Foundation species elevate local biodiversity and underpin critical ecological processes and functions. Kelp species are distributed along ~25% of the world’s coastlines, where they serve as foundation species in intertidal and subtidal habitats. As well as ameliorating environmental conditions and producing organic matter, they provide biogenic habitat for a vast array of associated organisms. Here, we investigated patterns of diversity and structure in assemblages associated with the stipe of the kelp Laminaria hyperborea in the NE Atlantic. Stipes were sampled at 4 study regions (with 3 sites nested within each region) in the UK, spanning ~9° of latitude. Stipe-associated communities were highly diverse (134 species) and abundant (16-4532 ind. stipe-1), with no obvious sequential shift in diversity or overall trends in abundance/biomass of assemblages with latitude. However, we observed high degrees of variability between sites from the same region and individuals within sites, indicating that processes working across smaller spatial scales were more important than those at regional scales. While we observed high between-site variability in assemblage structure, regional differences were also evident. Most notably, sites within our southernmost region (southern England) were largely devoid of amphipods that dominated all other regions. This study highlights the important role of L. hyperborea in elevating biodiversity at local to regional scales through a facilitative interaction. Moreover, given that L. hyperborea forests may be increasingly impacted by ocean warming, changes in coastal water quality and proposed exploitation, our study serves as an important benchmark against which to detect future changes.

1. Climate change can alter ecological communities both directly, by driving shifts in species distributions and abundances, and indirectly by influencing the strength and direction of species interactions. Within benthic marine ecosystems, foundation species such as canopy-forming macro-algae often underpin important cascades of facilitative interactions.2. We examined the wider impacts of climate-driven shifts in the relative abundances of foundation species within a temperate reef system, with particular focus on a habitat cascade whereby kelp facilitate epiphytic algae that, in turn, facilitate mobile invertebrates. Specifically, we tested whether the warm-water kelp Laminaria ochroleuca, which has proliferated in response to recent warming trends, facilitated a secondary habitat-former (epiphytic algae on stipes) and associated mobile invertebrates, to the same degree as the cold-water kelp Laminaria hyperborea.3. The facilitative interaction between kelp and stipe-associated epiphytic algae was dramatically weaker for the warm-water foundation species, leading to breakdown of a habitat cascade and impoverished associated faunal assemblages. On average, the warm-water kelp supported >250 times less epiphytic algae (by biomass) and >50 times fewer mobile invertebrates (by abundance) than the cold-water kelp. Moreover, by comparing regions of pre-and postrange expansion by L. ochroleuca, we found that warming-impacted kelp forests supported around half the biomass of epiphytic algae and one-fifth of the abundance of mobile invertebrates, per unit area, compared with unimpacted forests. We suggest that disruption to this facilitation cascade has the potential to

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