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Individuals in food webs: the relationships between trophic position, omnivory and among-individual diet variation.

Svanbäck R, Quevedo M, Olsson J, Eklöv P - Oecologia (2015)

Bottom Line: Yet, little is known about its variation among trophic levels and how such variation could affect phenotypic divergence within populations.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 also found that phenotypic divergence was negatively related to trophic position in a population.

View Article: PubMed Central - PubMed

Affiliation: Department of Ecology and Genetics/Limnology, Uppsala University, Norbyvägen 18D, 752 36, Uppsala, Sweden, richard.svanback@ebc.uu.se.

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.

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Related in: MedlinePlus

Hypothetical relationships between body size and the ratio of mortality (μ)/growth (g) expectancies of prey at different trophic positions. The figure shows a consumer’s “view” of the available prey environment. The conceptual model includes herbivores (c, f), intermediate carnivores (b, e) and top carnivores (a, d). Green lines indicate prey at the lowest trophic level (plants); then prey trophic level increases from blue through red and purple lines. The different types of resources are available to all consumers, yet using each resource type has a certain cost (μ) and benefit (g) ratio to the consumer expressed on the vertical axis, and this ratio varies for the consumer over its size range (horizontal axis). The different consumer types will have slightly different cost/benefit ratios at a specific size, reflected by dashed lines around an average (solid line) for each available resource. Scenarios of high a–c and low d–f variation among consumer types. Opportunities for among-individual diet variation by consumer types of different sizes are shown as light-grey bars (two resources available) and dark bars (three resources available). Any mechanism that causes consumer types to expand the variation in μ/g (interval width bounded by dashed lines) therefore increases the likelihood of among-individual diet variation of a consumer population by permitting alternate resources with equivalent fitness. One of the mechanisms that generates variants within a size class is trade-offs in foraging efficiencies, such as those observed across size classes (causing curvature in a resource function)
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Fig5: Hypothetical relationships between body size and the ratio of mortality (μ)/growth (g) expectancies of prey at different trophic positions. The figure shows a consumer’s “view” of the available prey environment. The conceptual model includes herbivores (c, f), intermediate carnivores (b, e) and top carnivores (a, d). Green lines indicate prey at the lowest trophic level (plants); then prey trophic level increases from blue through red and purple lines. The different types of resources are available to all consumers, yet using each resource type has a certain cost (μ) and benefit (g) ratio to the consumer expressed on the vertical axis, and this ratio varies for the consumer over its size range (horizontal axis). The different consumer types will have slightly different cost/benefit ratios at a specific size, reflected by dashed lines around an average (solid line) for each available resource. Scenarios of high a–c and low d–f variation among consumer types. Opportunities for among-individual diet variation by consumer types of different sizes are shown as light-grey bars (two resources available) and dark bars (three resources available). Any mechanism that causes consumer types to expand the variation in μ/g (interval width bounded by dashed lines) therefore increases the likelihood of among-individual diet variation of a consumer population by permitting alternate resources with equivalent fitness. One of the mechanisms that generates variants within a size class is trade-offs in foraging efficiencies, such as those observed across size classes (causing curvature in a resource function)

Mentions: It is well know that consumers trade-off foraging gain versus mortality risk, especially when these vary with habitat (Werner and Gilliam 1984). This trade-off will change over the ontogeny of individuals, as well as across species, according to the balance of predation mortality risk and resource benefits. Such a trade-off can be depicted by the ratio of mortality to growth expectancy (Werner and Gilliam 1984). Central to this trade-off is that selection should favor foragers that minimize the ratio by either minimizing mortality or maximizing growth. How may such a trade-off affect the degree of among-individual diet variation and omnivory across trophic levels? In Fig. 5 we show a conceptual model of how variation among individuals in the mortality/growth expectancy ratio may vary with size, by means of trade-offs in foraging efficiencies on distinct resources. The relationship of the mortality/growth ratio with size shows distinct shapes both among consumer species as well as among individuals within a species because competition and predation pressures experienced by foragers vary over ontogeny. Such differences may arise as a consequence of foraging efficiency trade-offs discussed above, trade-offs in defense strategies (Chipps et al. 2004; Ruxton et al. 2004; Svanbäck and Eklöv 2011), and differences in animal personalities that will affect both foraging and predation risk (Mittelbach et al. 2014; Sih et al. 2004). For a predator at a relatively low trophic position that specializes on lower trophic level prey and thus trade-offs efficiency on higher trophic level prey this results in the consumption of few resource types, a low degree of omnivory and a low degree of among-individual diet variation (Fig. 5c, f). A predator at an intermediate trophic position should have a higher degree of omnivory both across different size classes and across different phenotypes within a size class, resulting in a higher degree of among-individual diet variation because foraging on different prey items should result in different foraging predation risk trade-offs (Fig. 5b, e). A top predator should have a lower degree of omnivory, and a low among-individual diet variation because in aquatic systems they are mostly piscivores, which should specialize on capturing large prey sizes relatively early in their ontogeny, trading off foraging gain to smaller prey sizes later in their ontogeny (Fig. 5a, d). However, we know little about how the trade-off between resource use and predation risk of foragers across trophic levels affects opportunities for among-individual diet variation. The model in Fig. 5 shows at least two uncertainties about among-individual diet variation. Firstly, among-individual diet variation will only be favored by selection under equivalent fitness for feeding on different resources (i.e., grey bars in Fig. 5a, d, b, e). Thus, the question will be how likely this occurs relative to non-equivalence (Fig. 5c, f). Secondly, crucial to such equivalency will be the variation within a consumer type (variation between dashed lines; Fig. 5) relative to the variation between resource types (variation between solid lines; Fig. 5). Less variation between resources in mortality/growth and more variation within a consumer type yield greater opportunity for fitness equivalence (see the contrast between Fig. 5a–c, d–f).Fig. 5


Individuals in food webs: the relationships between trophic position, omnivory and among-individual diet variation.

Svanbäck R, Quevedo M, Olsson J, Eklöv P - Oecologia (2015)

Hypothetical relationships between body size and the ratio of mortality (μ)/growth (g) expectancies of prey at different trophic positions. The figure shows a consumer’s “view” of the available prey environment. The conceptual model includes herbivores (c, f), intermediate carnivores (b, e) and top carnivores (a, d). Green lines indicate prey at the lowest trophic level (plants); then prey trophic level increases from blue through red and purple lines. The different types of resources are available to all consumers, yet using each resource type has a certain cost (μ) and benefit (g) ratio to the consumer expressed on the vertical axis, and this ratio varies for the consumer over its size range (horizontal axis). The different consumer types will have slightly different cost/benefit ratios at a specific size, reflected by dashed lines around an average (solid line) for each available resource. Scenarios of high a–c and low d–f variation among consumer types. Opportunities for among-individual diet variation by consumer types of different sizes are shown as light-grey bars (two resources available) and dark bars (three resources available). Any mechanism that causes consumer types to expand the variation in μ/g (interval width bounded by dashed lines) therefore increases the likelihood of among-individual diet variation of a consumer population by permitting alternate resources with equivalent fitness. One of the mechanisms that generates variants within a size class is trade-offs in foraging efficiencies, such as those observed across size classes (causing curvature in a resource function)
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC4555210&req=5

Fig5: Hypothetical relationships between body size and the ratio of mortality (μ)/growth (g) expectancies of prey at different trophic positions. The figure shows a consumer’s “view” of the available prey environment. The conceptual model includes herbivores (c, f), intermediate carnivores (b, e) and top carnivores (a, d). Green lines indicate prey at the lowest trophic level (plants); then prey trophic level increases from blue through red and purple lines. The different types of resources are available to all consumers, yet using each resource type has a certain cost (μ) and benefit (g) ratio to the consumer expressed on the vertical axis, and this ratio varies for the consumer over its size range (horizontal axis). The different consumer types will have slightly different cost/benefit ratios at a specific size, reflected by dashed lines around an average (solid line) for each available resource. Scenarios of high a–c and low d–f variation among consumer types. Opportunities for among-individual diet variation by consumer types of different sizes are shown as light-grey bars (two resources available) and dark bars (three resources available). Any mechanism that causes consumer types to expand the variation in μ/g (interval width bounded by dashed lines) therefore increases the likelihood of among-individual diet variation of a consumer population by permitting alternate resources with equivalent fitness. One of the mechanisms that generates variants within a size class is trade-offs in foraging efficiencies, such as those observed across size classes (causing curvature in a resource function)
Mentions: It is well know that consumers trade-off foraging gain versus mortality risk, especially when these vary with habitat (Werner and Gilliam 1984). This trade-off will change over the ontogeny of individuals, as well as across species, according to the balance of predation mortality risk and resource benefits. Such a trade-off can be depicted by the ratio of mortality to growth expectancy (Werner and Gilliam 1984). Central to this trade-off is that selection should favor foragers that minimize the ratio by either minimizing mortality or maximizing growth. How may such a trade-off affect the degree of among-individual diet variation and omnivory across trophic levels? In Fig. 5 we show a conceptual model of how variation among individuals in the mortality/growth expectancy ratio may vary with size, by means of trade-offs in foraging efficiencies on distinct resources. The relationship of the mortality/growth ratio with size shows distinct shapes both among consumer species as well as among individuals within a species because competition and predation pressures experienced by foragers vary over ontogeny. Such differences may arise as a consequence of foraging efficiency trade-offs discussed above, trade-offs in defense strategies (Chipps et al. 2004; Ruxton et al. 2004; Svanbäck and Eklöv 2011), and differences in animal personalities that will affect both foraging and predation risk (Mittelbach et al. 2014; Sih et al. 2004). For a predator at a relatively low trophic position that specializes on lower trophic level prey and thus trade-offs efficiency on higher trophic level prey this results in the consumption of few resource types, a low degree of omnivory and a low degree of among-individual diet variation (Fig. 5c, f). A predator at an intermediate trophic position should have a higher degree of omnivory both across different size classes and across different phenotypes within a size class, resulting in a higher degree of among-individual diet variation because foraging on different prey items should result in different foraging predation risk trade-offs (Fig. 5b, e). A top predator should have a lower degree of omnivory, and a low among-individual diet variation because in aquatic systems they are mostly piscivores, which should specialize on capturing large prey sizes relatively early in their ontogeny, trading off foraging gain to smaller prey sizes later in their ontogeny (Fig. 5a, d). However, we know little about how the trade-off between resource use and predation risk of foragers across trophic levels affects opportunities for among-individual diet variation. The model in Fig. 5 shows at least two uncertainties about among-individual diet variation. Firstly, among-individual diet variation will only be favored by selection under equivalent fitness for feeding on different resources (i.e., grey bars in Fig. 5a, d, b, e). Thus, the question will be how likely this occurs relative to non-equivalence (Fig. 5c, f). Secondly, crucial to such equivalency will be the variation within a consumer type (variation between dashed lines; Fig. 5) relative to the variation between resource types (variation between solid lines; Fig. 5). Less variation between resources in mortality/growth and more variation within a consumer type yield greater opportunity for fitness equivalence (see the contrast between Fig. 5a–c, d–f).Fig. 5

Bottom Line: Yet, little is known about its variation among trophic levels and how such variation could affect phenotypic divergence within populations.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 also found that phenotypic divergence was negatively related to trophic position in a population.

View Article: PubMed Central - PubMed

Affiliation: Department of Ecology and Genetics/Limnology, Uppsala University, Norbyvägen 18D, 752 36, Uppsala, Sweden, richard.svanback@ebc.uu.se.

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.

Show MeSH
Related in: MedlinePlus