ORIGINAL PAPER Differences in stress tolerance and brood size between a non-indigenous and an indigenous gammarid in the northern Baltic Sea Jo ¨rg Sareyka • Patrik Kraufvelin • Mark Lenz • Magnus Lindstro ¨m • Ralph Tollrian • Martin Wahl Received: 31 January 2011 / Accepted: 18 April 2011 Ó Springer-Verlag 2011 Abstract Differences in stress tolerance and reproductive traits may drive the competitive hierarchy between non- indigenous and indigenous species and turn the former ones into successful invaders. In the northern Baltic Sea, the non-indigenous Gammarus tigrinus is a recent invader of littoral ecosystems and now occupies comparable ecolog- ical niches as the indigenous G. zaddachi. In laboratory experiments on specimens collected between June and August 2009 around Tva ¨rminne in southern Finland (59°50 0 N/23°15 0 E), the tolerances towards heat stress and hypoxia were determined for the two species using lethal time, LT 50 , as response variable. The brood size of the two species was also studied and some observations were made on maturation of juveniles. Gammarus tigrinus was more resistant to hypoxia and survived at higher temperatures than G. zaddachi. Brood size was also greater in G. tigrinus than in G. zaddachi and G. tigrinus matured at a smaller size and earlier than G. zaddachi. Hence, there are clear competitive advantages for the non-indigenous G. tigrinus compared to the indigenous G. zaddachi, and these may be further strengthened through ongoing environmental changes related to increased eutrophication and a warming climate in the Baltic Sea region. Introduction Human-driven introductions of non-indigenous species into new biogeographical ranges are occurring worldwide at increasing frequency in both terrestrial and aquatic habitats (Lodge 1993; Ruiz et al. 2000; Occhipinti–Ambrogi and Savini 2003). It is therefore urgent to elucidate the mech- anisms determining invasion success (McMahon 2002; Bruno et al. 2003; Stachowicz and Byrnes 2006; Lyons and Scheibling 2009). Generally, only a small number of introduced species become established and successfully invade their new habitat (Williamson and Fitter 1996). Knowledge about traits that turn a species into a successful invader is thus of great interest for ecologists and would help to predict and manage biological invasions. It is commonly assumed that a broad tolerance towards envi- ronmental stress, high reproductive rates, and plasticity (as capacity for change) increase invasiveness (Stachowicz and Byrnes 2006), although species’ abilities to respond to natural selection have been suggested to be even more important (Lee 2002). Even though many marine benthic habitats are inhabited by non-indigenous species that directly compete with indigenous species for common resources, the number of direct comparisons between both groups of organisms with Communicated by J. P. Grassle. J. Sareyka Á R. Tollrian Animal Ecology, Evolution and Biodiversity, Ruhr-University Bochum, Universita ¨ts-strasse 150, 44801 Bochum, Germany J. Sareyka Á P. Kraufvelin (&) Environmental and Marine Biology, A ˚ bo Akademi University, Artillerigatan 6, 20520 Turku/A ˚ bo, Finland e-mail: pkraufve@abo.fi P. Kraufvelin ARONIA, Coastal Zone Research Team, A ˚ bo Akademi University and Novia University of Applied Sciences, Raseborgsva ¨gen 9, 10600 Ekena ¨s, Finland M. Lenz Á M. Wahl IFM-GEOMAR, Leibniz-Institut fu ¨r Meereswissenschaften, Du ¨sternbrooker Weg 20, 24105 Kiel, Germany M. Lindstro ¨m Tva ¨rminne Zoological Station, J.A. Palme ´ns va ¨g 260, 10900 Hango ¨, Finland 123 Mar Biol DOI 10.1007/s00227-011-1708-5