DOI: 10.5061/dryad.gf1vhhmqz
title: Data from: Species interactions, environmental gradients and body size shape population niche width

description: Competition for shared resources is commonly assumed to restrict
 population-level niche width of coexisting species. However, the identity
 and abundance of coexisting species, the prevailing environmental
 conditions, and the individual body size may shape the effects of
 interspecific interactions on species’ niche width. Here we study the
 effects of inter- and intraspecific interactions, lake area and altitude,
 and fish body size on the trophic niche width and resource use of a
 generalist predator, the littoral-dwelling large, sparsely-rakered morph
 of European whitefish (Coregonus lavaretus; hereafter LSR whitefish). We
 use stable isotope, diet and survey fishing data from 14 subarctic lakes
 along an environmental gradient in northern Norway. The isotopic niche
 width of LSR whitefish showed a humped-shaped relationship with increasing
 relative abundance of sympatric competitors, suggesting widest population
 niche at intermediate intensity of interspecific interactions. The
 isotopic niche width of LSR whitefish tended to decrease with increasing
 altitude, suggesting reduced niche in colder, less productive lakes. LSR
 whitefish typically shifted to a higher trophic position and increased
 reliance on littoral food resources with increasing body size, although
 between-lake differences in ontogenetic niche shifts were evident. In most
 lakes, LSR whitefish relied less on littoral food resources than
 coexisting fishes and the niche overlap between sympatric competitors was
 most evident among relatively large individuals (&gt;250 mm).
 Individual niche variation was highest among &gt;200 mm long LSR
 whitefish, which likely have escaped the predation window of sympatric
 predators. We demonstrate that intermediate intensity of interspecific
 interactions may broaden species’ niche width, whereas strong competition
 for limited resources and high predation risk may suppress niche width in
 less productive environments. Acknowledging potential humped-shaped
 relationships between population niche width and interspecific
 interactions can help us understand species’ responses to environmental
 disturbance (e.g., climate change and species invasions) as well as the
 driving forces of niche specialization.
description: The 14 study lakes are relatively small (0.19–23.7
 km<sup>2</sup>), dimictic and meso- or oligotrophic lakes
 situated along an altitude gradient (275–540 m a.s.l.) in the Finnmark
 region in northern Norway. The study area has very limited human activity
 and the catchment areas consist mainly of mountain birch forest and bogs.
 No major fishery or any stocking activity occur in the study lakes.
 Besides whitefish, perch, grayling and charr, some lakes have relatively
 scarce populations of burbot (<i>Lota lota</i>), brown trout
 (<i>Salmo trutta</i>), pike (<i>Esox lucius</i>),
 European minnow (<i>Phoxinus phoxinus</i>) and/or nine-spined
 stickleback (<i>Pungitius pungitius</i>). LSR whitefish is
 present in all study lakes, whereas six of the lakes host polymorphic
 whitefish populations having the additional presence of a small pelagic,
 densely-rakered (DR) and/or a small profundal, sparsely-rakered (SSR)
 whitefish morph (Siwertsson et al., 2013; Häkli et al., 2018).
 Survey fishing was conducted between 8<sup>th</sup>
 August and 17<sup>th</sup> September in 2005­–2009 using the
 same multi-mesh gillnets set overnight in the littoral, profundal and
 pelagic habitats of each study lake. The used multi-mesh gillnets were 40
 m long and composed of eight randomly distributed 5 m panels of 10, 12.5,
 15, 18.5, 22, 26, 35 and 45 mm bar mesh sizes (knot to knot). In the
 littoral (0–8 m depth) and profundal (&gt;12 m depth) habitats, 1.5 m
 deep bottom nets were used, whereas 6 m deep floating nets were used in
 the pelagic zone in each lake. Supplementary material for stable isotope
 (hereafter SIA) and stomach contents (hereafter SCA) analyses were sampled
 using 30 m long and 1.5 m deep single-mesh (20, 22, 26, 32, 35 mm)
 gillnets. However, the supplementary samples were excluded from catch
 statistics. All study lakes were sampled in a single year between
 2007–2009, except Stuorajavri with main fish sampling in 2005 supplemented
 by profundal sampling in 2008, following the same sampling protocols and
 mostly by the same personnel. After removal from the
 gillnets, fish were killed by a sharp blow on the head, kept cool and
 brought to the field laboratory, where they were identified, measured
 (fork length, mm) and weighed (g). From a subsample of fish, a small piece
 of dorsal muscle tissue was dissected and stored at –20°C until subsequent
 preparation for SIA. The stomachs were removed, preserved in 96% ethanol,
 and later opened to visually estimate the total fullness on a percentage
 scale ranging from empty (0%) to full (100%) (Amundsen &
 Sánchez-Hernández, 2019). The prey items were identified to species, genus
 or family level and their relative contribution to the total fullness was
 estimated (Amundsen et al., 1996; Amundsen & Sánchez-Hernández,
 2019). All observed 26 prey taxa were included in the calculation of
 proportional diet similarity index (<i>PSi</i>) for each
 individual fish (with stomach fullness exceeding 10%), as well as the
 degree of among-individual diet variation (<i>V</i> = 1 ­–
 mean <i>PSi</i>) in each whitefish population following the
 equations described in Bolnick et al. (2002) and Svanbäck et al.
 (2015). Besides fish muscle tissue, qualitative samples
 of putative littoral and pelagic food resources were collected for SIA
 from each lake in August/September 2007–2009 to study the overall food-web
 structures (Figure 2) and particularly to estimate the relative littoral
 reliance (<i>LR</i>), trophic position (<i>TP</i>)
 and isotopic population niche width (cf. Petta et al., 2020) of LSR
 whitefish in the study lakes. The <i>LR</i> and
 <i>TP</i> estimates were calculated using the linear isotopic
 mixing models described in Post (2002). The <i>LR</i> and
 <i>TP</i> estimates estimates were used in calculation of
 population niche width, measured as the total convex hull area
 (<i>TA</i>; see Jackson et al., 2011 for details) encompassing
 the <i>LR</i> and <i>TP</i> values of all
 whitefish individuals in each lake. To validate the robustness of our
 results, we also present the sample-size corrected
 <i>SEA<sub>c</sub></i> and the Bayesian
 <i>SEAb</i> estimates of population niche width (Jackson et
 al., 2011) along with the <i>TA</i> estimates. Finally, we
 also calculated the mean nearest neighbour distances
 (<i>MNND</i>), based on Euclidean distances between individual
 data points in the <i>LR</i> – <i>TP</i> space, to
 measure trophic diversity among individuals (Jackson et al.,
 2011).
 <strong>References:</strong> Amundsen, P.-A., & Sánchez‐Hernández J. (2019). Feeding studies take guts – critical review and recommendations of methods for stomach contents analysis in fish. <i>Journal of Fish Biology</i>, 95, 1364–1373. https://doi.org/10.1111/jfb.14151 Amundsen, P.-A., Gabler, H.-M., & Staldvik, F.J. (1996). A new approach to graphical analysis of feeding strategy from stomach contents data –modification of the Costello (1990) method. <i>Journal of Fish Biology</i>, 48, 607–614. https://doi.org/10.1111/j.1095-8649.1996.tb01455.x Häkli, K., Østbye, K., Kahilainen, K.K., Amundsen, P.-A., & Præbel, K. (2018). Diversifying selection drives parallel evolution of gill raker number and body size along the speciation continuum of European whitefish. <i>Ecology and Evolution</i>, 8, 2317–2631. https://doi.org/10.1002/ece3.3876 Jackson, A.L., Inger, R., Parnell, A.C., & Bearhop, S. (2011). Comparing isotopic niche widths among and within communities: SIBER – Stable Isotope Bayesian Ellipses in R. <i>Journal of Animal Ecology</i>, 80, 595–602. https://doi.org/10.1111/j.1365-2656.2011.01806.x Post, D.M. (2002). Using stable isotopes to estimate trophic position: models, methods, and assumptions. <i>Ecology</i>, 83, 703–718. https://doi.org/10.1890/0012-9658(2002)083[0703:USITET]2.0.CO;2 Siwertsson, A., Knudsen, R., Præbel, K., Adams, C.E., Newton, J., & Amundsen, P.-A. (2013). Discrete foraging niches promote ecological, phenotypic, and genetic divergence in sympatric whitefish (<i>Coregonus lavaretus</i>). <i>Evolutionary Ecology</i>, 27, 547–564. https://doi.org/10.1007/s10682-012-9607-x
description: <strong>Data files</strong>
 <strong>Lake_modelling_data.csv
 </strong>: lake-specific data used in linear models predicting
 isotopic niche width (TA and SEAc), trophic diversity (MNND) and
 among-individual diet variation (V) of LSR whitefish as a function of
 interspecific (Inter) and intraspecific (Intra) interactions and lake
 altitude and surface area.
 <strong>Individual_SIA_data.txt</strong> : individual
 stable isotope data of LSR whitefish collected from the 14 study
 lakes.
 <strong>Individual_SCA_data.txt </strong>: individual
 stomach contents (diet) data of LSR whitefish collected from the 14 study
 lakes.
 <strong>All_fishes_SIA_data.txt</strong> : individual
 stable isotope data of the most abundant fish species collected from the
 14 study lakes used to plot size-dependent niche shifts in LR (Figure 4)
 and TP (Figure 5) estimates. <strong>Data in
 columns</strong>
 "<strong>waterBody</strong>" = lake
 name
 "<strong>waterBodyID</strong>" = unique
 lake number in Norwegian lake databases
 "<strong>Altitude</strong>" = altitude aka
 elevation in meters above the sea level
 "<strong>Area</strong>" = lake surface area
 in square kilometers (km<sup>2</sup>)
 "<strong>MaximumDepth</strong>" = lake
 maximum depth in meters (m).
  "<strong>Inter</strong>" = proxy for
 intensity of interspecific interactions measured as the proportion of fish
 species other than whitefish in the total multi-mesh survey gillnet
 catches (i.e., total fish biomass in grams including fish from all
 habitats) in each lake.
  "<strong>Intra</strong>" = proxy for
 intensity of intraspecific resource competition. This proxy variable was
 standardized for different sampling efforts in each study lake (Table S1)
 by calculating total catch per unit of effort (CPUE; measured as the
 number of whitefish individuals caught per 100 m<sup>2</sup>
 gillnet area per night), including all present whitefish morphs in each
 lake. "<strong>TA</strong>" =
 total convex hull area (<i>TA</i>; see Jackson et al., 2011
 for details) encompassing the <i>LR</i> and
 <i>TP</i> values of all LSR whitefish individuals in each lake
 used as a proxy for population (or "isotopic") niche
 width. "<strong>SEAc</strong>" =
 sample-size corrected standard ellipse area
 (<i>SEA<sub>c</sub></i>; Jackson et al., 2011)
 encompassing the core set of <i>LR</i> and
 <i>TP</i> values of LSR whitefish individuals in each lake,
 used as a proxy for population (or "isotopic") niche
 width. "<strong>V</strong>" = the
 degree of among-individual diet variation (<i>V</i> = 1 ­–
 mean <i>PSi</i>). See below the description for proportional
 diet similarity index <i>PSi.</i>
 "<strong>MNND</strong>" = mean nearest
 neighbour distance (<i>MNND</i>) based on Euclidean distances
 between individual data points in the <i>LR</i> –
 <i>TP</i> space and measuring trophic diversity among
 individuals (Jackson et al., 2011).
 "<strong>scientificName</strong>" = Latin
 (scientific) name of the fish species
 "<strong>Length</strong>" = total length of
 fish in millimeters (mm).
 "<strong>PSi</strong>" = proportional diet
 similarity index calculated for each individual fish with stomach fullness
 exceeding 10%. PSi indices were further used to calcualte the degree of
 among-individual diet variation (<i>V</i> = 1 ­– mean
 <i>PSi</i>) in each whitefish population following the
 equations described in Bolnick et al. (2002) and Svanbäck et al. (2015).
 The <i>PSi </i>index compares each individual’s diet to that
 of the entire population, with values ranging between 0 and 1. In
 populations where individuals specialize on different kinds of prey, the
 <i>PSi </i>values tend to be low and the resulting
 population-level <i>V</i> values tend to be high, i.e.
 approaching 1 (Bolnick et al., 2002; Svanbäck et al., 2015).
 "<strong>d13C</strong>" = stable carbon
 isotope value (δ<sup>13</sup>C) of fish muscle tissue used in
 calculation of littoral reliance estimates (<em>LR</em>, see
 descriptions below).
 "<strong>d15N</strong>" = stable nitrogen
 isotope value (δ<sup>15</sup>N) of fish muscle tissue used in
 calculation of trophic position estimates (<em>TP</em>, see
 description below).
 "<strong>LR</strong>" = estimate of the
 relative reliance of fish on littoral (versus pelagic) carbon sources
 calcualted using the two-source isotopic mixing model described in Post
 (2002). See more detailed description of <em>LR
 </em>calculation below.
 "<strong>TP</strong>" = estimate of the
 trophic position of fish in the lake food web calcualted using
 the two-source isotopic mixing model described in Post (2002). See more
 detailed description of <em>TP </em>calculation
 below. The <i>LR</i> (eqn 1) and
 <i>TP</i> (eqn 2) estimates were calculated using the linear
 isotopic mixing models described in Post (2002): <i>LR</i> =
 (δ<sup>13</sup>C<sub>fish</sub> –
 δ<sup>13</sup>C<sub>pel</sub>) /
 (δ<sup>13</sup>C<sub>lit</sub> –
 δ<sup>13</sup>C<sub>pel</sub>)                                                              eqn (1) <i>TP</i> = λ + (δ<sup>15</sup>N<sub>fish</sub> – [δ<sup>15</sup>N<sub>lit</sub> x LR + δ<sup>15</sup>N<sub>pel</sub> x (1 – LR)] / Δ<sub>n</sub>                                     eqn (2) where δ<sup>13</sup>C<sub>fish</sub> and δ<sup>15</sup>N<sub>fish</sub> refer to isotope values of individual fish; δ<sup>13</sup>C<sub>lit</sub>, δ<sup>15</sup>N<sub>lit</sub>, δ<sup>13</sup>C<sub>pel </sub>and δ<sup>15</sup>N<sub>pel</sub> refer to the lake-specific δ<sup>13</sup>C and δ<sup>15</sup>N values for the littoral and pelagic isotopic end-members; λ is the trophic position of the organisms used to estimate δ<sup>15</sup>N<sub>lit</sub> and δ<sup>15</sup>N<sub>pel</sub>, (here λ = 2 for primary consumers); and Δ<sub>n</sub> is the mean trophic fractionation of muscle tissue δ<sup>15</sup>N (i.e., 2.9‰; McCutchan et al., 2003). For <i>LR</i> calculation, δ<sup>13</sup>C<sub>fish</sub> were corrected for trophic fractionation by subtracting 1.3‰ from the original δ<sup>13</sup>C value (McCutchan et al., 2003). The littoral and pelagic isotopic end-members were defined as the mean isotope values of algae-grazing littoral benthic invertebrates (i.e., snails, <i>Gammarus lacustris</i> amphipods and chironomid larvae; δ<sup>13</sup>C ≥ -25‰) and pelagic zooplankton (δ<sup>13</sup>C ≤ -28‰), respectively. <strong>References</strong> Bolnick, D.I., Yang, L.H., Fordyce, J.A., Davis, J.M., & Svanbäck, R. (2002). Measuring individual-level resource specialization. <i>Ecology</i>, 83, 2936–2941. https://doi.org/10.1890/0012-9658(2002)083[2936:MILRS]2.0.CO;2 Jackson, A.L., Inger, R., Parnell, A.C., & Bearhop, S. (2011). Comparing isotopic niche widths among and within communities: SIBER – Stable Isotope Bayesian Ellipses in R. <i>Journal of Animal Ecology</i>, 80, 595–602. https://doi.org/10.1111/j.1365-2656.2011.01806.x McCutchan, J.H., Jr, Lewis, W.M., Jr, Kendall, C., & McGrath, C.C. (2003). Variation in trophic shift for stable isotope ratios of carbon, nitrogen, and sulfur. <i>Oikos</i>, 102, 378–390. https://doi.org/10.1034/j.1600-0706.2003.12098.x Post, D.M. (2002). Using stable isotopes to estimate trophic position: models, methods, and assumptions. <i>Ecology</i>, 83, 703–718. https://doi.org/10.1890/0012-9658(2002)083[0703:USITET]2.0.CO;2 Svanbäck, R., & Persson, L. (2004). Individual diet specialization, niche width and population dynamics: implications for trophic polymorphism. <i>Journal of Animal Ecology</i>, 73, 973–982. https://doi.org/10.1111/j.0021-8790.2004.00868.x