DOI: 10.5061/dryad.ghx3ffbs7
title: Svalbard reindeer winter diets (1995–2012) dataset

description: Arctic ecosystems are changing dramatically with warmer and wetter
 conditions resulting in complex interactions between herbivores and their
 forage. We investigated how Svalbard reindeer (Rangifer tarandus
 platyrhynchus) modify their late winter diets in response to long-term
 trends and interannual variation in forage availability and accessibility.
 By reconstructing their diets and foraging niches over a 17-year period
 (1995–2012) using serum δ13C and δ15N values, we found strong support for
 a temporal increase in the proportions of graminoids in the diets with a
 concurrent decline in the contributions of mosses. This dietary shift
 corresponds with graminoid abundance increases in the region and was
 associated with increases in population density, warmer summer
 temperatures and more frequent rain-on-snow (ROS) in winter. In addition,
 the variance in isotopic niche positions, breadths and overlaps also
 supported a temporal shift in the foraging niche and a dietary response to
 extreme ROS events. Our long-term study highlights the mechanisms by which
 winter and summer climate changes cascade through vegetation shifts and
 herbivore population dynamics to alter the foraging niche of Svalbard
 reindeer. Although it has been anticipated that climate changes in the
 Svalbard region of the Arctic would be detrimental to this unique
 ungulate, our study suggests that environmental change is in a phase where
 conditions are improving for this subspecies at the northernmost edge of
 the Rangifer distribution.
description: <strong>Sample & Data
 Collection</strong> We collected serum from
 Svalbard reindeer in late winter, which is the critical period for their
 winter survival and reconstructed their diets using serum
 δ<sup>13</sup>C and δ<sup>15</sup>N values. Serum
 samples were obtained from 1995 to 2012, excluding the years 2003 and
 2010, resulting in a total of 16 sampling campaigns. Altogether 232
 samples were collected from 182 individual reindeer, thus, some
 individuals were sampled only once and others up to three times during our
 study period. The number of samples per campaign ranged from 10–21, with
 the average value being 15. For reconstructing the diets of Svalbard
 reindeer, foliar δ<sup>13</sup>C and
 δ<sup>15</sup>N values of plausible (terrestrial) forage
 plants, including graminoids, mosses, dwarf shrubs
 (<em>S</em>. <em>polaris</em>;
 <em>D</em>. <em>octopetala</em>) and forbs were
 obtained from previous studies (Hansen, Lorentzen, et al., 2019; Zhao et
 al., 2019). Briefly, plant samples were collected in Semmeldalen
 (77°90' N, 15°20' E) during July 2009 and 2013 from
 <em>Luzula</em> heath and graminoid vegetation communities
 (Zhao et al., 2019), and from reindeer feeding craters in Adventdalen
 (78°10' N, 16°02' E) in March 2013 (Hansen, Lorentzen, et al.,
 2019). Foliar δ<sup>13</sup>C and δ<sup>15</sup>N
 values may vary over time due to differences in environmental conditions,
 so to control for this, vegetation was re-sampled in Adventdalen in
 February 2019 from five randomly selected 4 m<sup>2</sup>
 plots as no feeding craters were observed due to extreme ground icing. The
 plant samples collected were graminoids (n = 2), mosses (n = 4),
 <em>S</em>. <em>polaris</em> (n = 2) and
 <em>D</em>. <em>octopetala</em> (n = 5). Forbs (n=
 4) were collected as they were present in the plots but not present in or
 collected from the craters sampled in 2013. The
 δ<sup>13</sup>C and δ<sup>15</sup>N values were
 obtained after the serum samples were analysed at the Environment and
 Natural Resources Institute Stable Isotope Laboratory at the University of
 Alaska Anchorage (http://www.uaa.alaska.edu/enri/labs/sils) and the 2019 forage samples at the EcoCore Analytical Facility at the Colorado State University (https://ecocore.nrel.colostate.edu/ecocore-stable-isotope-analysis.html).   <strong>Data preparation and analysis</strong> Anthropogenic inputs of C from fossil fuel burning have resulted in measurable decreases in atmospheric δ<sup>13</sup>C known as the “Suess effect” and some authors utilise a time-dependent correction of -0.022 ‰ per year based on ice core records to account for this change. However, the discrimination of  δ<sup>13</sup>C during photosynthesis in C3 plants has been found to increase in response to increasing concentrations of atmospheric CO<sub>2</sub> (pCO<sub>2</sub>) and decreasing atmospheric δ<sup>13</sup>C<sub>CO2</sub> (Schubert & Jahren, 2012). Thus, we used the equation of Schubert & Jahren (2012) and annual pCO<sub>2</sub> (Tans & Keeling, 2022) to correct all serum and forage samples' δ<sup>13</sup>C values to expected 2012 levels.  The reindeer isotope data were used in conjunction with the forage isotope data and trophic discrimination factors derived in the R-package: <em>SIDER</em>, in Bayesian stable isotope mixing models (R-package: <em>simmr</em>) to estimate dietary proportions and isotopic niche space models (R-package: <em>SIBER</em>). In addition, we used linear regressions to explore how reindeer serum isotopic values were driven by variation in forage production (summer warming - July Average temperature), forage accessibility (Rain-on-Snow - ROS [mm]), intra-specific competition (population density), and intrinsic drivers like nutritional status (body mass [kg]) and status concerning pregnancy (as determined by Progesterone.  Population density in the Reindalen valley system the previous summer to account for density-dependent competitive effects (Albon et al., 2017). ROS (mm), measured as the sum of precipitation when the average daily temperature was ≥1°C during the winter (1st November to the 31<sup>st</sup> March), as a proxy for ground ice (Peeters et al., 2019). July average air temperatures at Svalbard Airport, obtained from the Norwegian Meteorological Institute (2021), as a proxy for summer plant biomass production (van der Wal & Stien, 2014). Population density and ROS were ln-transformed before being fitted as predictor variables in the model for δ<sup>13</sup>C.  <strong>References</strong> Albon, S. D., Irvine, R. J., Halvorsen, O., Langvatn, R., Loe, L. E., Ropstad, E., Veiberg, V., van der Wal, R., Bjørkvoll, E. M., Duff, E. I., Hansen, B. B., Lee, A. M., Tveraa, T., & Stien, A. (2017). Contrasting effects of summer and winter warming on body mass explain population dynamics in a food‐limited Arctic herbivore. <em>Global Change Biology, 23</em>(4), 1374–1389. https://doi.org/10.1111/gcb.13435 Hansen, B. B., Lorentzen, J. R., Welker, J. M., Varpe, Ø., Aanes, R., Beumer, L. T., & Pedersen, Å. Ø. (2019). Reindeer turning maritime: ice‐locked tundra triggers changes in dietary niche utilization. <em>Ecosphere, 10</em>(4), e02672. https://doi.org/10.1002/ecs2.2672 Norwegian Meteorological Institute. (2021). Seasonal temperatures for Svalbard Airport. Environmental monitoring of Svalbard and Jan Mayen (MOSJ). http://www.mosj.no/en/climate/atmosphere/temperature-precipitation.html Peeters, B., Pedersen, Å. Ø., Loe, L. E., Isaksen, K., Veiberg, V., Stien, A., Kohler, J., Gallet, J.-C., Aanes, R., & Hansen, B. B. (2019). Spatiotemporal patterns of rain-on-snow and basal ice in high Arctic Svalbard: detection of a climate-cryosphere regime shift. <em>Environmental Research Letters, 14</em>(1), 015002. https://doi.org/10.1088/1748-9326/aaefb3 Schubert, B. A., & Jahren, A. H. (2012). The effect of atmospheric CO<sub>2</sub> concentration on carbon isotope fractionation in C3 land plants. <em>Geochimica et Cosmochimica Acta, 96,</em> 29–43. https://doi.org/10.1016/j.gca.2012.08.003 Tans, P. & Keeling, R. (2022). Mauna Loa CO<sub>2</sub> annual mean data. NOAA/GML & Scripps Institution of Oceanography. https://gml.noaa.gov/ccgg/trends/data.html van der Wal, R., & Stien, A. (2014). High-arctic plants like it hot: a long-term investigation of between-year variability in plant biomass. <em>Ecology</em><em>, 95</em>(12), 3414–3427. https://doi.org/10.1890/14-0533.1.sm Zhao, L. Z., Colman, A. S., Irvine, R. J., Karlsen, S. R., Olack, G., & Hobbie, E. A. (2019). Isotope ecology detects fine-scale variation in Svalbard reindeer diet: implications for monitoring herbivory in the changing Arctic. <em>Polar Biology, 42</em>(4), 793–805. https://doi.org/10.1007/s00300-019-02474-8
description: Microsoft Excel All analyses were done in R
 version 4.05. (R Core Team, 2021) in R Studio (version
 1.4.1106). R-packages: 
 <em> simmr</em> version 0.4.5 
 <em>SIDER</em> version 1.0.0.0 
 <em>SIBER</em> (Stable Isotope Bayesian Ellipses in R package)
 version 2.1.5 <em>lme4</em> version
 1.1–27.1 <em>cAIC4</em> version
 1.0 <em>piecewiseSEM</em> version
 2.1.2 <em>redres</em> version
 0.0.0.9