1 Intraspecific trait variability (ITV) maintains functional diversity in populations and communities, and plays a crucial role in ecological and evolutionary processes such as trophic cascades or speciation. Furthermore, functional variation within a species and its populations can help buffer against harmful environmental changes. Trait variability within species can be observed from differences among populations, and between‐ and within individuals. In animals, ITV can be driven by ontogeny, the environment in which populations live and by within‐individual specialization or variation unrelated to growth. However, we still know little about the relative strength of these drivers in determining ITV variation in natural populations. 2 Here, we aimed to (a) measure the relative strength of between‐ and within‐individual effects of body size on ITV over time, and (b) disentangle the trophic changes due to ontogeny from other sources of variability, such as the environment experienced by populations and individual preferences at varying temporal and spatial scales. 3 We used as a model system the endangered marble trout Salmo marmoratus, a freshwater fish living in a restricted geographical area (<900 km2) that shows marked changes in diet through ontogeny. We investigated two trophic traits, trophic position and resource use, with stable isotopes (δ15N and δ13C), and followed over time 238 individually tagged marble trout from six populations to estimate the trophic changes between and within individuals through ontogeny at three different time‐scales (short term: 3 months, medium term: 1 year and long term: 2 years). 4 We found that the relative strength of between‐ and within‐individual effects of body size on trophic position and resource use change strongly over time. Both effects played a similar role in ITV over medium‐ and long‐term time‐scales, but within‐individual effects were significantly driving trophic variability over short‐term scales. Apart from ontogenetic shifts, individuals showed variability in trophic traits as big as the variability estimated between populations. 5 Overall, our results show how the relative strengths of ITV drivers change over time. This study evidences the crucial importance of considering effects of time‐scales on functional variability at individual, population and species levels.
