During the last decides European waters are subject to the influx of alien species mostly from North American and Ponto-Caspian regions . Amphipods are the most commonly occurring and successful invasive invertebrate organisms that are able to change diversity and community’s structure of the local species. One of such organisms is Ponto-Caspian Gammarid Pontogammarus robustoides . This is one of the alien species that has rapidly increased its distribution range not only by European water ways and invasion corridors, but also by introduction in the 1960s for increasing of fish feed resources in Latvia and Lithuania. In this research we investigated P. robustoides impact on species diversity of macroinvertebrates in upstream of Lielupe river and Daugava river and its reservoirs. Samples collected in Lielupe river were taken into account only for a comparison of the influence of native species, because species of alien gammarids were not found there. Quantitative sampling of gammarids and other macroinvertebrates was done in the littoral part of the river From May to September in 2016 and 2017. Handle net (25 x 25 cm, 500 μm) was used as equipment for sampling. For the estimation of diversity there have been calculated Shannon index, Simpson index (inverse) and effective number of species (conversion of Shannon index). Results of research showed that P. robustoides affects in a negative way or reduces the diversity of species. Values of Shannon index ranged from 0.23 to 2.22, with average value 1.22, in the habitat with alien species. Whereas, this index values ranged from 0.86 to 2.51, with average value 1.91, in the habitats of native gammarids (Gammarus pulex, Gammarus varsoviensis) where P. robustoides was not detected. Simpson index represent similar results and pointed to the dominance of one or some species in habitats of P. robustoides. Average value of this index in alien gammarid habitats was 0.52, but in habitat with native gammarids – 0.77. A tendency was observed that the number of effective species decreased with a larger number of P. robustoides specimens, what could not be observed in habitat with native gammarids.
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 Arbačiauskas K., Gumuliauskaitė S., 2007. Invasion of the Baltic Sea basin by the Ponto-Caspian amphipod Pontogammarus robustoides and its ecological impact. In: Gherardi F. (eds.) Biological invaders in inland waters: profiles, distribution, and threats. Invading Nature – Springer Series In Invasion Ecology, vol 2. Springer, Dordecht.
Research was supported by the national research program “The value and dynamic of Latvia’s ecosystems under changing climate – EVIDEnT” project “Non-native species distribution and impact on the Baltic Sea and freshwater ecosystems” sub-project „Non-indigenous species distribution and impact on freshwater ecosystems”.