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（ecological variables）的均值与方差依赖于采样属性，例如环境异质性（environmental heterogeneity）覆盖范围（即采样范围（sampling extent））与空间尺度（spatial scale）。营养生态位宽度（Trophic niche width）通常通过生物体全组织的碳（C）、氮（N）稳定同位素比值进行估算，即种群同位素生态位（population isotopic niche）。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（trophic variability），对同位素生态位宽度的塑造更为关键。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（如养分来源）驱动。 2. 为完善该假说，本研究通过刻画同位素尺度效应曲线的形态，对一组已发表的数据集展开分析——该数据集包含3种智利潮间带物种，分属不同摄食功能群（feeding guilds）：植食性螺类、滤食性贻贝（suspension feeding mussel）。随后，我们利用一组来自北爱尔兰、规模更大的全新数据集（包含3种功能同源的（functionally-analogous）潮间带物种）对前述假说进行验证。通过空间显式Bootstrap法（spatially-explicit bootstrap）生成同位素方差-面积曲线，并探究了环境-同位素关联的尺度依赖性（scale-dependency），其中包括波浪暴露程度（wave exposure）与亚生境异质性（sub-habitat heterogeneity）。 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（即二元C-N椭圆面积（bivariate C-N ellipse area））50%的变异。细尺度区间（滞后距离（lag）<1至10 km）可解释绝大多数变异。δ15N（ẟ15N）方差的尺度依赖性极强，其中超过40%的变异可通过线性系数（linear coefficients）模型得到解释。从受陆地影响的遮蔽型海湾（embayments）到浮游生物主导的（pelagic-dominated）开阔海岸线，δ15N基线梯度（或称同位素景观（isoscape））主导了δ15N方差的尺度效应模式。消费者δ13C（ẟ13C）方差的尺度依赖性较弱，在中尺度（mesoscales）范围（滞后距离>20 km）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范围，而采样范围决定了个体消费者所经历的环境异质性。若要使同位素生态位宽度能够准确反映营养生态位宽度，必须将环境异质性纳入考量范畴。在不同空间尺度下开展的研究，大概率会得到不同的环境-同位素关联结果。 5. 我们建议，应在采样设计中明确纳入空间尺度因素，最简便的方式是保持采样种群所在区域的滞后距离或面积一致。若有需要，可对已识别的同位素景观进行去趋势处理。