Abstract
Individual specialization in diet or foraging behavior within apparently generalist populations has been described for many species, especially in polar and temperate marine environments, where resource distribution is relatively predictable. It is unclear, however, whether and how increased environmental variability – and thus reduced predictability of resources – due to global climate change will affect individual specialization. We determined the within‐ and among‐individual components of the trophic niche and the within‐individual repeatability of δ13C and δ15N in feathers and red blood cells of individual female southern rockhopper penguins (Eudyptes chrysocome) across 7 years. We also investigated the effect of environmental variables (Southern Annular Mode, Southern Oscillation Index, and local sea surface temperature anomaly) on the isotopic values, as well as the link between stable isotopes and female body mass, clutch initiation dates, and total clutch mass. We observed consistent red blood cell δ13C and δ15N values within individuals among years, suggesting a moderate degree of within‐individual specialization in C and N during the prebreeding period. However, the total niche width was reduced and individual specialization not present during the premolt period. Despite significant interannual differences in isotope values of C and N and environmental conditions, none of the environmental variables were linked to stable isotope values and thus able to explain phenotypic plasticity. Furthermore, neither the within‐individual nor among‐individual effects of stable isotopes were found to be related to female body mass, clutch initiation date, or total clutch mass. In conclusion, our results emphasize that the degree of specialization within generalist populations can vary over the course of 1 year, even when being consistent within the same season across years. We were unable to confirm that environmental variability counteracts individual specialization in foraging behavior, as phenotypic plasticity in δ13C and δ15N was not linked to any of the environmental variables studied.
Highlights
There is increasing evidence for individual specialization in resource use within generalist species
We investigated the effect of environmental variables (Southern Annular Mode, Southern Oscillation Index, and local sea surface temperature anomaly) on the isotopic values, as well as the link between stable isotopes and female body mass, clutch initiation dates, and total clutch mass
The aims of this study were to investigate: (1) the degree of individual specialization by comparing the within- and among-individual variation in the total isotopic niche width; (2) the broad-sense repeatability in foraging behavior using d15N and d13C; (3) the level of phenotypic plasticity in trophic level and foraging area within individuals in response to several candidate environmental variables and to test whether it differs among individuals; and (4) whether within- and among-individual differences in isotopic compositions are linked to body mass, clutch initiation date, and clutch mass
Summary
There is increasing evidence for individual specialization in resource use within generalist species. According to optimal foraging theory, the level of individual specialization depends on the abundance and diversity of resources, as well as on individuals’ phenotypic traits: Reduced availability of preferred resources will lead to increased intraspecific competition and an expansion of the individual’s niche to include less valuable resources. It depends on the individuals’ preferences for different resources whether intraspecific competition will increase or decrease individual specialization (Araujo et al 2011). Individuals may on the other hand differ in their optimal diet, depending, for example, on their ability to detect, capture, and handle prey, the risk of predation involved in capturing a specific prey, physiological needs for specific nutrients, or differences in their boldness/shyness (Schoener 1971; Araujo et al 2011; Patrick and Weimerskirch 2014)
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