The age of rivalry: Eye lens stable isotopes reveal dynamic resource overlap throughout ontogeny in native and invasive fishes.

Competition for shared limiting resources within and between species is a powerful force in biological communities. Modern coexistence theory predicts that higher intra-specific, relative to inter-specific, competition results in species coexistence. However, resource use is rarely constant across animal development, whereby many species experience ontogenetic dietary shifts, suggesting that the relative strengths of competition may change throughout ontogeny. To quantify potential competition between invasive and native stream fishes throughout ontogeny, we measured isotopic niche overlap of individual lifetime trophic trajectories constructed using fish eye lens stable isotope analysis coupled with otolith growth measurements. Overlap of stable isotope ratios between individuals offers a proxy measurement for competition in resource space. We found that invasive green sunfish (Lepomis cyanellus) exhibited higher intra-specific isotopic niche overlap across ontogeny than inter-specific overlap with three native fish species (Gila robusta, Catostomus insignis, Catostomus clarkii) in Burro Creek, Arizona. These native fish species exhibited the highest isotopic niche overlap with green sunfish; a known superior competitor due to their generalist feeding habits and high rates of resource exploitation. The degree of resource overlap experienced by native species increased throughout ontogeny, while invasive green sunfish were freed from high interspecific resource overlap at older ages. Our results suggest that the native species are vulnerable to competitive exclusion by green sunfish. As invasive species continue to spread, contemporary ecological communities are increasingly being managed for coexistence, rather than invader eradication. Understanding how relative competition transforms throughout ontogeny supports the persistence of native species within mixed native-nonnative communities.