A comparison of terrestrial invertebrate communities in Spartina-invaded and restored Humboldt Bay salt marshes

Despite widespread concern regarding the impacts of invasive non-native vegetation on native salt marsh ecosystems, research investigating the effects of the Spartina densiflora invasion in Humboldt Bay on terrestrial invertebrate assemblages is non-existent. This study compared invertebrate assemblages in Spartina densiflora dominated salt marsh with those found in restored salt marsh along the Mad River Slough (Humboldt County, California). Results showed significant differences in invertebrate community structure in S. densiflora invaded marsh when compared to samples taken in restored marsh using a multi-response permutation (MRPP) method. Richness and evenness of epibenthic and low canopy invertebrates was highest in the restored marsh. Non-metric multi-dimensional scaling (NMDS) and Dufrene-Legendre indicator analysis revealed that both the invasive snail Myosotella myosotis and the native snail Littorina subrotundata displayed a strong association with invaded sites, while the native talitrid amphipod genus Orchestia was associated with restored sites. In restored Distichlis spicata dominated salt marsh, individuals in the order Hemiptera represented nearly 100% of all invertebrates sampled, the most abundant of which was in the family Delphacidae. The long-jawed orb weaver spider family Tetragnatha was found in relatively high abundance at invaded sites, but was nearly absent from Salicornia pacifica and D. spicata vegetation at restored sites. The taller height of S. densiflora (compared to native salt marsh vegetation) provides habitat niches and refugium at high tide not found in native salt marsh, and disrupts existing co-evolved relationships between invertebrates and native vegetation. Additional biotic and abiotic mechanisms can be invoked on a taxa-by-taxa basis to account for the shift in community structure between invaded and restored sites.