Population niches have long been used to classify species as generalist or specialist. However, a population of generalists that focus on the same prey may have a niche that is highly similar to that of a population of specialists, where each individual focuses on different prey items. Individual specialization has been shown in many studies, yet the factors driving it are so far not well understood. Biotic interactions like competition can have an influence, but abiotic factors could also play a role. Theory predicts that habitat stability could have a strong effect on specialization, but as of yet, there is little or no empirical evidence of this. To test for such an effect, we sampled trout from both stable and unstable habitats, as defined by hydromorphological characteristics. We used stomach content and stable isotope analysis to determine both long and short term feeding respectively, and thus obtained integrated measures of individual and population resource use. Specialization was measured by using Roughgarden’s measures of population and individual niche width on stomach contents and Layman’s measures for isotope data.
Fish from stable habitats showed significantly higher individual specialization. This was shown by both the between individual component of niche width as calculated from prey found in stomach contents, and mean nearest neighbor distance in isotopic space. Effect sizes for both measures were comparable, being roughly twice as high in stable, compared to unstable, habitats. Additional measures showed large effect sizes, suggesting that further investigation may be warranted. Notable among these measures is a classic indicator of specialization, the ratio of total niche width and within-individual component of dietary niche, which was more than 25% higher in stable environments. Our work therefore provides a novel case study showing among the first evidence for increased specialization in stable environments. We suggest this may stimulate further research that uncovers the precise mechanisms behind this effect, with a detailed focus placed on causative factors.