Two is better than one: combining gut content and stable isotope analyses to infer trophic interactions between native and invasive species.
Two complementary approaches are commonly used for analysing trophic interactions that allow inferences about consumed and assimilated resources: gut content (GCA) and stable isotope (SIA) analyses. We used these methods to assess the trophic ecology of a native (Pontastacus leptodactylus) and an invasive species (Faxonius limosus) of crayfish in the Lower Danube (Romania) by calculating five frequently used metrics (i.e. trophic position, omnivory index, elemental imbalance for C:N ratio, trophic niche width and overlap). The SIA approach indicated significantly different trophic niche widths between the crayfish species and potential resource partitioning that triggered lower niche overlap, aspects not depicted by GCA. The latter approach suggested higher omnivory indices and elemental imbalance for macronutrients, potentially because of high incidences of basal resources with low nitrogen content in the foreguts of both species. Although, as indicated by GCA, the two species seem to feed largely on the same resources, SIA suggested that the ingested food items were assimilated with differing efficiencies. These findings are of importance for studies of invasion biology, where the replacement of native species by invasive taxa is underpinned by mechanisms that should be explored by using the most appropriate approach.