DOI: 10.5061/dryad.dz08kprtz
title: Impacts of female body size on cannibalism and juvenile abundance in a dominant arctic spider

description: 1. Body size influences an individual’s physiology and the nature of its
 intra- and interspecific interactions. Changes in this key functional
 trait can therefore have important implications for populations as well.
 For example, among invertebrates, there is typically a positive
 correlation between female body size and reproductive output. Increasing
 body size can consequently trigger changes in population density,
 population structure (e.g., adult to juvenile ratio), and the strength of
 intraspecific competition. 2. Body size changes have been documented in
 several species in the Arctic, a region that is warming rapidly. In
 particular, wolf spiders, one of the most abundant arctic invertebrate
 predators, are becoming larger and therefore more fecund. Whether these
 changes are affecting their populations and role within food webs is
 currently unclear. 3. We investigated the population structure and feeding
 ecology of the dominant wolf spider species (Pardosa lapponica) at two
 tundra sites where adult spiders naturally differ in mean body size.
 Additionally, we performed a mesocosm experiment to investigate how
 variation in wolf spider density, which is likely to change as a function
 of body size, influences feeding ecology and its sensitivity to warming.
 4. We found that juvenile abundance is negatively associated with female
 size and that wolf spiders occupied higher trophic positions where adult
 females were larger. Because female body size is positively related to
 fecundity in P. lapponica, the unexpected finding of fewer juveniles with
 larger females suggests an increase in density-dependent cannibalism as a
 result of increased intraspecific competition for resources. Higher rates
 of density-dependent cannibalism are further supported by the results from
 our mesocosm experiment, in which individuals occupied higher trophic
 positions in plots with higher wolf spider densities. We observed no
 changes in wolf spider feeding ecology in association with short-term
 experimental warming. 5. Our results suggest that body size variation in
 wolf spiders is associated with variation in intraspecific competition,
 feeding ecology, and population structure. Given the widespread
 distribution of wolf spiders in arctic ecosystems, body size shifts in
 these predators as a result of climate change could have implications for
 lower trophic levels and for ecosystem functioning. 
description: <i>Field
 sampling</i> Wolf spider communities were
 sampled from two arctic tundra sites, Toolik (68°65′N,
 149°58′W) and Imnavait
 (68°62′N, 149°30′W) in the summer of 2012. The sites
 are 10.4 km apart in the northern foothills of the Brooks
 Range, Alaska,
 near
 Toolik Lake Field Station. At each site, we sampled wolf
 spiders two weeks, one month, and two months after snowmelt had occurred
 (Table S1 contains the dates of sampling, spring snowmelt, and fall snow
 accumulation during the study year). At two weeks and one month past
 snowmelt, we sampled each site using pitfall traps placed one meter apart
 on a grid of 10 x 10 meters. Due to concerns about depleting the local
 population and potential low sample sizes during the later part of the
 season, at two months past snowmelt, we sampled adjacent areas at each
 site using four 5 x 5 meter grids of pitfall traps; each grid was
 separated by at least thirty meters. Thus at each sampling time point,
 there were 100 pitfall traps per site. Pitfall traps contained 75% ethanol
 and were left out for 24 hours during each sampling period, except during
 the sampling at one month past snowmelt at Imnavait, where traps were open
 for 48 hours due to unforeseen poor weather. We therefore only used
 pitfall catch data from 15 and 60 days past snow melt to compare
 abundances of females between sites (see below). 
 <i>Sample
 processing</i> We identified all adult female
 specimens from Toolik and Imnavait to species according to
 Dondale and
 Redner (1990), except for the classification of
 <i>Pardosa concinna</i> and <i>Pardosa
 lapponica</i>, which are likely to be the same species
 (Sim et al.
 2014).
 Juveniles cannot be identified to the species level by morphology alone.
 As in Sackett
 et al. (2008), we assume that the relative
 abundance of juveniles of each species reflects that of the adult
 community. To compare body sizes (and hence, reproductive potential) at
 each site and to relate body size to trophic position, we measured the
 carapace width of every adult female and juvenile
 wolf spider using digital calipers (Diesella, Kolding, Denmark).
 We also
 dissected all egg sacs present from females sampled at Toolik and Imnavait
 in order to estimate parasitism rates at the two sites.
 <i>Mesocosm
 experiment</i> We conducted a fully
 factorial mesocosm experiment during June-July 2013 near Toolik Field
 Station to measure the effects of altered wolf spider densities and
 warming on wolf spider feeding ecology (Fig. S1). The mesocosms were 1.5
 meters in diameter and enclosed with aluminum flashing that was buried 20
 cm belowground and stood 20 cm above the soil surface. Plots were
 distributed among five blocks and randomly assigned to one of six spider
 density/warming treatments for a total of 30 plots (see Koltz et al. 2018
 <em>PNAS </em>for more detailed methods on the mesocosm
 experiment).  For
 the warming treatment, we covered the mesocosm openings with heavy gauge
 plastic sheeting that had regularly spaced, small openings to allow the
 passage of air and rainfall. We measured temperature at the soil surface
 in the plots every half hour from June 12 – July 22<sup>nd
 </sup>using iButtons (Maxim Integrated, San Jose, CA). Temperatures
 during this period were significantly higher in the experimentally warmed
 plots than in the ambient temperature plots (mixed effects model with plot
 nested in block; warming
 treatment: Estimate ± SE = 2.223 ± 0.381, df = 27, t = 5.845, p &lt;
 0.0001). Mean temperatures in the ambient temperature plots
 were 15.23 (± 2.35 SE)°C and those in the warmed plots were 17.45 (± 2.95
 SE)°C (Fig. S2). The spider density treatments
 included: 1) reduced wolf spider density; 2) control wolf spider density,
 and 3) high wolf spider density. We removed all wolf spiders from the
 reduced density plots at the beginning of the summer and continued to
 check and remove individuals throughout the experiment. High-density plots
 received enough additional spiders at the start of the experiment to bring
 wolf spider densities to approximately double the early season average
 density of control plots. We used pitfall traps at the end of July to
 verify that we had successfully manipulated wolf spider densities (ANOVA:
 F<sub>2,27</sub> = 6.20, <i>p</i> = 0.0061).
 Post-hoc Tukey tests for these data showed that high density plots had
 significantly more spiders than either low density (<i>p</i> =
 0.0061) or control plots (<i>p</i> = 0.044), although
 densities did not differ between low and control density plots
 (<i>p</i> = 0.68; Fig. S3a). Density estimates from the
 pitfall traps showed that there was an average of 0.3 ± 0.48 wolf spiders
 in low density plots, 0.6 ± 0.7 wolf spiders in control plots, and 1.5 ±
 1.08 wolf spiders in the high density plots. At the conclusion of the
 experiment, we destructively sampled the plots in order to
 opportunistically catch any remaining wolf spiders. We measured the body
 sizes (carapace width) of these specimens and of the wolf spiders caught
 earlier in pitfall traps and removed the legs from a subset of individuals
 for stable isotope analyses. <i>Wolf spider feeding ecology
 </i> We used stable isotope analyses of
 carbon (C) and nitrogen (N) to quantify the functional feeding roles of a
 subset of juveniles and non-egg sac carrying adult female <i>P.
 lapponica</i> from the Toolik and Imnavait field populations. We
 also conducted stable isotope analyses on all captured wolf spiders from
 the mesocosm experiment. While sample storage in ethanol can
 affect carbon stable isotope ratios in invertebrates, we assume here that any effects
 of ethanol storage were similar across samples and experimental
 treatments. We also measured the stable isotope
 ratios of a subset of Collembola that were removed from pitfall traps to
 use as site-specific isotopic baselines for Toolik and Imnavait
 (see Ponsard
 and Arditi 2000). We used Collembola as our
 isotopic baseline because this group is an important prey source for wolf
 spiders (Wise
 2004, Koltz et al. 2018b), and because as detritivores, they
 should reflect differences in <i>δ</i><sup>13</sup>C between the two sites due to any differences in vegetation. Values of <i>δ</i><sup>13</sup>C and <i>δ</i><sup>15</sup>N for the field-collected wolf spiders from Toolik and Imnavait are expressed as the measured values minus the site-specific mean value for Collembola. Wolf spider and Collembola specimens were dried for 48 hours at 60 °C and homogenized with a mortar and pestle prior to stable isotope analysis. We used whole bodies of specimens from the Toolik and Imnavait field populations and between 4-6 legs from wolf spiders collected from the mesocosm experiment. Because of their small size, up to 13 Collembola were pooled per sample. Stable isotope ratios of animals were then determined at the Duke Environmental Stable Isotope Laboratory (DEVIL) using a Carlo-Erba NA1500 elemental analyzer feeding a Thermo Finnigan Delta Plus XL continuous flow mass spectrometer system. Ratios were calculated as <i>δ</i>X in per ml (vs. atmospheric N<sub>2</sub> for <sup>15</sup>N and vs. VPDB for <sup>13</sup>C) as ((R<sub>sample</sub>− R<sub>standard</sub>)/R<sub>standard</sub>) x 1000, whereby R represents the heavy to light isotope ratio (i.e. <sup>13</sup>C/<sup>12</sup>C or <sup>15</sup>N/<sup>14</sup>N) and X is the target isotope. Reference materials from The National Institute of Standards and Technology (NIST) and the U.S Geological Survey (USGS), as well as internally calibrated standards were used for 3-point normalization of raw isotope data. The ratio of standards to samples analyzed was approximately 1:6.