Abstract
Invasive species are often in focus in the non-indigenous species (NIS) research while low-abundance species receive relatively little attention. However, tracking NIS dynamics since the early stages of an invasion provides valuable information on the ecology of invasions. In the current paper, we investigated the invasion history and population dynamics of the small-bodied cladoceran Evadne anonyx G. O. Sars, 1897 in the Gulf of Riga (Baltic Sea) almost since its first detection in 2000. The species already was widespread the Gulf of Riga in 2001 and has been found in nearly every subsequent sample collected during summer months. However, the abundance of the species remained low, seldom 100 individuals m -3 . Both, salinity and water temperature affected the spatial distribution and population abundance of E. anonyx. The species was found to occur only sporadically at salinities below ca. 6. To obtain reliable presence/absence and density estimates on this small-bodied cladoceran, the entire zooplankton sample needed to be analysed.
Highlights
Invasive species are a component of non-indigenous species (NIS) that can alter the structure and function of marine ecosystems and may result in appreciable socio-economic impacts to humans (Bax et al 2003); they have become a special area of study
E. anonyx was already widely distributed in the Gulf of Riga (GoR) during 2001 – it was present in 126 of 132 samples
The Baltic NIS has been colonized by several pelagic invertebrates during the past few decades
Summary
Invasive species are a component of non-indigenous species (NIS) that can alter the structure and function of marine ecosystems and may result in appreciable socio-economic impacts to humans (Bax et al 2003); they have become a special area of study. NIS that are currently rare might subsequently become invasive should environmental conditions become more suited for their expansion, but this might take several decades Our current knowledge of invasive processes in many ecosystems is far from sufficient to predict invasive species expansions and their overall impacts with changing environmental conditions (Richardson 2011). Due to the scarcity of basic inventory studies in many marine waters (ICES 2012), marine invasions can go unnoticed, perhaps for several decades. A second important factor is the lack of personnel with the ability to recognise and distinguish NIS from native organisms (e.g. Geller et al 2010 and references therein)
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