Effects of the non-indigenous Harris mud crab on eelgrass growth
Engström, Linn (2020)
Engström, Linn
Åbo Akademi
2020
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https://urn.fi/URN:NBN:fi-fe2020062645976
https://urn.fi/URN:NBN:fi-fe2020062645976
Tiivistelmä
Biological invasions are occurring at increasingly fast rates, and they are among the most serious threats to biodiversity. Introduced species interact with local communities, potentially altering the food web structure and functioning. The Harris mud crab (Rhithropanopeus harrisii) is an introduced species in the Baltic Sea, where its distribution range is expanding. Since there are no native crab species in the northern Baltic Sea, the mud crab is expected to affect the structure and functioning of native ecosystems. It utilises a range of habitats and was recently discovered in eelgrass (Zostera marina), an important foundation species, in its introduced range. Mud crab predation on invertebrate grazers and mussels could induce a trophic cascade through top-down processes, which would negatively affect eelgrass by promoting filamentous algae and altering sediment nutrient levels. Previous studies suggest the mud crab can induce trophic cascades in the Baltic Sea, where the simple food web structures are vulnerable to change. Effects of the mud crab in eelgrass have not previously been studied. Here, I conducted a field survey to measure mud crab densities in an eelgrass meadow. Then, I studied the potential occurrence of a mud crab induced trophic cascade and direct physical disturbance to eelgrass through an aquarium experiment. The density of mud crabs in an eelgrass meadow in the Archipelago Sea was 21 crabs/m-2, which was five times higher than the last sampling in 2015. The mud crabs readily consumed invertebrates in the aquarium experiment, but no trophic cascade occurred; the mud crab did not affect porewater nutrients, algal growth or eelgrass growth. Light availability was instead the determining factor for eelgrass and algal growth. The burrowing behaviour of the mud crab did disturb eelgrass and caused uprooting of planted shoots. This is the first record of mud crabs uprooting eelgrass shoots. The density of mud crabs in the Baltic Sea is generally poorly known and the data available have been collected using varying methods, making comparisons difficult. Here, I present an estimate of mud crab densities based on core samples that sample both burrowed and active crabs to accurately reflect the population structure and density. Mud crab burrowing behaviour could disturb eelgrass, especially transplanted shoots that are not stabilised by complex root and rhizome structures. Further studies are needed to determine the occurrence of trophic cascades and uprooting of shoots in field conditions.