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.