Hybridisation as an ecological process: admixed individuals contribute substantially to canopy biomass in an intertidal macroalgal hybrid zone

<jats:p>Beyond its relevance for evolutionary biology and genetics, hybridisation is increasingly recognised as an ecological process that can modify community structure, yet the demographic contribution of admixed individuals in natural primary-producer systems remains poorly resolved. On sheltered shores of the Swedish west coast, the canopy-forming foundation species Fucus spiralis (upper intertidal) and F. vesiculosus (mid intertidal) meet across a steep, spatially compressed intertidal gradient. Combining fine-scale spatial sampling at two sites with high-resolution genomic data, we asked whether morphologically ambiguous individuals are admixed, how ancestry varies with shore height, and how much admixed individuals contribute to standing canopy biomass across a 30-cm depth gradient. Genome-wide structure resolved two parental ancestry groups and a sharply bounded, intermediate centimetre-scale zone of admixed individuals whose elevated heterozygosity is consistent with recent admixture. They contributed about one-third of total standing canopy biomass while comprising only about one-sixth of sampled individuals, and appear to be an environmentally constrained demographic component dependent on both parental species rather than a self-sustaining lineage. Admixed individuals therefore form a substantial part of the canopy in the narrow band where neither parental species performs well. Whether that contribution alters canopy continuity or ecosystem function at the boundary is beyond what these data can establish, and we set it out as a prediction for direct testing rather than as a result. More broadly, hybridisation may act as an ecological process shaping spatial structure in foundation-species systems, even where its evolutionary outcome is transient.</jats:p>