Facilitation cascades, in which one facilitator enhances another to support broader communities, are key drivers of biodiversity in stressful environments. Rocky intertidal zones are dynamic examples, yet few studies have quantified these cascades over spatial and temporal gradients, particularly where the secondary facilitator is an obligate epiphyte. This thesis addresses these gaps along the Norwegian coast. Epifaunal assemblages associated with the macroalgal host Ascophyllum nodosum and varying loads of its obligate epiphyte Vertebrata lanosa were evaluated over three sampling timepoints, two wave-exposure regimes, and two vertical intertidal bands. V. lanosa biomass emerged as the primary positive driver of epifaunal abundance, taxon richness, and diversity. This facilitation was strongly modulated by environmental gradients; positive effects peaked in the lower intertidal due to reduced desiccation, while wave exposure and seasonal cycles jointly altered temporal community patterns. Species composition varied across sites without reducing overall diversity supported by the cascade. These findings demonstrate that the A. nodosum–V. lanosa cascade functions as a flexible ecological structure sustaining diverse assemblages under contrasting conditions, underscoring the conservation value of both species as an interdependent functional unit. Protecting such units is increasingly critical as climate change intensifies physical stressors in coastal systems.
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