Invasive mangroves alter macrofaunal community structure and facilitate opportunistic exotics

Mangroves were introduced to the Hawaiian Islands in 1902, providing an unusual opportunity to examine the impacts of introduced vascular plants on coastal ecosystems. Despite >100 yr residence in Hawaii, little is known regarding how mangroves alter coastal ecosystem structure. We conducted a case study of 2 Rhizophora mangle habitats in Hawaii, comparing habitat parameters and macrofaunal community structure in introduced mangroves and nearby control sandflats at a similar tidal elevation. Mangrove sediments had finer sediments and higher organic carbon concentrations and porewater salinities than sandflats. Emergent mangrove roots were colonized by the introduced barnacles Chthamalus proteus, Balanus reticulatus, and B. amphitrite and the introduced sponges Suberites zeteki, Sigmadocia caerulea, and Gelloides fibrosa. Higher densities of non-native macrofauna were found in mangrove transects than in sandflat controls, indicating that invasive mangroves facilitate the persistence of non-native fauna in Hawaii. Mangrove habitats also had higher macrofaunal species richness and diversity, as well as greater dominance by subsurface deposit feeders. Introduced mangroves substantially altered benthic community structure, in part by enhancing the structural complexity of the Hawaiian coastal environment. Because macrobenthos provide a variety of ecosystem services, e.g. serving as prey for fish and birds and promoting detrital decomposition, mangrove-induced changes in sediment community composition will likely have far-reaching consequences in Hawaii. Similar consequences of mangrove invasion are likely in other regions, as mangrove habitats expand with climate warming and increased coastal sedimentation.