Unraveling how keystone niche individuals contribute disproportionately to the population niche

Individual diet specialization is prevalent in wild populations and scales
up to drive ecological processes at the population and community levels.
As the population trophic niche emerges from the combination of foraging
decisions made by individuals, specific individuals with particular niches
can disproportionately contribute to the niche of their population, i.e.,
keystone niche individuals. We investigated whether individuals of South
American fur seals and sea lions differ in their contribution to the
trophic niche width of their populations. To quantify trophic variation
within and between individuals over long time frames, we analyzed stable
isotopes (δ13C and δ15N) of serially sampled whiskers (18 individuals of
O. flavescens and 20 of A. australis females; average of 33 measures per
individual). We used these repeated observations per individual to model
population-level trophic niches in the isotopic bivariate space,
accounting for between and within-individual variation. Then, we estimated
niche width, position, and orientation for each individual in the
population. Finally, we quantified the relative contribution of each
individual to the niche of its population by estimating how niche width
changes when a given individual is removed from its population. While most
conspecifics make negligible contributions, a few keystone niche
individuals contribute disproportionately to population niche width.
Individuals with niches located close to the population centroid promote
the contraction of the population trophic niche, while individuals with
niches located far from the population centroid promote its expansion.
Further, wide individual niches with a divergent orientation compared to
the population niche tend to expand the population niche width. Our
findings reveal how intraspecific niche variation can shape population
niches from the inside out, highlighting that individuals differ
substantially in their contribution to patterns at higher levels of
biological organization.

个体食性特化广泛存在于野生种群中，其效应可通过尺度拓展驱动种群与群落层级的生态过程。种群营养生态位（trophic niche）由所有个体的觅食决策共同塑造，因此部分拥有特定生态位的个体，会不成比例地对种群的营养生态位产生贡献——这类个体即被称为关键生态位个体（keystone niche individuals）。
本研究以南美海狗和海狮的个体为研究对象，探究它们对种群营养生态位宽度的贡献是否存在差异。为量化长期尺度下个体内部与个体间的营养变异，我们对连续采样的触须样本进行了稳定同位素（stable isotope）分析，检测其δ¹³C与δ¹⁵N值；样本涵盖18头南美海狮（O. flavescens）雌性个体与20头南美海狗（A. australis）雌性个体，每头个体平均获得33项检测数据。
我们利用每头个体的重复观测数据，在同位素双变量空间中构建种群层级的营养生态位模型，并纳入个体间与个体内变异的影响。随后，我们估算了种群中每头个体的生态位宽度、位置与取向。最后，我们通过估算移除某一特定个体后种群生态位宽度的变化量，量化了每头个体对其种群生态位的相对贡献。
尽管多数同种个体的贡献可忽略不计，但少数关键生态位个体仍会不成比例地影响种群生态位宽度。生态位靠近种群质心的个体，会推动种群营养生态位的收缩；而生态位远离种群质心的个体，则会促进种群营养生态位的扩张。
此外，与种群生态位取向相异的宽幅个体生态位，往往会扩大种群的生态位宽度。本研究结果揭示了种内生态位变异如何由内而外地塑造种群生态位，同时强调：不同个体对更高层级生物组织格局的贡献存在显著差异。