Report assessing the effects of key NIS on ecosystem functioning. BIO-C3 Deliverable, D2.3

Executive Summary

The deliverable shed light on the direct and indirect food-web effects of increasing abundance and expanding ranges of certain invasive non-indigenous species on native populations and ecosystem functioning. The results support predictions about impact of invasive and non-indigenous species performed in WP3 and WP4. Focus was given on the invasive species which are expanding along the coastal ecosystems, affecting a range of trophic interactions including direct predation, competition with native species and population regulation by their predators and parasites. Round goby (Neogobius melanostomus) was the most widely studied invasive species due to its wide range and potential large effects on the food web functioning. In addition, the role of Harris mud crab (Rhithropanopeus harrisii), grass prawn (Palaemon elegans), red-gilled mud worm (Marenzelleria spp.), and comb jelly (Mnemiopsis leidyi) were assessed by field sampling, laboratory experiments and stable isotope analysis. Mnemiopsis results have been partly utilized also in 1.3 and 2.2 reports concerning adaptation potential and impacts on herring larvae.

The diet and prey preference of the round goby (Neogobius melanostomus) and the potential competition with native species was studied in various locations throughout the Baltic Sea. Round goby seems to be an opportunistic predator feeding on a variety of prey species, the diet depending on the local prey community available. Furthermore, the deliverable gained knowledge on the predators and parasites of the round goby, which could have potential to regulate the populations and further spread in the invasive range in the Baltic Sea. With these results the role of the round goby in the Baltic Sea food webs can be assessed more accurately than before.

Another invasive species the deliverable shed light on was the Harris mud crab (Rhithropanopeus harrisii) which have been part of the fauna in the southern parts of the Baltic Sea for more than a century but which suddenly begun spreading to new areas in late 2000’s. The impacts of this species were investigated in newly invaded areas in the Northern Baltic Sea, in Finland and in Estonia. The results show that Harris mud crab has established also in the northern areas and shows to prefer the keystone habitats. Their isotopic signature places them amongst second-degree consumers and the experiments show that they predate preferentially on grazers, especially gastropods, which may have implications on the health of the coastal ecosystems. Furthermore, in areas where the crabs are abundant, they are also experiencing predation from local fish species. The Harris mud crab has therefore found and established in a niche and has become a part of the coastal systems.

The third invasive species studied was the Grass prawn Palaemon elegans, which has been colonizing the Baltic Sea since early 2000’s. In the southern Baltic their success may have contributed to a decline of the native shrimp P. adspersus, but similar pattern has not been observed in the northern Baltic Sea range yet. In addition, based on field samplings in the north both species are abundant in similar environments and in the experiments it was observed that the non-native P. elegans showed more aggression towards P. adspersus than vice versa. If competition will be more intense in the future, there might be impacts on the native shrimps also in the north. The role of P. elegans as a prey was also studied. It was found to be a frequent prey for perch and cod in the central Baltic Sea whereas in the north perch didn’t eat this novel prey. It seems that fish may need more time to find this prey.

The deliverable also gained knew knowledge on the benthic invasive Marenzelleria spp. polychaetes which are among the few invaders potentially impacting the Baltic Sea positively. They have an ability to burrow much deeper into the sediments than native species and can therefore oxygenate sediments and impacting also biogeochemical processes such as nutrient cycling. In the Central Baltic, in the Vistula Lagoon they have begun to dominate the benthos and significantly altered the benthic habitats. Species distribution modelling indicates that the abundance of Marenzelleria ssp. in shallow water sediments makes them potential prey for coastal fish and in these areas competition with native species is likely. Furthermore, they have potential to increase phosphorus retention in bottom deposits due to deeper oxygen penetration into sediments and formation of a deeper oxidized layer. This may contribute to significant changes in nutrient cycling.

The deliverable also reports on the effective strategy of the non-indigenous comb jellyMnemiopsis leidyi, which has been present in the Northern Europe since 2005. We show for the first time that low winter temperatures have a significant effect on the range expansion of M. leidyi and re-seeding of animals from high abundance hot-spots has been documented to be extremely fast with more than 2,000 km year-1. Also, we show that M. leidyi in invasive sub-populations show selection for traits to optimize its population growth, compared to native sub-populations which optimize for individual life time reproductive success. Combined, these two major findings show that M. leidyi is a very potent invasive species due to its fast population growth but also its swift re-colonization following local extinctions.