Abstract
A population's dietary niche, including individual variation and specialization, shapes the scope and strength of its trophic linkages. Individual diet variation may emerge in response to spatial variation in the selective pressures that shape trophic morphologies, such as food availability or competition. Therefore, characterizing dietary niche variation and its link to morphological differences among individuals is a key focus in evolutionary ecology. We examined how diet composition and individual specialization varied across space and trophic morphology in threespine stickleback (Gasterosteus aculeatus) in Lake Mývatn, Iceland. We found that trophic morphology and diet varied spatially, which may reflect dietary niche partitioning or environmental food availability. Diet variation correlated with body size and trophic morphology: smaller fish consumed more cladocerans, while larger fish ate more chironomid larvae, mollusks, and stickleback eggs. Gut length had the next strongest relationship with diet, with longer guts linked to harder prey and shorter guts associated with stickleback eggs. Gill raker length and head shape were also associated with diet variation, suggesting functional morphological feeding differences. Most stickleback were dietary generalists, but individuals from a relatively warm habitat were more specialized than the rest of the population. More specialized individuals tended to feed more on crustaceans or mollusks, as opposed to chironomids. However, specialization was not linked to trophic traits measured in this study, implying that it is underpinned by other traits. Our findings demonstrate spatial variation in diet and specialization in Mývatn stickleback, despite the population's generalist feeding and limited spatial genetic differentiation. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10452-026-10299-x.