Biological invasions disrupt the relationship between size spectrum and trophic interactions in freshwater fish communities.

The size spectrum, which describes the relationship between abundance (or biomass) and body size, is an ataxic approach that can provide insights into energy fluxes across trophic levels. However, anthropogenic perturbations can alter the relationship between body size and trophic position, and therefore, the predator-prey mass ratio (PPMR). In this study, we used body size distribution and stable isotope analyses to investigate the relationship between size spectrum and the PPMR in lake fish communities across various eutrophication and invasion levels. Our results revealed that, although size spectrum and PPMR co-varied (i.e. resulting in flatter size spectrum when PPMR was low), this effect was modulated by the level of biological invasion in the community. This was likely caused by differences in trophic niche between native and non-native species: small non-native species exhibited higher trophic positions than small native species, while large non-native species can have lower trophic positions than their native counterparts. These findings suggest that the relationship between size structure and trophic interactions in lake fish communities may be blurred by anthropogenic perturbations, challenging core assumptions of size-based ecology in estimating energy fluxes within freshwater food webs.