Data from: The effects of spatial scale and isoscape on consumer isotopic niche width

1. The mean and variance of ecological variables are dependent on sampling attributes such as the coverage of environmental heterogeneity (sampling extent) and spatial scale. Trophic niche width is often approximated by bulk tissue stable isotopes of C and N, i.e. the population isotopic niche. However, recent studies suggest that environmental heterogeneity (experienced by individuals) may be more important in defining the isotopic niche width than trophic variability. We hypothesised that isotopic niche width will increase monotonically with spatial scale, largely produced by environmental variation, e.g. nutrient source.
2. To refine this hypothesis, by describing the shapes of isotope scaling curves, we explored a previously published dataset describing three Chilean intertidal species representing different feeding guilds (grazing snails, suspension feeding mussel). We tested these hypotheses on a new, larger dataset describing three functionally-analogous intertidal species from Northern Ireland. We generated isotopic variance-area curves from a spatially-explicit bootstrap and investigated the scale-dependency of environment-isotope relationships, including wave exposure and sub-habitat heterogeneity.
3. Spatial scale explained 50% of the variance in population isotopic niche widths (bivariate C-N ellipse area) by simple, non-linear relationships. Finer scales (< 1 to 10 km lag) accounted for most variance. Scale dependence was strong for ẟ15N variance, of which > 40% was explained by modelling linear coefficients. A ẟ15N baseline gradient, or isoscape, dominated ẟ15N variance scaling patterns, from sheltered, terrestrially-influenced embayments to exposed, pelagic-dominated coastline. Consumer ẟ13C variance had a weaker scale-dependence, plateauing at mesoscales (> 20 km lag).
4. We show that isotopic niche width is strongly dependent on sampling spatial extent, which controls the environmental heterogeneity experienced by individual consumers. Environmental heterogeneity must be accounted for before isotopic niche width can be considered to accurately represent trophic niche width. Studies conducted at different spatial scales are likely to identify different environment-isotope relationships.
5. We recommend that spatial scale should be incorporated into sampling designs explicitly, easiest by maintaining a consistent lag distance or area within which populations are sampled. Identified isoscapes can be de-trended, where necessary.

1. 生态变量的均值与方差，取决于采样属性，例如环境异质性覆盖范围（采样幅度，sampling extent）与空间尺度。营养生态位宽度（trophic niche width）通常通过碳（C）、氮（N）的整体组织稳定同位素来近似，即种群同位素生态位。然而，近期研究表明，个体所经历的环境异质性，相较于营养变异性，对同位素生态位宽度的界定更为关键。本研究提出假说：同位素生态位宽度将随空间尺度呈单调递增趋势，这一现象主要由环境变异（如营养源）所驱动。
2. 为细化该假说，本研究通过刻画同位素标度曲线的形态，对一组已发表的数据集展开分析——该数据集涵盖3种智利潮间带物种，分别代表不同的取食功能群：植食性螺类、滤食性贻贝。随后，我们利用一组来自北爱尔兰的、规模更大的全新数据集，对上述假说进行验证，该数据集包含3种功能相似的潮间带物种。我们通过空间显式自举法生成了同位素方差-面积曲线，并探究了环境-同位素关系的尺度依赖性，其中涉及波浪暴露度与亚生境异质性两类因子。
3. 空间尺度通过简单的非线性关系，解释了种群同位素生态位宽度（二元C-N椭圆面积）中50%的变异。细尺度范围（滞后距离<1至10km）贡献了绝大多数的变异。δ15N的方差表现出极强的尺度依赖性，其中超过40%的变异可通过线性系数模型进行解释。从受陆地影响的隐蔽型海湾到浮游生物主导的开阔海岸，δ15N基线梯度（即同位素景观（isoscape））主导了δ15N方差的尺度变化模式。消费者的δ13C方差则表现出较弱的尺度依赖性，在中尺度范围（滞后距离>20km）时趋于平稳。
4. 本研究证实，同位素生态位宽度强烈依赖于采样空间幅度，而采样空间幅度决定了个体消费者所经历的环境异质性。在将同位素生态位宽度视为营养生态位宽度的准确表征之前，必须先纳入环境异质性的影响。在不同空间尺度下开展的研究，大概率会得到不同的环境-同位素关系结论。
5. 本研究建议，应将空间尺度明确纳入采样设计中，最简便的方式是保持种群采样的滞后距离或采样面积一致。若有必要，可对已识别的同位素景观进行去趋势化处理。