Beyond the littoral and pelagic dichotomy: A finer pattern of resource partitioning in the threespine stickleback

Natural populations often exhibit variation for adaptation to alternate habitats and/or resources. For instance, in many north temperate lacustrine fishes, individuals vary in their propensity to use littoral versus pelagic habitats and prey. Such variation may give rise to discrete morphological groups. Alternatively, variation may be more continuous and detectable only by contrasting the morphology of individuals collected in divergent habitats. Here, we provide evidence for an even more subtle level of variation in a population of the three-spine stickleback (Gasterosteus aculeatus). Based on complex network theory, we developed a measure of the degree to which a population is composed of discrete clusters of individuals with similar diets. We applied our method to data from an experimental manipulation of population density. Notably, we observe littoral/pelagic differentiation even when individuals were held in a homogenous habitat (9 m² enclosures, 2 m deep). Still more remarkably, we find that intraspecific competition led to the partition of resources within littoral and within pelagic environments, in which individuals form dietary clusters. This higher level of diet variation is achieved not only through increased clustering of individuals with respect to diet, but also the addition of novel diet items. Morphometric analysis reveals that these dietary clusters are morphologically differentiated groups. We are documenting previously unrecognized levels of diet and morphological variation within sticklebacks in which individuals form ‘microguilds’ that represent a finer partition of the broad “littoral” and “pelagic” resource categories.