DOI: 10.5061/dryad.8cq43
title: Data from: The effects of spatial scale and isoscape on consumer isotopic niche width

description: 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 (&lt; 1 to 10 km lag) accounted
 for most variance. Scale dependence was strong for ẟ15N variance, of which
 &gt; 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 (&gt; 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.
description: Table S4. Northern Irish
 environment and consumers’ stable isotope dataThis file contains all
 consumer (ME = Mytilus, LL = Littorina littorea, PV = Patella vulgata)
 stable isotope values alongside environmental variables for the
 site.FEreddinST4.csvTable S5. Northern Irish macroalgal stable isotope data.This file contains macroalgal (Laminaria digitata, Ulva spp., Fucus serratus) stable isotope values.FEreddinST5.csvR code for isotope scaling curves and plottingDirectories need changing. All files imported are also made in this R code from the original data files. The file 'Covariogrammodels.csv' is attached separately for consistencyIsotope scaling curves.RCovariogram model parametersThis file is needed to plot the covariograms, but is also made in 'Isotope scaling curves.R'Covariogrammodels.csvR code for modelling the effect of sample sizeFiles needed for this code are either made within it or in 'Isotope scaling curves.R' from the original data.SampleSizeModelling.R