Biogeographic patterns of community diversity associated with an introduced alga

Non-native foundation species may alter physical environments and provide habitat, thereby impacting recipient communities. Along the US east coast, we assessed biogeographic patterns of free-living and parasitic community diversity associated with the non-native red alga Gracilaria vermiculophylla , which is characterized by fixed (with holdfast) or free-floating thalli depending on the availability of hard substratum. In summer 2019, we surveyed 17 sites across 3 biogeographic regions. We used a random quadrat design to collect G. vermiculophylla and associated mobile macroinvertebrates per site, and we took abiotic measurements. We also haphazardly collected 100 Ilyanassa obsoleta snails per site to assess trematode diversity. In the lab, macroinvertebrates were removed from thalli and identified to lowest taxonomic level, and snails were dissected to determine trematode prevalence and diversity. Biotic and abiotic variables were analyzed for the best sets of predictors for species richness, abundance, and diversity of macroinvertebrates and trematodes across bioregions. Gracilaria vermiculophylla biomass was used as an offset in free-living analyses. Across all our US east coast sites, we detected 10,113 free-living (mobile) macroinvertebrates across 39 taxa. Three Gammaridean amphipods ( Gammarus mucronatus , Ampithoe longimana , and Gammarus lawrencianus ) comprised >50% of all detected organisms. We found biogeographic region to be a key predictor of macroinvertebrate abundance and richness. Trematode prevalence and richness were best explained by G. vermiculophylla biomass, while biogeographic region best explained diversity. As a widespread invader, our study provides evidence for associations that have formed as this foundation species has become established outside its native range. Over time, the presence and spread of G. vermiculophylla could continue to impact macroinvertebrate structure and diversity, and future work should directly compare macroinvertebrate communities with G. vermiculophylla to other foundation species along coastlines it is now common.