DOI: 10.5068/d1km3p
title: When form does not predict function: empirical evidence violates functional form hypotheses for marine macroalgae

description: 1) Functional groups are widely used to reduce complexity and generalize
 across ecological communities. These models assume that shared traits
 among species correspond to some ecological role, process, or function,
 and that these traits can be leveraged to generate meaningful and distinct
 functional groups so that intergroup trait variation exceeds intragroup
 variation. 2) We sought to validate the assumptions of the widely used
 Functional Group Model (FGM) for marine macroalgae, which groups species
 based on morphological complexity, by testing the predictions of the FGM
 for several traits assumed to correspond with morphological complexity.
 The FGM predicts increased resistance to disturbance and herbivory as
 morphological complexity (tensile strength and thallus toughness,
 respectively) increases. The FGM also predicts a tradeoff between
 complexity and growth rate. To test predictions, we measured: 1) thallus
 toughness (force to penetrate), 2) tensile strength (force to break) and
 3) relative growth for both tropical and temperate macroalgae from
 different functional groups. 3) Penetration strength followed model
 predictions at the functional group level, though there was significant
 variability among species. However, the model did not predict tensile
 strength at any level for either tropical or temperate macroalgae.
 Further, relative growth did not follow predictions; rather it was highly
 variable among species and functional groups. 4) Synthesis. The
 assumptions of the FGM that differences in morphological complexity can be
 used to generate distinct functional groups and that intergroup trait
 variation outweighs intragroup variation were violated, providing strong
 evidence that individual species responses need to be considered. Further,
 violations of assumptions indicate that functional groups should not be
 used to predict community responses to ecological drivers and/or species
 contributions to ecosystem function. Our study challenges the usefulness
 of functional form groups for marine macroalgae and emphasizes the need
 for a different conceptual framework.
description: <em>Model
 predictions</em> The
 FGM offers several measurable predictions of traits that are indicative of
 ecosystem function and species’ response to environmental drivers. We
 focus on the FGM predictions of toughness (weight to penetrate), tensile
 strength (weight to break), and growth (change in weight) (Fig. 1). These
 traits are assumed to be a measure of species’ resistance to herbivory,
 resistance to physical disturbance, and recovery from disturbance/role in
 ecological succession, respectively, as different strategy types (for
 review of terrestrial plant strategies, see Westoby et al. 2002). These
 strategies can be in turn be related to ecological functions such as
 productivity and food chain support (Littler and Littler 1980). The FGM
 model predicts that toughness and tensile strength increase with
 morphological complexity, while growth is the opposite (Fig. 1).
 Therefore, morphologically complex species should be more resistant to
 herbivory and disturbance and last in the line of succession while
 morphologically simple species are predicted to be involved in early
 successional stages, and vulnerable to herbivory and physical disturbance
 (Littler and Littler 1983b). <em>Study
 system</em> We
 tested the assumptions of the FGM using 33 total species and five
 functional groups from both tropical and temperate locales. We collected
 tropical species from a fringing coral reef lagoon. These systems are
 generally characterized by low productivity (Borer et al. 2013; Huston and
 Wolverton 2009), lower physical disturbance, and high herbivory pressure
 (Floeter et al. 2005; Vergés et al. 2014). Temperate species were
 collected from intertidal and subtidal reefs, which are generally
 characterized by higher productivity (Borer et al. 2013; Huston and
 Wolverton 2009), higher physical disturbance (Littler and Littler 1984),
 and lower herbivory pressure (Floeter et al. 2013; Vergés et al. 2014). As
 the FGM is utilized in a variety of habitats in temperate and tropical
 systems that vary in these environmental contexts, we collected species
 from different site types in both locales to test assumptions in a variety
 of systems. <em>Algal
 collection</em> Many
 algal species exhibit complex life cycles, where different generations can
 vary in morphology, ploidy, and sex (Thornber 2006; for life cycle types
 of collected algae, see Table 2). While little is known regarding how
 responses to ecological drivers vary between generations, there is some
 evidence these differences may be profound (Krueger-Hadfield 2020;
 Lubchenco and Cubit 1980; Martinez and Santelices 1998; Thornber 2006).
 Thus, we have identified what generation we collected, where possible, in
 Table 2. Thirteen species
 of tropical algae representing five functional groups were haphazardly
 collected between January 21 and February 14, 2018 via snorkel from a
 common site at approximately 2-3-m depth in the patch reef zone of a
 fringing reef in Cook’s Bay in Mo’orea, French Polynesia (Table 2, Fig.
 2B). Collected algal species included <i>Dictyota
 bartayresiana</i>, <i>Padina boryana</i>, <i>Ulva
 intestinalis</i>, <i>Ulva lactuca</i> (sheetlike),
 <i>Caulerpa serrulata</i>, <i>Spyridia</i>
 <i>filamentosa</i> (filamentous), <i>Amansia
 rhodantha</i>, <i>Acanthophora spicifera</i> (coarsely
 branched), putatively <i>Gracilaria parvispora </i>(hereafter
 <i>Gracilaria parvispora</i>), <i>Sargassum pacificum,
 Turbinaria ornata</i> (thick and leathery), <i>Galaxaura
 fasciculata, </i>and <i>Halimeda opuntia</i> (jointed
 calcareous).  For the purposes of this study, only whole, macroscopic
 thalli (a term defined as the body form of an alga) that were attached to
 the benthos and appeared healthy were collected. Approximately 20 thalli
 were collected per species for toughness and tensile experiments while
 approximately 15 thalli were collected per species for growth experiments
 (see below). Algae were immediately transferred to an outdoor flow-through
 water table. Using ambient seawater, thalli were cleaned of sediment and
 other organisms and processed within 24 hours of initial
 collection. Twenty species
 of temperate algae representing four functional groups were collected
 between April 15 and August 1, 2018 at three sites throughout central and
 southern California, United States (Table 2, Fig. 2A). Collected species
 included <i>Pyropia perforata, Dictyopteris undulata, Zonaria
 farlowii, Dictyota binghamiae, Dictyota coriacea</i> (sheetlike),
 <i>Endocladia muricata</i>, <i>Mastocarpus
 papillata</i>, <i>Prionitis sternbergii, Laurencia
 pacifica</i>, <i>Pterocladiella capillacea, Plocamium
 pacificum, Colpomenia sinuosa</i> (coarsely branched),
 <i>Egregia menziessi</i>, <i>Stephanocystis
 doica</i>, <i>Sargassum horneri</i>, <i>Sargassum
 palmeri</i>, <i>Silvetia compressa</i>,
 <i>Stephanocystis osmundaceae</i> (thick and leathery),
 <i>Bossiella orbigniana</i>, and <i>Calliarthron
 tuberculosum</i> (jointed calcareous). Species were collected from
 intertidal sites in Cambria and Palos Verdes, immediately placed in
 coolers filled with seawater, and transported back to the University of
 California, Los Angeles where they were kept in indoor aquaria. Species
 from Santa Catalina Island were collected from a subtidal site at
 approximately 2-3-m depth via snorkel and immediately transported back to
 the University of Southern California Wrigley Institute of Environmental
 Science (WIES) where they were placed in outdoor flow-through water tables
 with ambient seawater. Replication, algal cleaning, and algal processing
 were performed as above. <em>Thallus
 toughness</em> To
 test the prediction that thallus toughness of species within functional
 groups was similar and toughness between groups increases from the
 simplest to the most complex algal thalli, we chose 10 whole thalli of
 each species. As many complex algal species exhibit apical growth,
 toughness was tested on blades or tissue from the middle of each thallus,
 where applicable, to prevent testing younger, potentially weaker, areas.
 To measure thallus toughness, we secured each subsample below a
 penetrometer so that the needle of the penetrometer rested on the thallus
 surface. We added weight until the penetrometer just pierced the thallus
 surface. This process was repeated for 10 thalli of each
 species. <em>Thallus
 tensile strength</em> To test the same predictions as above, but
 for thallus tensile strength, we selected 10 thalli of each species. To
 control for the very different algal morphologies, we used the whole algal
 thallus for each species, from the apex to the base. The basal end of each
 thalli was secured to a spring scale while just below the apical end was
 pulled until the thallus broke. The force (weight) required to break the
 thallus was used as a measure of tensile strength. Data were not used if
 thalli broke near where the thallus was secured to the spring scale, or
 where the thallus was held near the apical end. This process was repeated
 for each of the 10 replicate thalli per species. <i>Relative
 growth</i> To test
 model predictions for relative growth of tropical species, we conducted
 field experiments from January 25-February 4, 2018 using six species, with
 two in each of three functional groups (Table 2). Algae were spun in a
 salad spinner for one minute and wet-weighed into eight replicate 1-g
 (<i>Acanthophora spicifera </i>and <i>Amansia
 rhodantha</i>) or 3-g (<i>Sargassum pacificum</i>,
 <i>Galaxaura fasciculata</i>, <i>Halimeda
 opuntia</i>) subsamples by trimming whole thalli into appropriate
 weights, taking care to avoid removing apical meristems. Subsample weights
 varied by species due to different volume to mass ratios for each species
 (Mantyka and Bellwood 2007). Thus, in order to achieve similar subsample
 volumes among focal species, subsample weights had to be increased for
 more dense (<i>S. pacificum</i>) and calcifying algae
 (<i>G. fasciculata</i> and <i>H. opuntia</i>). As
 <i>Turbinaria ornata</i> is sensitive to trimming, eight
 replicate individuals of similar size (2.79±0.41SE-g, reproductively
 mature) were collected and weighed without normalizing to a standard
 initial weight. We secured
 each replicate to the bottom of fully-enclosed, cylindrical cages (12-cm
 diameter x 10-cm height) constructed of hardware cloth with
 1-cm<sup> </sup>openings that have been shown to have few cage
 artefacts and to limit herbivory in previous experiments (e.g., Fong et
 al. 2006, Smith et al. 2010). Cages were randomly attached to rope with at
 least 0.5-m spacing between replicates and secured to the benthos. Algae
 in cages were secured in an upright growth position and ropes were secured
 to the benthos with coral rubble at ~2-m depth on the same fringing reef
 as collection. Cages were cleaned of fouling every other day and recovered
 after 10 days when algae were spun and wet-weighed. Percent change in
 biomass was calculated as [(final weight-initial weight)/initial
 weight]*100 and then expressed on a per day basis. To test the same predictions for relative
 growth of temperate algae, we used five species, with two in sheetlike,
 one in<i> </i>coarsely branched, and two in thick and leathery
 functional groups as they were abundant in the collection site near Santa
 Catalina Island, California. Algal thalli were spun in a salad spinner for
 one minute and wet weighed into eight replicate 1-g
 (<i>Pterocladiella capillacea</i>) or 2-g
 (<i>Dictyopteris undulata, Zonaria farlowii, Stephanocystis doica,
 </i>and <i>Sargassum palmeri</i>) subsamples. As before,
 subsample weights were adjusted to roughly match volumes. Cages were
 deployed from July 10-July 20, 2018, as above in ~2-m depth in a sheltered
 cove just west of WIES (same site as collection) and cleaned every other
 day. All cages were removed after 10 days, and algae spun and wet-weighed.
 Per cent change in biomass per day was calculated as above.
 <i>Analysis</i>
 As environmental conditions are
 markedly different between temperate and tropical regions (see
 <i>Study system</i> section for overview) and species were
 collected from each region at different points in time, we analysed
 the temperate and tropical
 species separately. For each response variable, species means
 were calculated as the average response (weight to penetrate, weight to
 break, % change in biomass) over all replicates. To compare responses
 among functional groups for each variable, species’ means were used to
 calculate functional group means. Transformations were applied where
 necessary to meet the assumptions of parametric statistics. If the data
 still did not meet parametric assumptions following transformation,
 nonparametric statistics were utilized (described below). To compare species means within and
 between functional groups for the response variables of thallus toughness,
 tensile strength, and growth, data for measures from individual thalli
 that met assumptions for parametric statistics were analysed with a nested
 ANOVA, with functional group as a fixed factor and species nested within
 functional group. Data that did not meet parametric assumptions were
 analysed with a nested PERMANOVA, with functional group as a fixed factor
 and species nested within functional group. As there was only one species
 in the coarsely branched group for the temperate growth experiment, this
 group was omitted from nested analyses. Significant nested analyses were followed
 by 1-factor ANOVAs for parametric data or 1-factor Kruskal-Wallis tests
 for nonparametric data to separately compare functional group and species
 means. For pairwise comparisons, significant ANOVAs were followed by a
 Tukey HSD post-hoc test and significant Kruskal-Wallis tests were followed
 by Wilcoxon post-hoc tests. P-values were adjusted with Bonferroni’s
 correction for multiple comparisons. All analyses were conducted using
 base functions in R Statistical Software (R Core Team 2017), except for
 PERMANOVAs, which were conducted using the “vegan” package for R (Oksanen
 et al. 2019).