Elucidating the mechanisms that enable sympatric species to coexist in complex communities is an important challenge in community ecology. Niche partitioning allows for multiple species to coexist in the same space through differences in the utilization of various resource dimensions. Quantifying resource overlap is one tool used to inform how resources, such as food and habitat, are partitioned among species. In the present study, we analyzed the stomach contents of three syntopic predator species, reticulate sculpin (Cottus perplexus), cutthroat trout (Onchoryhchus clarkii), and coastal giant salamander (Dicamptodon tenebrosus) from three Western Oregon streams. The aim of our study was to compare prey composition and quantify the degree of niche breadth and dietary overlap, which have not been previously assessed between these species. Dietary niche breadth was calculated according to Levins’ standardized measure. Dietary overlap was calculated according to the Shoener index.
We identified the stomach contents of 780 C. perplexus, 194 O. clarkii, and 43 D. tenebrosus and found a total of 54 different prey categories belonging to at least 17 orders and 54 families. Ephemeroptera (mainly Baetidae) and Diptera (mainly Chironomidae) larvae were the most commonly consumed prey in all three predators. C. perplexus exhibited the narrowest dietary niche breadth (0.070), whereas O. clarkii (0.102) and D. tenebrosus (0.121) exhibited slightly wider feeding breadth. The largest degree of dietary overlap was observed between C. perplexus and O. clarkii (70.8%), followed by O. clarkii versus D. tenebrosus (58.8%) and C. perplexus versus D. tenebrosus (56.8%). Hemipterans and other surface prey (terrestrial and emerged aquatic insects) constituted a larger proportion of O. clarkii (15.8%) and D. tenebrosus (8.4%) diets than that of C. perplexus diet (1.5%), indicating vertical segregation in the utilization of prey in the water column. Our study suggests these species utilize the same dietary resources to a large degree, though morphological and behavioral differences likely facilitate the partitioning of food resources. Future analyses will explore patterns in spatial and intraspecific overlap, as well as seasonal differences.