Individuals in food webs: The relationships between trophic position, omnivory and among-individual variation
Among-individual diet variation is common in natural populations and may occur at any trophic level within a food web. Yet, little is known about its variation among trophic levels and how such variation could affect phenotypic divergence within populations. In this study we investigated the relationships between trophic position (population’s range and average) and among-individual diet variation. First, we tested for diet variation among individuals and across size classes of Eurasian perch (Perca fluviatilis), a widespread, Eurasian predatory freshwater fish that undergoes ontogenetic niche shifts. Second, we investigated among-individual diet variation within fish and invertebrate populations in two different lake communities using stable isotopes. Third, we tested potential evolutionary implications of population trophic position by assessing the relationship between the proportion of piscivorous perch (populations of higher trophic position) and the degree of phenotypic divergence between sub-populations of littoral and pelagic perch.
We show that among-individual diet variation is highest at intermediate trophic positions, and that this variation likely causes an increase in the range of trophic positions among individuals. Both omnivory and range in littoral-pelagic foraging in the populations showed a quadratic relationship with the average trophic position. We also found that phenotypic divergence was negatively related to trophic position in a population. This study shows that trophic position is related to and may be important for among-individual diet variation. This variation in diet can be an important driver of phenotypic divergence at intermediate trophic levels.