Data from: Determinants of individual foraging specialisation in large marine vertebrates, the Antarctic and Subantarctic fur seals

1. 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. 2. 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. 3. 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. 4. 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. 5. 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.

1. 野生种群中个体的资源利用特化程度差异悬殊，种群内个体从完全泛化的觅食者到高度特化者均有分布。这类个体间变异对诸多生态与进化过程具有深远影响。近期一项综述提出了个体特化的四大主要生态成因：种间竞争、种内竞争、生态机会与捕食作用。
2. 本研究通过对经二次采样获取的触须进行同位素特征（isotopic signature）分析，探究了三大成因（种间竞争、种内竞争与生态机会）对两种海狗种群生态位宽度以及个体觅食特化程度的影响：研究对象为南极海狗（Arctocephalus gazella）与亚南极海狗（Arctocephalus tropicalis），研究周期横跨多个年份。
3. 当两种海狗在异域分布（种间竞争水平较低）时，其种群生态位宽度显著高于同域分布时，或是海狗在高密度稳定繁殖群落繁殖时（种内竞争水平较高）的情况。符合生态位变异假说（niche variation hypothesis, NVH）的预期：种群生态位宽度越高，个体间生态位变异程度也越高。但与该假说相悖的是，所有南极海狗雌性在繁殖间隔期的生态位宽度均有所提升，此时它们可利用的觅食场域与关联猎物类群更为广泛（生态机会较高），这表明所有个体均扩散至了相似的高生产力海域。
4. 个体特化程度在不同种群间以及年度周期内均存在差异。种间竞争较低或种内竞争较高的情境下，个体间变异程度最高。而关于生态机会的影响，研究结果则存在分歧：根据海狗物种的不同，雌性个体在繁殖间隔期的个体特化程度要么更高，要么更低，这反映了该时期内物种特异性的生物学约束。
5. 上述研究结果表明，生态相互作用对种群生态位宽度与个体特化程度具有显著影响。个体层面的这类变异可能是物种可塑性的重要影响因素，对物种响应环境变化的方式具有重要意义。