Abstract: The degree of individual specialization in resource use differs widely among wild populations where individuals range from fully generalized to highly specialized. This interindividual variation has profound implications in many ecological and evolutionary processes. A recent review proposed four main ecological causes of individual specialization: interspecific and intraspecific competition, ecological opportunity and predation. Using the isotopic signature of subsampled whiskers, we investigated to what degree three of these factors (interspecific and intraspecific competition and ecological opportunity) affect the population niche width and the level of individual foraging specialization in two fur seal species, the Antarctic and subantarctic fur seals (Arctocephalus gazella and Arctocephalus tropicalis), over several years. Population niche width was greater when the two seal species bred in allopatry (low interspecific competition) than in sympatry or when seals bred in high-density stabilized colonies (high intraspecific competition). In agreement with the niche variation hypothesis (NVH), higher population niche width was associated with higher interindividual niche variation. However, in contrast to the NVH, all Antarctic females increased their niche width during the interbreeding period when they had potential access to a wider diversity of foraging grounds and associated prey (high ecological opportunities), suggesting they all dispersed to a similar productive area. The degree of individual specialization varied among populations and within the annual cycle. Highest levels of interindividual variation were found in a context of lower interspecific or higher intraspecific competition. Contrasted results were found concerning the effect of ecological opportunity. Depending on seal species, females exhibited either a greater or lower degree of individual specialization during the interbreeding period, reflecting species-specific biological constraints during that period. These results suggest a significant impact of ecological interactions on the population niche width and degree of individual specialization. Such variation at the individual level may be an important factor in the species plasticity with significant consequences on how it may respond to environmental variability.

Abstract: Variations in sea ice occurrence for the Barents Sea since c. AD 1700 have been determined by analysis of the abundance of the sea ice biomarker IP25 in three marine sediment box cores obtained from locations in the north, southeast and southwest parts of the region. Depth/age models for each core were established using excess 210Pb activity profiles. Comparisons between these depth/age models with those reported previously for the Barents Sea, suggest that the proxy sea ice record may, alternatively, extend back to c. AD 1500--1600. Sedimentation accumulation rates and bulk densities were combined with IP 25 concentrations, resulting in temporal changes to IP25 fluxes, which have been interpreted in terms of sea ice variability. The IP25 sea ice proxy data are also compared with other bulk organic geochemical parameters (total organic carbon and C/N ratios) and with sediment particle size distributions. The data indicate ice-free conditions for the southwest Barents Sea for the past c. 300 yr, consistent with previous reports based on historical sea ice records. In contrast, the combined proxy data from the southeast and north Barents Sea suggest variable spring sea ice occurrence on a c. 10--50 yr timescale, with reduced sea ice over the last c. 40--100 yr. In the early record, an enhanced sea ice occurrence is observed for the southeast Barents Sea during the mid--late nineteenth century, but as early as c. AD 1780 for the northern region. The outcomes of this study are broadly consistent with those obtained from ice edge position determinations derived previously from observational records. The study also demonstrates the potential of using IP25 as a sea ice proxy for longer-term palaeo sea ice determinations (e.g. the Holocene) for the Barents Sea.