Historical Impacts of Invasive Species May Not Always Be What They Seem: A Genetic Perspective From an Endangered Native Species

Aim: Invasive species can affect native species detrimentally, causing localised population declines, range contractions and extinctions. Given the costs and difficulties of controlling invasive species, sound knowledge of the potential benefits to native species is essential. However, sometimes the impacts of invasive species have been inferred by proxy, based on allopatric distribution with native species. Galaxias johnstoni, an endemic freshwater fish in Tasmania, is one such species. It has a restricted range and is believed to have experienced fragmentation and decline due to the introduction of brown trout, Salmo trutta, in 1864. This study examines patterns of genetic diversity in contemporary G. johnstoni populations and tests for genetic signatures of impact by introduced brown trout. Location: Central highlands Tasmania. Methods: Nuclear Single Nucleotide Polymorphisms and mitochondrial DNA sequences were used to assess genetic isolation of populations. Approximate Bayesian Computation was employed to test alternative historical demographic scenarios for the observed patterns of genetic variation. Results: Genetic isolation of the sampled populations was confirmed. However, brown trout do not appear to have caused genetic isolation of contemporary G. johnstoni populations and received equivocal support for reductions in population size. Instead, habitat preferences and post-glacial recolonisation from local refugia appear more likely to explain the contemporary patterns of G. johnstoni genetic variation. Main Conclusions: Our study highlights how genetic approaches can evaluate historical impacts of invasive species, especially where native and invasive species are presently allopatric. We also illustrate potential limitations of this approach: (1) the possibility that other processes have influenced native species coincident with the arrival of invasive pests, (2) the overshadowing of invasive species impacts by signatures from earlier demographic events and (3) the inability to recover signatures from genetically isolated populations that do not survive to the present. Multiple lines of evidence are critical to inform appropriate conservation actions.