Effects of Non-indigenous Oysters on Ecosystem Processes Vary with Abundance and Context

摘要

Invasive species can alter the structure and functioning of ecosystems and affect the quality of the services they provide. Effects on biodiversity are well documented, but less is known about their impacts on ecosystem functioning and how these change as their populations increase. Invasive oysters, Crassostrea gigas, were added at increasing abundances to two different estuarine habitats (mussel-beds and mud-flats) and biotic and functional changes were assessed after 15 months. Sediment-water fluxes and benthic turnover rates of ammonium were greatest at medium cover of C. gigas in both habitats, but for silicate they increased with increasing cover of oysters in mud-flats but, by contrast, decreased at the highest cover of oysters in mussel-beds. Community respiration increased with the greatest cover of oysters in both habitats. Biodiversity was increased by C. gigas in mud-flats, but decreased with the greatest cover of oysters in mussel-beds. Assemblage structure differed between habitats and among different covers of oysters and 33% of the total variation in assemblage structure correlated with the variation in nutrient fluxes and concentrations, suggesting possible direct or indirect effects of C. gigas on the relationship between biodiversity and ecosystem functioning. C. gigas can alter biodiversity and benthic turnover rates of important limiting nutrients, and may therefore affect provision of ecosystem services. Predicting the effects of invasive species requires knowledge of how their influence varies with abundance and among receiving environments.