anthropogenic stressors, whose interactive effects are likely to become more severe with global climate change.