Effect of invasive species on the structure and function of the Sylt-Rømø Bight ecosystem, northern Wadden Sea, over three time periods.
Three quantitative energy flow models have been constructed for the Sylt-Rømø Bight representing the stocks and flows in the ecosystem during 1995, 2007 and 2010. The 1995 model consists of 59 compartments, while the 2007 and 2010 models have 63 compartments each. The 4 additional compartments in the 2007 and 2010 models are 2 native barnacle species Semibalanus balanoides and Balanus crenatus and 2 invasive species namely the Pacific oyster Crassostrea gigas and the Australasian barnacle Austrominius modestus. Oyster and A. modestus biomass increased from virtually zero in 1995 to 15 and 1.3 gC m-2, respectively, in 2007, and then subsequently decreased to 0.6 and 0.12 gC m-2, respectively, in 2010. Reasons for the increases in oysters and A. modestus are related to climate change which favored strong recruitment of the invasive species during unseasonably warm summers, and the subsequent decline to weak recruitment of the invasive species and an unseasonably cold 2009/2010 winter. We used ecological network analysis (ENA) to quantify ecosystem structure and function through analyses of energy (or carbon) transfer between living and non-living compartments in each of the 3 network models. Results from ENA show an increase in most of the system attributes, but a decline in number of cycles, the trophic efficiency, the ascendency ratios, and relative redundancy from 1995 to 2007; these are ascribed to the impact of the invasive species on system organization and function. Following the dramatic decline of the invasive species from 2007 to 2010 most of the system attributes decreased. The variance in the biomass and associated flows of other compartments in addition to that of the invasive species has also played a role in the changes in system attributes and function.