DOI: 10.5061/dryad.sn02v6xd0
title: Data from: Latitudinal variation in the constitutive and inducible defenses of a canopy-forming rocky intertidal seaweed

description: A long-standing theory in biogeography is that stronger biotic
 interactions at lower latitudes select for better-defended phenotypes.
 However, greater environmental variability at temperate latitudes may also
 shape defensive strategies by increasing temporal or local spatial
 variation in consumer pressure and thus selecting for greater phenotypic
 plasticity, or inducible defenses. Distinguishing between inducible and
 constitutive defense strategies is therefore necessary to test for
 latitudinal defense gradients, but also for understanding how species
 interactions and community dynamics vary across ecological and
 evolutionary scales.  We investigated latitudinal variation in
 antiherbivore defenses of a cosmopolitan rocky intertidal seaweed (Fucus
 vesiculosus) against a similarly common and abundant grazer (periwinkle
 snails, Littorina littorea). We used a multi-phase common garden
 experiment with seaweeds from three different regions along the US
 Atlantic coast and snails from the southern- and northern-most study
 sites. We manipulated and measured snail grazing on seaweeds in a series
 of no-choice and two-choice assays to identify regional differences in
 seaweed functional defenses (i.e., reduced grazing).  Across all
 assays, grazing rates declined with seaweed latitude. Prior grazing
 reduced the palatability of southern, but not northern, seaweeds. Changes
 in seaweed nutritional content (C:N) and phlorotannins (a putative
 chemical defense) correlated with induced but not constitutive functional
 defenses.   Our results indicate that the constitutive
 anti-herbivore traits of F. vesiculosus increase with latitude and
 negatively covary with defense plasticity. This result suggests that the
 selective pressure of herbivory is stronger for northern seaweed
 populations, while southern populations may face a different set of
 tradeoffs leading to defense plasticity, such as increasing environmental
 stress.   The strength of trophic interactions plays an
 important role in community dynamics and food web stability. Our findings
 add to a growing literature highlighting the importance of ecological
 context in shaping trophic interactions and suggests that estimates and
 comparisons of interaction strength need to consider spatiotemporal
 variation in prey defenses. Defensive traits that vary with latitude or
 along environmental stress gradients may be particularly important for
 predicting the effects of climate change on trophic interactions and their
 consequences for community dynamics and ecosystem function. 
description: <em><strong>Organism
 Collections-</strong></em> We collected
 <em>F.</em> <em>vesiculosus</em> seaweeds from
 nine sites spanning &gt;750km of NW Atlantic coastline and three
 regions in and around the Gulf of Maine (Fig. 1). Seaweed individuals (one
 10-20cm thallus and its holdfast) were collected from the mid-intertidal
 zone, removing each from the substrate at its holdfast with a metal
 scraper. Individual holdfasts were separated by &gt;0.25m to avoid
 sampling genetically identical thalli (<em>N=</em> 80-100
 individuals site-1). We avoided seaweeds that were reproductive or visibly
 damaged. Upon collection, we froze subset of seaweeds
 (<em>n</em>= 10 site-1) at -20°C for later analysis of
 ‘<em>in situ</em>’ tissue chemistry. <em>L.
 littorea</em> snails were collected from one southern and one
 northern field site (<em>N</em>= 400 site-1; shell length=
 15-25 mm). All snails and seaweed were maintained under ambient light and
 seawater conditions in shallow flow-through tanks (water temperature range
 19-21°C). Snails were kept in separate tanks and fed <em>ad
 libitum</em> with non-experimental sympatric
 <em>Fucus</em> until 24h before starting
 experiments.   <em><strong>Relaxation
 Phase</strong></em> <em>–</em> Seaweeds were
 cleared of all epiphytes and epifauna and maintained in the absence of
 herbivores for 10d beginning 21August 2019. This “relaxation phase” allows
 for defenses that may have been induced in the field to relax or dissipate
 (Underwood et al., 2002), which can take as little as 4d (Rohde &
 Wahl 2008). After 10d, samples of “relaxed” seaweeds
 (<em>n</em>= 6-10 site-1) were frozen as above, while the
 remainder were used in the induction phase (<em>n</em>= 56
 site-1).   <em><strong>Induction Phase
 -</strong></em> Pairs of relaxed, sympatric seaweed
 individuals were maintained in independent flow-through “induction units”
 in the presence or absence of herbivorous snails for 12d. Each induction
 unit consisted of a 1.2L mesh-sided container submerged within a 2.5L
 bucket receiving its own supply of running seawater. A mesh panel divided
 the inner container to separate the two individual seaweeds. Induction
 units were assigned randomly to one of three treatments: Southern Inducer
 Snails, Northern Inducer Snails, or No-Snail Control
 (<em>n</em>= 7, 7, or 14 site-1, respectively;
 <em>N</em>= 252 induction units). Units assigned to inducer
 snail treatments received 3 snails from the respective source population
 in one side of the container, allowing the other seaweed to serve as an
 autogenic growth control (Long et al. 2013). The second seaweed in the
 no-snail controls ensured that total seaweed biomass was similar across
 all induction units and treatments (see Fig. S1).  
 Each seaweed’s tissue mass was measured at the beginning and end
 of the induction phase (mean ± SE= 1.068 ± 0.014g, <em>N=</em>
 504). Individuals were blotted dry with paper towels to remove all visible
 moisture prior to weighing. Inducer snails (<em>n</em>= 3
 unit-1) were also blotted dry with paper towels and weighed, as a group,
 for total biomass per unit (Bunit). At the end of the induction phase and
 after measuring final mass, two apical tips were clipped from each seaweed
 for use in choice assays (Sotka et al., 2002, Taylor et al., 2002), with
 additional tips sampled and frozen for later analysis of “post-induction”
 tissue chemistry.   The total amount of seaweed grazed
 by snails within a given unit (Gunit) was estimated as Si(Af/Ai)-Sf, where
 Si and Sf are the initial and final mass, respectively, of the seaweed
 with snails, while Ai and Af are those of the paired autogenic control
 (Long et al., 2013). Because northern snails were slightly larger than
 southern snails (mean ± SE individual mass, 3.82 ± 0.05 vs. 3.16 ± 0.04
 g), we also calculated mass-adjusted, <em>per capita</em>
 daily grazing rates (Gpc) normalized for an individual 3.5g snail using
 the formula 3.5*(Gunit/Bunit)/Dunit, where Bunit is the total biomass of
 the three snails in the unit, and Dunit is the induction phase duration
 (11.6 – 12.0 d). We analysed Gunit and Gpc with separate linear mixed
 models (“LMMs”) that included seaweed region, inducer snail population,
 and their interaction as fixed effects. Seaweed collection site (within
 region) and the site x inducer interaction were included in the model as
 random intercepts to account for the nested experimental design. Seaweed
 growth rates in the no-snail control treatment were calculated as
 (Cf-Ci)/Ci where Cf and Ci are the final and initial mass, respectively.
 We calculated then analysed the average growth rate in each induction unit
 using a similar LMM that included region (fixed effect) and seaweed
 collection site (random effect). Seaweeds from 5 units were excluded from
 analyses and choice assays: two units experienced water flow issues and
 mortality and three units were missing seaweeds. 
 <em><strong>Choice Phase</strong></em>
 <em>–</em> We conducted two types of choices assays to assess
 differences in relative palatability (1) between regions (“inter-region”
 assays) and (2) between induction treatments within each region
 (“intra-region” assays). Choice assays were conducted in the same
 flow-through setup as the induction phase but with single-compartment mesh
 containers (“choice units”). Each choice unit contained a pair of apical
 tips from seaweeds exiting the induction phase and either three northern
 or three southern “chooser” snails (Fig. S1). Seaweed collection sites
 within each region were pooled and seaweeds were chosen at random for each
 choice replicate. (<strong>1</strong>)
 <em>Inter-region</em> assays offered snails a choice between
 one lower- and one higher-latitude tip from no-snail control seaweeds.
 Three paired region choices (lower/higher: South/Central, South/North,
 Central/North) were crossed with the two chooser snail populations (N, S)
 in a 3 x 2 design with <em>n=</em> 12 choice units for each
 choice x chooser combination (<em>N</em>= 72 inter-region
 choice units). (<strong>2</strong>)
 <em>Intra-region</em> assays offered snails choice between two
 seaweed tips from the same region but different induction treatments: one
 tip was from a seaweed in the no-snail control treatment and the other was
 from a seaweed in either the N- or S-inducer snail treatment. Two paired
 control/inducer choices (Control/N Inducer, Control/S Inducer) for each of
 the three seaweed regions were offered to N and S chooser snails in a 2 x
 3 x 2 factorial design. Seaweeds from two induction units (S seaweed in
 the N inducer treatment) could not be included in choice assays because
 high grazing rates during induction removed most or all apical tissues. In
 addition to mortality losses described above (see “Induction Phase”), this
 led to an unbalanced design with 7-12 replicates for each inducer x region
 x chooser treatment combination (<em>N=</em> 138 intra-region
 choice units).   In both choice assays, the different
 choice combinations (e.g., South/Central, Control/N-induced, etc.) were
 replicated by randomly selecting one seaweed (i.e., induction unit) from
 each of the two corresponding regions or treatments. Source sites within
 each region were pooled and selected randomly. One apical tip from each
 seaweed/induction unit was then placed in each of two choice units, which
 were then randomly assigned to receive either northern or southern chooser
 snails (see Fig. S1 for an example). In this way, we ensured that any
 among-unit differences (i.e., variation among individual seaweeds) within
 each induction treatment were distributed uniformly between the two
 chooser snail populations.   All choice assays began on
 13 September and lasted 3-6d. Units were inspected daily to visually
 assess grazing on apical tips. Assays were terminated on a unit-by-unit
 basis when either tip appeared ~50% consumed, or after a maximum of 6d
 (&gt;70% of units crossed the 50% threshold by day 6). The flexible
 assay duration minimized potentially confounding effects of preferred
 resource limitation (i.e., grazers consuming all of a preferred tip) or
 seaweed trait changes (i.e., defense relaxation or induction) during the
 choice assay.   Seaweed tips were individually weighed
 immediately before and after each assay to calculate tissue mass grazed by
 snails (Gtip= initial-final mass) and standardized <em>per
 capita</em> daily grazing rates as Gpc= 3.5g*(Gtip/Bunit)/Dunit,
 where Dunit is the choice unit-specific assay duration (3-6d). Grazing
 rates on each tip were analysed with LMMs that included choice unit,
 seaweed source site, and induction unit as random intercepts. Grazing
 rates were square root transformed prior to analysis to satisfy
 assumptions of normality and homoscedasticity. Analysis of inter-region
 choice data included seaweed tip relative latitude (low or high), chooser
 snail population (N or S), seaweed region combination (S-N, S-C, C-N), and
 their interactions as fixed effects. Intra-region analyses included
 seaweed tip induction treatment (induced or control), inducer snail
 population (i.e., induced tip exposed to N or S grazers during the
 induction phase), chooser snail population (N or S snail grazing during
 the choice assay), and all interactions as fixed effects. We then
 conducted joint contrast tests on estimated marginal means (R package
 <em>emmeans</em>; Lenth 2023) to test for main and interactive
 effects of induction treatment and inducer snail population within each
 region x chooser combination.  
 <em><strong>Tissue Chemistry
 -</strong></em> Seaweed apical tips from different experiment
 phases were freeze dried, homogenized, and ground into fine powders. We
 extracted phlorotannins from pre-weighed subsamples (~15mg) following Kurr
 & Davies (2019) and spectrophotometrically analysed liquid
 extracts following Pavia & Toth (2000) using Folin-Ciocalteu
 reagent with phloroglucinol (1, 3, 5-trihydroxybenzene, Sigma-Aldrich
 P3502) as a standard. An additional subsample of
 <em>Fucus</em> powder (~1.5mg) was weighed on a
 semi-microbalance (± 0.001mg) and passed through a Fisons NA 1500 Series 2
 Elemental Analyzer (Costech Analytical, Inc.) to estimate carbon and
 nitrogen mass based on curves produced using acetanilide standards
 (C8H9NO, Costech). C:N ratios and phlorotannin concentrations (proportion
 dry mass) of samples collected before (<em>in situ</em>,
 relaxed) or after the induction phase were analysed with LMMs and GLMMs,
 respectively, that included region, relaxation state (or induction
 treatment), and their interaction as fixed effects plus site and site x
 relaxation state (or site x induction treatment) as random intercepts.
 GLMMs of phlorotannin concentrations (proportions) were fit with a beta
 distribution and logit-link. 
 <em><strong>Field Surveys</strong></em>
 <em>–</em> We conducted quadrat surveys of herbivore and
 fucoid assemblages in the late spring/early summer 2020 at 7 of the 9
 collection sites (Fig.1a). Quadrats (0.5 x 0.5 m, <em>n=</em>
 10 site-1) were haphazardly placed in the mid-intertidal zone at each
 site, avoiding tide pools and maintaining &gt;1m between quadrats. For
 each quadrat, we estimated canopy % cover for each algal species using a
 25-point-intercept grid and recorded identity and density (no. 0.25m-2) of
 fucoid holdfasts and herbivores. We measured the maximum frond lengths of
 fucoids located within all or part of the quadrat (up to 30 individuals
 quadrat-1). Densities of the two most common fucoids (<em>F.
 vesiculosus</em> and <em>Ascophyllum nodosum</em>) were
 analysed with negative binomial GLMMs (log-link function) that included
 the region, species, and their interaction as fixed effects, with site and
 the site x quadrat interaction as random intercepts. Frond lengths and
 herbivore densities (<em>Littorina littorea</em> and
 <em>L. obtusata</em>) were analysed with similar GLMMs.
 Percent cover data were logit-transformed for analysis with a similar
 LMM.   <em><strong>Statistical
 Analyses</strong></em> <em>–</em> All statistical
 analyses were performed in R (R Core Team 2024). Models were fit via REML
 with type 3 contrasts to account for unequal sample sizes. Fixed effects
 were evaluated with marginal Wald c2 (GLMMs) F tests on Kenward-Roger df
 (LMMs). Diagnostic tests and residual plots were used to verify
 assumptions of homoscedasticity and normality for LMMs, with data
 transformations and/or weighted variance structures applied as necessary
 following Zuur et al. (2009). We evaluated significant interactions
 (<em>p</em> &lt; 0.05) using linear contrasts on estimated
 marginal means and adjusted contrast p-values for the false discovery
 rate. Details of each model and analysis are given with corresponding
 results in the supporting information (Tables S1-6).  
description: # Data from: Latitudinal variation in the constitutive and inducible
 defenses of a canopy-forming rocky intertidal seaweed **Table of
 Contents**: **File Name**: 1_induction_phase_data.csv **Corresponding
 Experiment Phase**: Induction Phase **Summary**: Compiled data from
 experimental snails and seaweeds during the induction phase used in
 analyses of grazing rates and post-induction seaweed tissue chemistry
 presented in Figures 2a, 2b, 2d, 2f, Appendix Tables S1 and S3 and Figure
 S2. **File Name**: 2_pre_induction_phloro.csv **Corresponding Experiment
 Phase**: Collection and Relaxation Phase **Summary**: Compiled data from
 seaweeds frozen before and after the relaxation phase presented in Figure
 2c and Appendix Table S2a. **File Name**: 3_pre_induction_cn.csv\
 **Corresponding Experiment Phase**: Collection and Relaxation Phase\
 **Summary**: Compiled data from seaweeds frozen before and after the
 relaxation phase presented in Figure 2e and Appendix Table S2b. **File
 Name**: 4_inter_region_choice_phase_data.csv **Corresponding Experiment
 Phase**: Choice Phase **Summary**: Compiled data from experimental snails
 and seaweeds during the inter-regional choice assay phase presented in
 Figure 3a, Appendix Table S4. **File Name**:
 5_intra_region_choice_phase_data.csv\ **Corresponding Experiment Phase**:
 Choice Phase\ **Summary**: Compiled data from experimental snails and
 seaweeds during the intra-regional choice assay phase presented in Figure
 3b, Appendix Table S5. **File Name**: 6_seaweed_canopy_survey_data.csv
 **Corresponding Experiment Phase**: Field Surveys **Summary**: Compiled
 data from field surveys of seaweed canopy cover presented in Figure 4a and
 Appendix Table S6a. **File Name**:
 7_seaweed_density_length_survey_data.csv\ **Corresponding Experiment
 Phase**: Field Surveys\ **Summary**: Compiled data from field surveys of
 fucoid seaweed holdfast density and length presented in Figure 4b and 4c
 and Appendix Table S6b and S6c. **File Name**: 8_snail_survey_data.csv
 **Corresponding Experiment Phase**: Field Surveys **Summary**: Compiled
 data from field surveys of herbivore assemblages presented in Figure 4d
 and Appendix Table S6d. ## Description of the data and file structure ###
 File: 1_induction_phase_data.csv **Variable Name (row 1)**: indunit
 **Variable Type or Units (row 2)**: ID factor **Description**: number used
 to identify an individual induction unit (1-252) **Variable Name (row
 1)**: indsubunit **Variable Type or Units (row 2)**: ID factor
 **Description**: letter used to identify each of the two seaweeds within
 the induction unit (a,b) **Variable Name (row 1)**: region **Variable Type
 or Units (row 2)**: ID factor **Description**: Seaweed source region
 (south, central, north); see also Figure 1 **Variable Name (row 1)**: site
 **Variable Type or Units (row 2)**: ID factor **Description**: Seaweed
 collection site (n = 3 sites region-1); see also Figure 1 **Variable Name
 (row 1)**: ind.type **Variable Type or Units (row 2)**: ID factor
 **Description**: Type of induction treatment (no-snail control, induced,
 or autogenic growth control) applied to each indunit or indsubunit
 **Variable Name (row 1)**: ind.treat **Variable Type or Units (row 2)**:
 ID factor **Description**: Specific induction treatment applied to each
 indsubunit (no-snail control, southern or northern inducer snail or
 corresponding autogenic growth control) **Variable Name (row 1)**: initial
 mass **Variable Type or Units (row 2)**: unit = grams **Description**:
 initial seaweed mass (grams) in the corresponding induction subunit prior
 to the start if the induction phase; corresponds to Si, Ai, or Ci for
 inducer snail, autogenic growth, and no-snail control induction subunit
 treatments. **Variable Name (row 1)**: final mass **Variable Type or Units
 (row 2)**: unit = grams **Description**: final seaweed mass (grams) in the
 corresponding induction subunit at the end of the induction phase;
 corresponds to Sf, Af, or Cf for inducer snail, autogenic growth, and
 no-snail control induction subunit treatments. NA indicates that a
 measurement was not taken. **Variable Name (row 1)**: Bunit **Variable
 Type or Units (row 2)**: unit = grams **Description**: Induction unit
 snail biomass; total mass (grams) of the three snails in the northern or
 southern inducer treatment induction units. NA indicates that the value is
 not applicable, if the induction unit did not contain inducing snails
 (ind.type = "Control" or "Autogenic Growth Control"),
 or was not calculated if there were snails present (ind.type =
 "Induced") **Variable Name (row 1)**: startdate **Variable Type
 or Units (row 2)**: YYYYMMDD HH:MM **Description**: Start date of
 induction phase **Variable Name (row 1)**: enddate **Variable Type or
 Units (row 2)**: YYYYMMDD HH:MM **Description**: End date of induction
 phase **Variable Name (row 1)**: Dunit **Variable Type or Units (row 2)**:
 unit = days **Description**: Unit-specific induction phase duration (days)
 **Variable Name (row 1)**: Gunit **Variable Type or Units (row 2)**: unit
 = mg **Description**: Total seaweed grazed by inducers (mg); Gunit =
 (Si(Af/Ai)-Sf)*1000mg/g, where Si = initial mass = Sf = final mass and Ai
 and Af are the initial and final mass, respectively of the paired
 autogenic growth control seaweed (same indunit); as in Figure 2a and
 Appendix Table S1a. NA indicates that the measurement was not calculated.
 **Variable Name (row 1)**: Gpc **Variable Type or Units (row 2)**: unit =
 mg d-1 3.5g-snail-1 **Description**: Mass-standardized per capita daily
 grazing rate (mg d-1 3.5g snail-1); Gpc = 3.5g*Gunit/Bunit/Dunit; as in
 Figure 2b, Appendix Table S1b. NA indicates that the value is not
 applicable, if the induction unit did not contain inducing snails
 (ind.type = "Control" or "Autogenic Growth Control"),
 or was not calculated if there were snails present (ind.type =
 "Induced") **Variable Name (row 1)**: growth **Variable Type or
 Units (row 2)**: unit = % of initial mass **Description**: (final mass –
 initial mass)/initial mass; e.g., (Cf-Ci)/Ci for autogenic growth controls
 and no-snail controls. NA indicates that the value was not calculated, if
 the induction unit did not contain inducing snails (ind.type =
 "Control" or "Autogenic Growth Control"), or was not
 applicable if there were snails present (ind.type = "Induced")
 **Variable Name (row 1)**: exclusions **Variable Type or Units (row 2)**:
 ID factor **Description**: Induction units or subunits excluded from
 induction phase analyses and subsequent choice assays **Variable Name (row
 1)**: notes **Variable Type or Units (row 2)**: ID factor **Description**:
 reasons for exclusions. NA indicates not applicable, no notes for this
 unit. **Variable Name (row 1)**: phloro **Variable Type or Units (row
 2)**: unit = % dry mass **Description**: Phlorotannin concentration (% dry
 mass) in sampled apical tip of corresponding induction subunit seaweed; as
 in Figure 2d and Appendix Table S3a. NA indicates that the value was not
 calculated. **Variable Name (row 1)**: N **Variable Type or Units (row
 2)**: unit = % dry mass **Description**: Nitrogen content (% dry mass) in
 sampled apical tip of corresponding induction subunit seaweed. NA
 indicates that the value was not calculated. **Variable Name (row 1)**: C
 **Variable Type or Units (row 2)**: unit = % dry mass **Description**:
 Carbon content (% dry mass) in sampled apical tip of corresponding
 induction subunit seaweed. NA indicates that the value was not calculated.
 **Variable Name (row 1)**: CN **Variable Type or Units (row 2)**: unit =
 NA; ratio, C:N **Description**: Carbon to nitrogen ratio, C:N, (C/N) in
 sampled apical tip of corresponding induction subunit seaweed; Figure 2f,
 Appendix Table S3b. NA indicates that the value was not calculated. ###
 File: 2_pre_induction_phloro.csv **Variable Name (row 1)**: region
 **Variable Type or Units (row 2)**: ID factor **Description**: Seaweed
 source region (south, central, north); see also Figure 1 **Variable Name
 (row 1)**: site **Variable Type or Units (row 2)**: ID factor
 **Description**: Seaweed collection site (n = 3 sites region-1); see also
 Figure 1 **Variable Name (row 1)**: relaxation.state **Variable Type or
 Units (row 2)**: ID factor **Description**: Relaxation state of each
 seaweed prior to chemical analyses, either Relaxed (frozen after the
 relaxation phase) or InSitu (frozen upon collection). **Variable Name (row
 1)**: sample.no **Variable Type or Units (row 2)**: ID factor
 **Description**: Number used to identify samples (1-10) from each site,
 relaxation state combination. **Variable Name (row 1)**: phloro.pct
 **Variable Type or Units (row 2)**: unit = % dry mass **Description**:
 Phlorotannin concentration (% dry mass) in sampled apical tip of
 corresponding induction subunit seaweed; as in Figure 2c and Appendix
 Table S2a. ### File: 3_pre_induction_cn.csv **Variable Name (row 1)**:
 region\ **Variable Type or Units (row 2)**: ID factor\ **Description**:
 Seaweed source region (south, central, north); see also Figure 1
 **Variable Name (row 1)**: site\ **Variable Type or Units (row 2)**: ID
 factor\ **Description**: Seaweed collection site (n = 3 sites region-1);
 see also Figure 1 **Variable Name (row 1)**: relaxation.state\ **Variable
 Type or Units (row 2)**: ID factor\ **Description**: Relaxation state of
 each seaweed prior to chemical analyses, either Relaxed (frozen after the
 relaxation phase) or InSitu (frozen upon collection). **Variable Name (row
 1)**: sample.no\ **Variable Type or Units (row 2)**: ID factor\
 **Description**: Number used to identify samples (1-10) from each site,
 relaxation state combination. **Variable Name (row 1)**: N **Variable Type
 or Units (row 2)**: unit = % dry mass **Description**: Nitrogen content (%
 dry mass) in sampled apical tip of corresponding induction subunit seaweed
 **Variable Name (row 1)**: C **Variable Type or Units (row 2)**: unit = %
 dry mass **Description**: Carbon content (% dry mass) in sampled apical
 tip of corresponding induction subunit seaweed **Variable Name (row 1)**:
 CN **Variable Type or Units (row 2)**: unit = NA; ratio, C:N
 **Description**: Carbon to nitrogen ratio, C:N, (C/N) in sampled apical
 tip of corresponding induction subunit seaweed; Figure 2e and Appendix
 Table S2b. ### File: 4_inter_region_choice_phase_data.csv **Variable Name
 (row 1)**: indunit **Variable Type or Units (row 2)**: ID factor
 **Description**: number used to identify an individual induction unit
 (1-252); as in “1_induction_phase_data.csv” **Variable Name (row 1)**:
 indsubunit **Variable Type or Units (row 2)**: ID factor **Description**:
 letter used to identify each of the two seaweeds within the induction unit
 (a,b); as in “1_induction_phase_data.csv” **Variable Name (row 1)**:
 choiceunit **Variable Type or Units (row 2)**: ID factor **Description**:
 Number used to identify an individual choice unit (1-216) **Variable Name
 (row 1)**: choicetip **Variable Type or Units (row 2)**: ID factor
 **Description**: Number (1 or 2) assigned to each seaweed piece in a
 choice unit **Variable Name (row 1)**: combo **Variable Type or Units (row
 2)**: ID factor **Description**: Code used to identify each region
 combination in interregion choice assays (R1R2, R2R3, R1R3) testing
 constitutive differences in palatability between regions (i.e., Figure 3a,
 Appendix Table S4) **Variable Name (row 1)**: region **Variable Type or
 Units (row 2)**: ID factor **Description**: Seaweed source region (south,
 central, north); as in “1_induction_phase_data.csv” **Variable Name (row
 1)**: site **Variable Type or Units (row 2)**: ID factor **Description**:
 Seaweed collection site (n = 3 sites region-1); as in
 “1_induction_phase_data.csv” **Variable Name (row 1)**: ind.type
 **Variable Type or Units (row 2)**: ID factor **Description**: Type of
 induction treatment (no-snail control or induced) applied to each indunit
 or indsubunit; as in “1_induction_phase_data.csv” **Variable Name (row
 1)**: ind.treat **Variable Type or Units (row 2)**: ID factor
 **Description**: Type of induction treatment (No-Snail Control, Southern
 Inducer, or Northern Inducer) applied to each indunit or indsubunit; as in
 “1_induction_phase_data.csv” **Variable Name (row 1)**: inducer **Variable
 Type or Units (row 2)**: ID factor **Description**: Origin of inducer
 snail (No-Snail Control for interregion choice combinations and either
 Southern Inducer or Northern Inducer for intraregion choice combinations)
 applied to each choiceunit **Variable Name (row 1)**: chooser **Variable
 Type or Units (row 2)**: ID factor **Description**: Origin of chooser
 snail (Northern Choice Snail or Southern Choice Snail) applied to each
 choiceunit **Variable Name (row 1)**: initial mass **Variable Type or
 Units (row 2)**: unit = grams **Description**: initial seaweed mass
 (grams) of the corresponding choice seaweed tip prior to the start if the
 choice assay phase **Variable Name (row 1)**: final mass **Variable Type
 or Units (row 2)**: unit = grams **Description**: final seaweed mass
 (grams) of the corresponding choice seaweed tip after the choice assay
 phase **Variable Name (row 1)**: csnailmass **Variable Type or Units (row
 2)**: unit = grams **Description**: Total mass (g) of the chooser snails
 in the choice assay phase, applied to each choiceunit. Referred to as
 Bunit in formula for Gpc below. **Variable Name (row 1)**: start time
 **Variable Type or Units (row 2)**: MMDDYY HHMM **Description**: Date and
 time at which the choice assay began for each choiceunit **Variable Name
 (row 1)**: end time **Variable Type or Units (row 2)**: MMDDYY HHMM
 **Description**: Date and time at which the choice assay ended for each
 choiceunit **Variable Name (row 1)**: Dunit **Variable Type or Units (row
 2)**: unit = days **Description**: Unit-specific choice assay phase
 duration (days) **Variable Name (row 1)**: Gtip **Variable Type or Units
 (row 2)**: unit = mg **Description**: Total seaweed tissue grazed for each
 choicetip within each choiceunit. Calculated as initial-final mass.
 **Variable Name (row 1)**: Gpc **Variable Type or Units (row 2)**: unit =
 mg d-1 3.5g snail-1 **Description**: Standardized per capita daily grazing
 rates (mg d-1 3.5g snail-1) on each choicetip calculated as Gpc =
 3.5g*(Gtip/Bunit)/Dunit; as in Figure 3a and Appendix Table S4 ### File:
 5_intra_region_choice_phase_data.csv **Variable Name (row 1)**: indunit\
 **Variable Type or Units (row 2)**: ID factor\ **Description**: number
 used to identify an individual induction unit (1-252); as in
 “1_induction_phase_data.csv” **Variable Name (row 1)**: indsubunit\
 **Variable Type or Units (row 2)**: ID factor\ **Description**: letter
 used to identify each of the two seaweeds within the induction unit (a,b);
 as in “1_induction_phase_data.csv” **Variable Name (row 1)**: choiceunit\
 **Variable Type or Units (row 2)**: ID factor\ **Description**: Number
 used to identify an individual choice unit (1-216) **Variable Name (row
 1)**: choicetip\ **Variable Type or Units (row 2)**: ID factor\
 **Description**: Number (1 or 2) assigned to each seaweed piece in a
 choice unit **Variable Name (row 1)**: combo\ **Variable Type or Units
 (row 2)**: ID factor\ **Description**: Code used to identify each region
 combination in intraregion choice assays (R1I, R2I, R3I)\ testing
 palatability differences between control or induced seaweeds within a
 region (i.e., Figure 23, Appendix Table S5). **Variable Name (row 1)**:
 region\ **Variable Type or Units (row 2)**: ID factor\ **Description**:
 Seaweed source region (south, central, north); as in
 “1_induction_phase_data.csv” **Variable Name (row 1)**: site\ **Variable
 Type or Units (row 2)**: ID factor\ **Description**: Seaweed collection
 site (n = 3 sites region-1); as in “1_induction_phase_data.csv” **Variable
 Name (row 1)**: ind.type\ **Variable Type or Units (row 2)**: ID factor\
 **Description**: Type of induction treatment (no-snail control or induced)
 applied to each indunit or indsubunit; as in “1_induction_phase_data.csv”
 **Variable Name (row 1)**: ind.treat\ **Variable Type or Units (row 2)**:
 ID factor\ **Description**: Type of induction treatment (No-Snail Control,
 Southern Inducer, or Northern Inducer) applied to each indunit or
 indsubunit; as in “1_induction_phase_data.csv” **Variable Name (row 1)**:
 inducer\ **Variable Type or Units (row 2)**: ID factor\ **Description**:
 Origin of inducer snail: Southern Inducer or Northern Inducer for
 intraregion choice combinations) applied to each choiceunit **Variable
 Name (row 1)**: chooser\ **Variable Type or Units (row 2)**: ID factor\
 **Description**: Origin of chooser snail (Northern Choice Snail or
 Southern Choice Snail) applied to each choiceunit **Variable Name (row
 1)**: initial mass\ **Variable Type or Units (row 2)**: unit = grams\
 **Description**: initial seaweed mass (grams) of the corresponding choice
 seaweed tip prior to the start if the choice assay phase **Variable Name
 (row 1)**: final mass\ **Variable Type or Units (row 2)**: unit = grams\
 **Description**: final seaweed mass (grams) of the corresponding choice
 seaweed tip after the choice assay phase **Variable Name (row 1)**:
 csnailmass\ **Variable Type or Units (row 2)**: unit = grams\
 **Description**: Total mass (g) of the chooser snails in the choice assay
 phase, applied to each choiceunit. Referred to as Bunit in formula for Gpc
 below. **Variable Name (row 1)**: start time\ **Variable Type or Units
 (row 2)**: MMDDYY HHMM\ **Description**: Date and time at which the choice
 assay began for each choiceunit **Variable Name (row 1)**: end time\
 **Variable Type or Units (row 2)**: MMDDYY HHMM\ **Description**: Date and
 time at which the choice assay ended for each choiceunit **Variable Name
 (row 1)**: Dunit\ **Variable Type or Units (row 2)**: unit = days\
 **Description**: Unit-specific choice assay phase duration (days)
 **Variable Name (row 1)**: Gtip\ **Variable Type or Units (row 2)**: unit
 = mg\ **Description**: Total seaweed tissue grazed for each choicetip
 within each choiceunit. Calculated as initial-final mass. **Variable Name
 (row 1)**: Gpc\ **Variable Type or Units (row 2)**: unit = mg d-1 3.5g
 snail-1\ **Description**: Standardized per capita daily grazing rates (mg
 d-1 3.5g snail-1) on each choicetip calculated as Gpc =
 3.5g*(Gtip/Bunit)/Dunit; as in Figure 3b and Appendix Table S5 ### File:
 6_seaweed_canopy_survey_data.csv **Variable Name (row 1)**: region
 **Variable Type or Units (row 2)**: ID factor **Description**: Region
 (south, central, north) of survey; see also Figure 1 **Variable Name (row
 1)**: site **Variable Type or Units (row 2)**: ID factor **Description**:
 Site (n = 3 sites region-1) of survey; see also Figure 1 **Variable Name
 (row 1)**: quadrat **Variable Type or Units (row 2)**: ID factor
 **Description**: Number (1-10) assigned to each quadrat survey conducted
 at each site **Variable Name (row 1)**: pts.asco **Variable Type or Units
 (row 2)**: unit = NA; number **Description**: Number of grid points (out
 of 25) corresponding to *Ascophyllum nodosum* in point-intercept quadrat
 surveys; as used in Figure 4a and Appendix Table S6a **Variable Name (row
 1)**: pts.fucus\ **Variable Type or Units (row 2)**: unit = NA; number\
 **Description**: Number of grid points (out of 25) corresponding to
 *Fucus *in point-intercept quadrat surveys; as used in Figure 4a and
 Appendix Table S6a **Variable Name (row 1)**: pts.red\ **Variable Type or
 Units (row 2)**: unit = NA; number\ **Description**: Number of grid points
 (out of 25) corresponding to red algae species in point-intercept quadrat
 surveys. **Variable Name (row 1)**: pts.green\ **Variable Type or Units
 (row 2)**: unit = NA; number\ **Description**: Number of grid points (out
 of 25) corresponding to green algae species in point-intercept quadrat
 surveys. **Variable Name (row 1)**: pts.rock\ **Variable Type or Units
 (row 2)**: unit = NA; number\ **Description**: Number of grid points (out
 of 25) corresponding to bare rock in point-intercept quadrat surveys.
 **Variable Name (row 1)**: pts.invert\ **Variable Type or Units (row 2)**:
 unit = NA; number\ **Description**: Number of grid points (out of 25)
 corresponding to sessile invertebrate species in point-intercept quadrat
 surveys. ### File: 7_seaweed_canopy_survey_data.csv **Variable Name (row
 1)**: region\ **Variable Type or Units (row 2)**: ID factor\
 **Description**: Region (south, central, north) of survey; see also
 Appendix S1 **Variable Name (row 1)**: site\ **Variable Type or Units (row
 2)**: ID factor\ **Description**: Site (n = 3 sites region-1) of survey;
 see also Appendix S1 **Variable Name (row 1)**: quadrat\ **Variable Type
 or Units (row 2)**: ID factor\ **Description**: Number (1-10) assigned to
 each quadrat survey conducted at each site **Variable Name (row 1)**:
 species\ **Variable Type or Units (row 2)**: ID factor\ **Description**:
 Name of species identified in 2020 quadrat surveys. Either Fucus or
 Ascophyllum nodosum **Variable Name (row 1)**: measured\ **Variable Type
 or Units (row 2)**: ID factor\ **Description**: Measured for length,
 "Yes" or "No" **Variable Name (row 1)**: length\
 **Variable Type or Units (row 2)**: unit = cm\ **Description**: Length
 (cm) of individual seaweed within a quadrat. NA indicates that the value
 was not calculated. **Variable Name (row 1)**: density\ **Variable Type or
 Units (row 2)**: unit = NA; number\ **Description**: Number of individuals
 of each seaweed species (counted as number of holdfasts) within a quadrat
 (number 0.25m-2); as used in Figure 3a and Appendix S2 Table S6a. NA
 indicates that the value was not calculated. **Variable Name (row 1)**:
 Survey Date\ **Variable Type or Units (row 2)**: unit= Date MMDDYYY\
 **Description**: The date on which the survey was conducted. ### File:
 8_snail_survey_data.csv **Variable Name (row 1)**: region **Variable Type
 or Units (row 2)**: ID factor **Description**: Region (south, central,
 north) of survey; see also Figure 1 **Variable Name (row 1)**: site
 **Variable Type or Units (row 2)**: ID factor **Description**: Site (n = 3
 sites region-1) of survey; see also Figure **Variable Name (row 1)**:
 quadrat **Variable Type or Units (row 2)**: ID factor **Description**:
 Number (1-10) assigned to each quadrat survey conducted at each site
 **Variable Name (row 1)**: l.littorea\ **Variable Type or Units (row 2)**:
 unit = NA; number\ **Description**: Number of *Littorina littorea* counted
 within the quadrat; as used in Figure 4d and Appendix Table S6d.
 **Variable Name (row 1)**: l.obtusata\ **Variable Type or Units (row 2)**:
 unit = NA; number\ **Description**: Number of *Littorina obtusata* counted
 within the quadrat; as used in Figure 4d and Appendix Table S6d.