Specialization among amphipods: the invasiveGammarus tigrinushas narrower niche space compared to native gammarids

Human-mediated invasions of nonindigenous species are modifying global biodiversity. Despite significant interest in the topic, niche separation and specialization of invasive and closely related native sympatric species are not well understood. It is expected that combined use of various methods may reveal different aspects of niche space and provide stronger evidence for niche partitioning as compared to a single method. We applied the species marginality index (OMI) and species distribution modeling (SDM) in the northern Baltic Proper to determine (1) if environmental niche spaces at habitat scale differ between taxonomically and functionally closely related invasive and native gammarid species, and (2) whether the observed pattern relates to the species distribution overlap. Both methods agreed in notably narrower and more segregated realized niche of invasive Gammarus tigrinus compared to the studied native gammarids. Among native species, the distribution of G. zaddachi overlapped the most with G. tigrinus. Our results confirm that widespread colonization does not require a wide niche of the colonizer, but may rather be a function of other biological traits and/or the saturation of the recipient ecosystem. The niche divergence and wider environmental niche space of native species are likely to safeguard their existence in habitats less suitable for G. tigrinus