The arrival of an invasive species of mud crab - Rhitropanopeus harrisii - in the Baltic Sea, has rapidly and permanently shifted the control of resources within the food web according to a study in Scientific Reports. The findings suggest that species abundance is now controlled by predation, rather than by the availability of nutrients, food and space in this environment.
Jonne Kotta and colleagues used a combination of long-term data monitoring targeted censuses of mud crabs, and field experiments to record the impact of this invasive species. The authors found that within two years of arriving in the Baltic Sea in 2011, crab populations had expanded rapidly, while benthic (sea floor) invertebrate biomass and richness declined by 61% and 35%, respectively. They also observed a decline in the numbers of dominant bivalves and the disappearance of clams, cockles, snails and slugs - which make up the bulk of the crabs’ diet - at sites where crabs were present. The biomass of two other invasive species - the bivalve Dreissena polymorpha and the marine worm Laonome sp. - increased, as did nutrient concentrations in seawater.
The authors note that D. polymorpha has a much harder shell than native bivalves and is not as easily preyed upon my mud crabs. This means the crabs are more likely to prey on native bivalves, indirectly increasing the biomass of phytoplankton normally consumed by native bivalves. The authors suggest this provides a basis for the population growth of the other invasive species.