DOI: 10.25338/b8306p
title: Temporal changes in predator density are linked to shifts in prey behavior, mortality, and abundance in the field

description: Predators suppress prey populations and elicit defensive phenotypes in
 prey. The magnitude of predator effects depends upon several factors,
 including the density of predators, and their cue concentrations, in the
 environment. Predator density manipulations have often relied on
 laboratory studies that use unrealistic densities of predators and prey
 over unnatural temporal and spatial scales. Field studies can provide
 insights into predator-prey interactions under more realistic scenarios.
 However, field studies linking predator density and prey populations are
 limited by the challenge of manipulating predator densities or predicting
 predator densities in dynamic/stochastic environments. We exploited a
 somewhat predictable rise in predatory crab communities associated with
 ocean warming to evaluate the impacts of swimming crab density on
 ecologically important horn snails. Our approach combined long-term
 monitoring of crabs and snails with snail behavior surveys and snail
 tethering experiments repeated during and after a marine heat wave. Near
 the end of a marine heat wave in 2016, swimming crabs were found in 25% of
 marsh tidal creeks. No swimming crabs were found in these tidal creeks
 during cooler water conditions in 2012 and 2021. When swimming crabs were
 more abundant (e.g. 2016), horn snails experienced 7,533% more mortality
 and were 91% less abundant. The proportion of snails climbing vegetation
 was 454% higher when swimming crabs were more abundant. Thus, higher
 predator densities occurring during a marine heat wave were associated
 with changes in snail abundance, mortality, and behavior. Such changes
 could influence marsh food webs and nutrient cycling. Our findings
 highlight the value of exploiting climatic anomalies to understand
 ecological patterns linked to the top-down effects in ecosystems - a
 critical need in predator-prey ecology.
description: Study system and site California horn snails
 (hereafter, horn snails) are ubiquitous in tidal marshes and mudflats
 across the Southern California Bight (Lorda & Lafferty 2012).
 Throughout this region, horn snails are prey for several predators
 (Armitage & Fong 2006; Lorda et al. 2016)and are intermediate
 hosts for parasitic trematodes (Hechinger & Lafferty 2005;
 Lafferty et al. 2006b). Horn snails can be predated upon by predatory
 swimming crabs during high tides, when tidal inundation facilitates crab
 movements into mud flat and marsh habitats (Cote et al. 2001; Belgrad
 & Smith 2014). In southern California, the abundance of these
 swimming crabs (e.g., the Xantus’ swimming crab, Portunus xantusii, and
 the arched swimming crab, Callinectes arcuatus; hereafter swimming crabs)
 is connected to sea water temperature. Warm water events (e.g., marine
 heat waves, El Niños) are associated with influxes of swimming crabs
 (Zedler et al. 1992; Williams et al. 2001). Consistent
 with observations during warm water events in 1982-83 and 1997-98 (Zedler
 et al. 1992; Williams et al. 2001), the San Onofre Nuclear Generating
 Station (SONGS) Mitigation Monitoring Program found elevated abundances of
 swimming crabs in San Dieguito Lagoon (hereafter SDL) during the 2014-2016
 Pacific marine heat wave. SDL is a restored 123 acre brackish tidal marsh
 located in Del Mar, California (32° 58’ 40.4’’N, 117° 14’ 32.8’’W)
 comprised of marsh, mud flats, and subtidal habitats (e.g., tidal creeks).
 Tidal marsh is the dominant habitat in SDL and is defined by robust
 populations of cordgrass (Spartina foliosa) and pickleweed (Sarcocornia
 pacifica) that were started via transplants in 2008 (Walker et al. 2021;
 Beheshti et al. 2022). Mud flats and subtidal habitats account for 18 and
 31 acres of SDL, respectively (Beheshti et al. 2022). Horn snails and
 swimming crabs co-occur on mud flats and marsh habitats, particularly at
 high tide. Lab Assay-Consumption: To
 assess the relative predation-risk of swimming crabs and Lined shore crabs
 on horn snails, we conducted a short feeding trial. We offered horn snails
 to individual adult swimming crabs [n = 5; biomass:  127 ± 19 g (mean ±
 1SE)] and Lined shore crabs (n = 5; biomass:  15 ± 1 g) in plastic
 containers (30 x 25 x 25 cm, l x w x h) with flow through seawater. To
 standardize the number of snails offered to these different sized crabs,
 we added 95 and 10 horn snails to swimming crab and Lined shore crab
 treatments, respectively. The horn snails offered to crabs were 5– 28 mm
 in length (measured as the distance between the shell aperture and apex).
 This range is representative of the natural range of sizes observed at
 SDL. We systematically added snails to each replicate to make sure large
 and small snails were evenly distributed across swimming crab and Lined
 shore crab treatments. We assessed crab predation rates on horn snails 24
 hours later. Lab Assay- Crab Abundance:
 In August 2016, we used laboratory mesocosms to understand if
 waterborne chemical cues from swimming crabs elicit defensive behaviors in
 horn snails. We exposed snails to one of three cues: No Crab, Swimming
 Crab, and Lined shore Crab (n = 7). We included cues from Lined shore
 crabs (Pachygrapsus crassipes) to compare to cues from swimming crabs
 because Lined shore crabs consume horn snails, have been reported to
 elicit defensive phenotypes in horn snails, and are abundant at SDL
 (Armitage & Fong 2006; Lorda et al. 2016; Walker et al. 2021). All
 invertebrates used in our laboratory assays were collected from SDL. We
 collected horn snails and Lined shore crabs by hand, and we collected
 swimming crabs with baited, vinyl-coated crab traps [65 × 49 × 25 cm, l x
 w x h; covered with mesh (3.8 cm openings)] with 2 trap door entrances (16
 × 8 cm, w x h). All replicates consisted of an upstream
 (9 x 16.5 x 9 cm, l x w x h) and downstream container (15 x 14cm, h x
 diameter) that received flow through seawater from San Diego Bay (32° 43’
 48.1’’N, 117° 12’ 51.0’’W). Upstream containers (1 L) drained into
 downstream containers (2.36 L) at a rate of ~1 L minute-1. Four drainage
 holes (2.5 cm diameter each) were drilled into the downstream containers
 9.5 cm above the container floor. Each downstream container held five horn
 snails (≥ 10 mm length). To prevent snail escape, holes and the top of the
 containers were covered with plastic mesh (6 mm opening). We randomly
 assigned upstream containers to one of three treatments (No Crab, Swimming
 Crab, Lined shore crab). No Crab treatments were left empty. In Swimming
 Crab and Lined shore crab treatments, we included a single adult swimming
 crab or Lined shore crab, respectively [Swimming crab biomass: 138 ± 14 g
 individual-1 (mean ± 1SE); Lined shore crab biomass: 15 ± 1 g
 individual-1].  Crabs were not fed during the assay but were provided horn
 snails ad libitum prior to the study. We exposed horn
 snails in the downstream buckets to waterborne cues from upstream
 containers for 48 h. Then, we quantified the proportion of snails found
 above the waterline (i.e., the proportion of snails with their entire foot
 above the waterline). We monitored the proportion of snails climbing above
 the waterline because previous studies suggested that horn snails may seek
 refuge by crawling out of water containing predators and their associated
 cues (Belgard and Smith 2014). Lab Assay- Crab
 Biomass: Because adult swimming crabs are considerably
 larger than Lined shore crabs [138 ± 14 g individual-1 and 15 ± 1 g
 individual-1 (mean ± 1SE); respectively], we conducted a second behavioral
 assay in September 2016 to test if horn snails respond to Lined shore crab
 cues when crab biomass is increased. We included two treatments: No Crab
 (n = 7) and Lined Shore Crab (n = 6). All replicates
 consisted of an upstream and downstream container and were set up as
 described in the Lab Assay-Abundance Methods section. However, in Lined
 Shore Crab treatments, instead of adding a single crab, we added 113 ± 5 g
 of Lined shore crabs, equal to 12 ± 1 individual Lined shore crabs. We
 targeted this biomass because we observed strong effects of individual
 swimming crabs in our previous assays and their biomass was similar
 (105-194 g, n = 9). We exposed horn snails in the
 downstream containers to waterborne cues from upstream containers for 48
 hours. After 48 hours, we quantified 1) if each snail was climbing and 2)
 if the snail had climbed above the waterline (i.e., climbed &gt; 9.5
 cm). Temporal monitoring data:
             Horn snail and swimming crab relative abundance data
 at SDL were obtained from the SONGS Mitigation Monitoring Program. This
 monitoring effort began in 2012 and surveys sites on an annual basis in
 the late summer or early fall (July-November; Beheshti et al. 2022). Each
 year, six tidal creeks were monitored except in 2016 when eight tidal
 creeks were monitored. Within each tidal creek, snail and crab presence
 were monitored at five sampling stations spaced 10-20 m apart. At each
 sampling station within each tidal creek, horn snail presence was
 evaluated along two transects placed perpendicular to the water mark.
 Along each transect, three quadrats (0.06 m2) were placed between the
 lower limit of vegetation (~ 1.3 ft NGVD) and the thalweg of the tidal
 creek (~50-60 cm water depth). This resulted in a total of six quadrats
 per station, and 30 quadrats per tidal creek. Swimming crab presence
 (presence/absence) was assessed at all five sampling stations in each of
 the six tidal creeks using a circular enclosed trap (0.9 m tall x 0.74 m
 diameter; 0.43 m2 area) and a beach seine (2 m tall x 7.6 m wide, 3.2 mm
 mesh) covering a ~46 m 2 area (see (Steele et al. 2006b, a). The
 proportional abundance of horn snails and swimming crabs was then
 calculated by dividing the number of tidal creeks where each species was
 observed in any quadrat (snails) or any trap/seine (crabs) by the total
 number of tidal creeks sampled (n = 6 or n = 8 in 2016).
 Mortality study: In September 2016 and 2021,
 we evaluated horn snail predation-risk at three tidal elevations of a
 common site (32° 58’ 39.8’’N, 117° 14’ 40.9’’W) using a tethering
 experiment. Specifically, we deployed 30 - 0.5 m2 PVC frames containing 25
 tethered horn snails each. Each PVC frame included five lengths of braided
 fishing line (rated to 50lbs) running parallel across the frame at 10 cm
 intervals. Five horn snails were tethered to each of the five fishing
 lines attached to every PVC frame. Horn snails were individually tethered
 using a 5 cm length of braided fishing line (rated to 50lbs) attached to
 the apex of each snails’ shell with cyanoacrylate. We
 deployed tethered snails at three horizontal distances landward of MLLW
 (1.5, 3.0, and 4.5 m). These horizontal distances were associated with an
 increase in elevation. At each distance, plots were either open to allow
 predators access to tethered snails or were protected by a plastic mesh
 cage (1 cm opening) to act as mortality controls (open plots, n = 5 both
 years; caged plots, n = 5 and n = 2 for 2016 and 2021, respectively). We
 deployed fewer caged plots in 2021 because no snails were lost from these
 in 2016 when predation was high in open plots. We surveyed the number of
 live tethered horn snails remaining in each plot after 48 h. Additionally,
 we counted the number of tethered horn snails that had buried into the
 soil in caged plots because previous studies suggested that horn snails
 bury in response to predatory crabs (Lorda et al. 2016).
 Mortality Study- Crab Traps: To characterize
 the subtidal predator community in tidal creeks adjacent to our tethering
 plots, we deployed two crab traps within one week of our tethering studies
 in both 2016 and 2021. Vinyl-coated crab traps [65 × 49 × 25 cm, l x w x
 h; covered with mesh (3.8 cm openings)] with 2 trap door entrances (16 × 8
 cm, w x h) were placed in the center of the tidal creek adjacent to the
 location of our tethering site. Traps were placed 10 m apart and were
 baited with squid (0.23 kg each). After 48 h, we counted the number of
 trapped swimming crabs (P. xantussii and C. arcuatus).
 Field Survey & Field Survey- Climbing Logit:
 To understand how changes in swimming crab abundance may be
 linked to the abundance, distribution, and climbing behavior of horn
 snails in the field, we conducted transect surveys spanning the intertidal
 zone at a single site (32° 58’ 39.8’’N, 117° 14’ 40.9’’W) at SDL in 2016
 and 2021. Specifically, we placed five - 10 m transects perpendicular to
 the waterline starting at MLLW and extending landward. Individual
 transects were spaced 2 m apart. Along each transect, we recorded the 1)
 density of non-climbing horn snails, 2) density of climbing horn snails,
 and 3) percent cover of plants (mainly pickleweed) every meter using a 0.5
 x 0.5m quadrat. Horn snails were considered climbing if their entire foot
 was attached to vegetation, rather than the bare sediment. Since horn
 snails are known to bury in the presence of crab predators (Armitage
 & Fong 2006; Lorda et al. 2016), we searched the top 2 cm of soil
 in quadrats for buried horn snails.  We also evaluated the propensity of
 horn snails to migrate landward from the MLLW in 2016 and 2021 using a
 mark and recapture study (see Horn snail migration study in Appendix
 S2). Migration Study: We evaluated
 the propensity of horn snails to migrate out of “high-risk” habitats close
 to the MLLW using a mark-and-capture study. In September 2016 and 2021, we
 collected 100 horn snails from San Dieguito Lagoon (SDL). Snails were
 transported to the Coastal and Marine Institute Laboratory where their
 shells were dabbed dry and painted with red nail polish. The nail polish
 dried for at least 12 hours before snails were returned to SDL and
 released inside a 0.5 x 2.0 m plot at Mean Lower Low Water (MLLW). The
 corners of the plot were marked with PVC stakes. We surveyed snail
 distribution and behavior 48 hours later. We systematically searched the
 area around the plot (see Fig. S2 for search area schematic), raking our
 fingers through the top 1-2cm of soil to recover any snails that may have
 buried themselves. For each recovered snail, we estimated the minimum
 horizontal distance traveled. The minimum horizontal distance was defined
 as the distance between the snail’s foot and the closest 2.0 m side of the
 deployment plot.