DOI: 10.5061/dryad.jq2bvq868
title: Predator population size structure alters consumption of prey from epigeic and grazing food webs

description: Numerous studies have found that predators can suppress prey densities and
 thereby impact important ecosystem processes such as plant productivity
 and decomposition. However, prey suppression by spiders can be highly
 variable. Unlike predators that feed on prey within a single energy
 channel, spiders often consume prey from asynchronous energy channels,
 such as grazing (live plant) and epigeic (soil surface) channels. Spiders
 undergo few life cycle changes and thus appear to be ideally suited to
 link energy channels, but ontogenetic diet shifts in spiders have received
 little attention. For example, spider use of different food channels may
 be highly specialized in different life stages and thus a species may be a
 multichannel omnivore only when we consider all life stages. Using stable
 isotopes, we investigated whether wolf spider (Pardosa littoralis,
 henceforth Pardosa) prey consumption is driven by changes in
 spider size. Small spiders obtained &gt; 80% of their prey from the
 epigeic channel, whereas larger spiders used grazing and epigeic prey
 almost equally. Changes in prey consumption were not driven by changes in
 prey density, but by changes in prey use by different spider size classes.
 Thus, because the population size structure
 of Pardosa changes dramatically over the growing season,
 changes in spider size may have important implications for the strength of
 trophic cascades. Our research demonstrates that life history can be an
 important component of predator diet, which may in turn affect community-
 and ecosystem-level processes.
description: Study site and organisms We conducted this
 study at a salt marsh near Tuckerton, New Jersey, USA (39° 30.8′ N, 74°
 19.0′ W) that is dominated by the cordgrass Spartina alterniflora (Denno
 et al. 2002, now reclassified as Sporobolus alterniflorus). We focused on
 the dominant species in the live plant and epigeic (algal and detrital)
 food webs; relationships among these dominant arthropods have been well
 characterized (Finke and Denno 2002, 2003, 2004, 2005; Wimp et
 al. 2013). Pardosa hatch and grow during the summer, overwinter as
 late-instar juveniles, mature to adults in the spring and begin
 reproduction in early summer (Dobel et al. 1990). The grazing herbivore
 assemblage in S. alterniflora is dominated by two congeneric planthopper
 species, Prokelisia dolus and P. marginata, which constitute upwards of
 80% of herbivore biomass (Denno et al. 2000). The planthopper Delphacodes
 penedetecta and the mirid bug Trigonotylus uhleri rank as the next most
 abundant herbivores. The mirid bug Tytthus vagus consumes planthopper eggs
 exclusively, and frequently falls prey to Pardosa. We
 focused on three species to represent the marsh epigeic food web because
 of their abundance at the study marsh, similar to the high relative
 abundances of the herbivore species listed above: Orchestia
 grillus, Venezillo parvus, and Saldula interstitialis. The amphipod O.
 grillus is abundant and feeds on surface epiphyton as well
 as Spartina detritus (Agnew et al. 2003; Lopez et al. 1977); although
 amphipods may not be part of a typical terrestrial detrital food web, they
 are commonly found in systems with a terrestrial–aquatic interface, both
 freshwater and marine. The isopod V. parvus is less abundant than O.
 grillus at this marsh, but ingests substantial amounts of Spartina litter
 (Zimmer et al. 2004). Finally, the saldid bug S. interstitialis is
 saprophagous in early instars, while older nymphs and adults are active
 predators feeding on surface and subsurface arthropods, including
 amphipods (Griesinger and Bauer 1990). Arthropod
 samples Four times during the summer of 2009 we sampled
 marsh arthropods from six blocks, each composed of three
 10-m<sup>2</sup> sample plots. Within each block, the three
 plots were located in different habitats. One plot was located near the
 upland border of S. alterniflora with S. patens (this border occurs at
 mean high water level), a second plot was located in the center of the S.
 alterniflorameadow, and the third in intermediate form S.
 alterniflora near a tidal creek. We sampled the entire arthropod community
 on 16 Jun 2009, 20 Jul 2009, 18 Aug 2009, and 15 Sept 2009 using a D-vac
 suction sampler that we placed in ten different locations within each plot
 for 3-s periods following the methods of Wimp et al. (2013) and Murphy et
 al. (2012). We immediately placed the collected arthropods into closed
 containers with ethyl acetate, then transported them to the laboratory in
 coolers with dry ice and stored in a − 20 °C freezer until they were
 processed. We sorted arthropod samples by species,
 counted the abundance of each species, cleaned them of foreign debris, and
 dried them at 60 °C for 3 days. Pardosa were then weighed and assigned to
 one of six size classes by dry weight (Appendix A). Size classes were
 chosen to divide the collected Pardosa into roughly equally represented
 groups. Stable isotope analysis We
 used stable isotopes of carbon to assess the level of multichannel
 omnivory by Pardosa over the course of the summer, by measuring
 δ<sup>13</sup>C values of Pardosa and their grazing and
 epigeic prey. While δ<sup>15</sup>N values are useful for
 determining the trophic level of an organism and we measured them in this
 study for this purpose, δ<sup>13</sup>C values reflect changes
 in food web use. Carbon isotopes have proven useful in determining the
 relative importance of dual food webs in the diets of generalist predators
 (Newsome et al. 2007; Wimp et al. 2013; Wise et al. 2006), because carbon
 isotopic signatures, δ<sup>13</sup>C, remain relatively
 unchanged from food source to consumer (Fry 2006). Therefore, if the
 δ<sup>13</sup>C values of the bases of two food webs are
 sufficiently distinct, a predator’s δ<sup>13</sup>C value
 reflects its relative consumption from the two webs even when intermediate
 feeding links are unclear (DeNiro and Epstein 1981; Post 2002). In
 previous work, we found that δ<sup>13</sup>C values in salt
 marsh grazing and epigeic webs differed substantially for at least part of
 the summer (Wimp et al. 2013). In the present study, we examined both
 δ<sup>13</sup>C to examine whether Pardosa diet changes across
 different size classes, and δ<sup>15</sup>N to confirm trophic
 position. We had to combine individuals of the smaller size classes to
 obtain sufficient tissue for stable isotope analysis, but only individuals
 collected in the same plot on the same day were combined. We then ground
 samples and obtained their isotopic values as described in Wimp et al.
 (2013). Statistical analyses We
 performed an ANOVA on Pardosa δ<sup>15</sup>N values across
 the different collection months to confirm predator status and see how
 this status might change during the season or depending on the S.
 alterniflora habitat type (center, intermediate, edge; Appendix B). In all
 ANOVA and regressions involving Pardosa isotopes, we treated
 all Pardosa recovered from a plot as repeated measures from that plot. We
 also examined how Pardosa δ<sup>15</sup>N changed with spider
 size using a correlation analysis (Appendix B). To
 examine how multichannel omnivory was related to the size of Pardosa, we
 used carbon stable isotopes. Initially, we collected Pardosa from
 different habitats, but we wanted to determine whether carbon isotopes
 differed according to habitat or could be combined. Thus, we performed a
 repeated measures ANOVA by SAS proc mixed (SAS 2002), which showed that
 habitat within block had no effect on the δ<sup>13</sup>C of
 grazing prey (F<sub>2,17.1</sub> = 0.03, P = 0.97), epigeic
 prey (F<sub>2,14.3</sub> = 2.45, P = 0.12),
 or Pardosa (F<sub>2,12</sub> = 1.32, P = 0.30), and that
 habitat had no effect on Pardosa size
 (F<sub>2,15.5</sub> = 1.24, P = 0.32). Thus, the different
 habitats in each of our transects did not affect
 δ<sup>13</sup>C isotopic ratios and were combined for further
 analysis. We were able to collect a large range
 of Pardosa sizes only in June and July (Appendix A), so we examined the
 relationship between δ<sup>13</sup>C and size only during
 those months. Because we had to combine individuals of the smaller size
 classes (1–3) to obtain sufficient tissue for stable isotope analysis
 (explained above), we were not able to collect enough individuals of the
 smaller size classes to obtain even a single sample for the months of
 August and September, and we only had a single sample in size class 4
 (Appendix A, Figs. A1, A2). We performed separate ANOVA analyses in June
 and July with size class as a discrete explanatory variable and
 δ<sup>13</sup>C as the response. We included only size classes
 for which we had at least five measurements in a given month. Variances
 were not equal across size classes, so the ANOVA was performed using
 separate variance estimates for each size class with SAS proc
 mixed.