Abstract
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 investigate the relationships between trophic position (the population’s range and average) and among-individual diet variation. We test for diet variation among individuals and across size classes of Eurasian perch (Perca fluviatilis), a widespread predatory freshwater fish that undergoes ontogenetic niche shifts. Second, we investigate among-individual diet variation within fish and invertebrate populations in two different lake communities using stable isotopes. Third, we test 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 littoral and pelagic perch sub-populations. We show that among-individual diet variation is highest at intermediate trophic positions, and that this high degree of among-individual variation likely causes an increase in the range of trophic positions among individuals. We also found that phenotypic divergence was negatively related to trophic position in a population. This study thus shows that trophic position is related to and may be important for among-individual diet variation as well as to phenotypic divergence within populations.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-014-3203-4) contains supplementary material, which is available to authorized users.
Highlights
Among-individual diet variation is common in natural populations and may occur at any trophic level within a food web (Bolnick et al 2003), and could be important for both ecological and evolutionary processes (Bolnick et al 2011, 2003; Quevedo et al 2009)
We found a negative relationship between the proportion of piscivorous perch in the populations and the degree of divergence between perch sampled in the littoral and pelagic habitats (Fig. 4; R2 = 0.412, n = 11, P = 0.033)
Among-individual diet variation is common in animal populations (Bolnick et al 2003), and may be driven by intraspecific competition (Jones and Post 2013; Svanbäck and Bolnick 2007; Svanbäck and Persson 2004; Svanbäck et al 2011), predation (Eklöv and Svanbäck 2006), or other interactions such as interspecific competition and intra-guild predation (Bolnick et al 2010)
Summary
Among-individual diet variation is common in natural populations and may occur at any trophic level within a food web (Bolnick et al 2003), and could be important for both ecological and evolutionary processes (Bolnick et al 2011, 2003; Quevedo et al 2009) Both competition for food and predation influence the degree of among-individual diet variation (Eklöv and Svanbäck 2006; Svanbäck and Bolnick 2007; Svanbäck and Persson 2004). Hairston et al (1960) proposed that in tri-trophic food chains carnivores suppress herbivores indirectly allowing plants to grow unimpeded by predation (i.e., a trophic cascade) This indirect effect was later generalized by Oksanen et al (1981) and found to be valid for systems of up to five trophic levels. If among-individual diet variation increases with intraspecific competition (Svanbäck and Bolnick 2007; Svanbäck and Persson 2004) and decreases with predation (Eklöv and Svanbäck 2006), we would expect top predators and herbivores in a four-trophic food chain to show higher among-individual diet variation than intermediate predators
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