Contribution of non-native species to soft-sediment marine community structure of San Francisco Bay, California

While non-native species (NIS) are important components in many coastal bays and estuaries, quantitative measures that characterize their effects on community structure at bay-wide scales are rare. In this study, we measure species composition and abundance for soft-sediments to assess the contribution of NIS to multiple dimensions of community structure, focusing on one of the most highly invaded bays in the world, San Francisco Bay. Benthic macrofauna was sampled in the high salinity, muddy shallow subtidal (2 m depth) across 10 sites, using replicate 0.1 m(2) Van Veen grabs. Invertebrates retained on a 1 mm sieve were identified, counted, and used to estimate the overall contribution of NIS to (a) abundance (b) species richness, and (c) community similarity. Soft-sediment communities were dominated numerically by NIS, which accounted for 76 % of all organisms detected and had a mean bay-wide abundance that was three and half-fold higher than native biota. Overall, NIS contributed to 36 % of observed taxa and 24-29 % of total estimated regional diversity. Native species accounted for 21 % of total abundance and 45 % of total species richness. Compared to native species, NIS occurred more frequently among samples and also explained more of the variation in community structure among sites. NIS dominate several key attributes of the soft-sediment infaunal community in San Francisco Bay. Percent contribution of NIS to species richness was at least two-fold higher than reported from two decades ago. Unique to this bay, these measures establish a quantitative baseline on the state of invasions and provide an important model for evaluating the extent of NIS in estuaries. Application of this approach across estuaries, with repeated measures over time, is critically needed to advance scientific understanding of invasions and also evaluation of efficacy and gaps in management to reduce new invasions.