ORIGINAL PAPER Does the introduced signal crayfish occupy an equivalent trophic niche to the lost native noble crayfish in boreal lakes? Fabio Ercoli • Timo J. Ruokonen • Heikki Ha ¨ma ¨la ¨inen • Roger I. Jones Received: 30 October 2013 / Accepted: 23 January 2014 Ó Springer International Publishing Switzerland 2014 Abstract The introduced North-American signal crayfish (Pacifastacus leniusculus) has become wide- spread throughout Europe where it has often replaced the native noble crayfish (Astacus astacus). The impact of this replacement on ecosystem processes in boreal lakes is still largely unknown. We compared the trophic niches of these two crayfish species in 16 small to medium sized boreal lakes in southern Finland; eight lakes with noble crayfish and eight lakes where the native crayfish populations had been lost and replaced by signal crayfish. We analysed carbon and nitrogen stable isotopes from samples of the crayfish and their putative food sources, and used stable isotope models to compare trophic niche widths of the two species of crayfish and to quantify the food sources used by them. At species level the signal crayfish exhibited a substantially larger trophic niche than that of the noble crayfish, but within-lake populations of the species did not differ in their niche widths. The isotopic niches of the two species strongly overlapped, and while the estimated proportions of food resources (profundal and littoral macroinverte- brates, terrestrial leaf detritus and macrophytes) used by crayfish varied considerably among individual populations, they did not differ consistently between the species. Our results suggest that, contrary to often expressed concerns, replacement of lost noble crayfish populations by the signal crayfish may not greatly alter the littoral food web structure in boreal lakes. Keywords Invasive species  Niche width  Stable isotopes  Food sources  Boreal lakes Introduction The introduction of alien species is recognized as one of the greatest biological threats to global biodiversity, since they can affect the distribution and abundance of native species as well as ecosystem function (Lodge et al. 1998; Mack et al. 2000; Sala et al. 2000; Hoper et al. 2005; McCarthy et al. 2006). Freshwater ecosystems are considered particularly susceptible to introductions of alien species (Dudgeon et al. 2006; Strayer 2010) which in many cases are able to become established, reach high densities and have a major impact on the whole ecosystem at many ecological levels (Light 2003; McCarthy et al. 2006). The success of any introduced species is related to its adaptability to the new habitat and to how it responds to biological and physical factors such as resource availability Electronic supplementary material The online version of this article (doi:10.1007/s10530-014-0645-x) contains supple- mentary material, which is available to authorized users. F. Ercoli (&)  T. J. Ruokonen  H. Ha ¨ma ¨la ¨inen  R. I. Jones Department of Biological and Environmental Science, University of Jyva ¨skyla ¨, P.O. Box 35, 40014 Jyva ¨skyla ¨, Finland e-mail: faercoli@jyu.fi 123 Biol Invasions DOI 10.1007/s10530-014-0645-x