Seaweed functional diversity revisited: Confronting traditional groups with quantitative traits

Abstract

1. Macroalgal (seaweed) beds and forests fuel coastal ecosystems and are rapidly reorganizing under global change, but quantifying their functional structure still relies on binning species into coarse groups on the assumption that they adequately capture relevant underlying traits. 2. To interrogate this 'group gambit', we measured 12 traits relating to competitive dominance and resource economics across 95 macroalgal species collected from the UK and widespread on NorthEast Atlantic rocky shores. We assessed the amount of trait variation explained by commonly used traditional groups-(a) two schemes based on gross morphology and anatomy and (b) two categorizations of vertical space use-and examined species reclassification into post hoc, so-called emergent groups arising from the functional trait dataset. We then offer an alternative, emergent grouping scheme of macroalgal functional diversity. 3. (a) Morphology and anatomy-based groups explained slightly more than a third of multivariate trait expression with considerable group overlap (i.e. low precision) and extensive mismatch with underlying trait expression (i.e. low accuracy). (b) Categorizations of vertical space use accounted for about a quarter of multivariate trait expression with considerable group overlap. Nonetheless, turf species tended to display attributes of opportunistic forms. (c) A nine-group emergent scheme provided a highly explanatory and parsimonious alternative to traditional functional groupings. 4. Synthesis. Our analysis using a comprehensive dataset of directly measured functional traits revealed a general mismatch between traditional groups and underlying traits, highlighting the deficiencies of the group gambit in macroalgae. While existing grouping schemes may allow first order approximations, they risk considerable loss of information at the trait and, potentially, ecosystem levels. Instead, we call for further development of a trait-based approach to macroalgal functional ecology to capture unfolding community and ecosystem changes with greater accuracy and generality.

Functional diversity of habitat formers declines scale-dependently across an environmental stress gradient

“…Functional diversity studies in marine species have traditionally been limited by a lack of appropriate trait data. However, these are now emerging for corals (Madin et al 2016 ), seagrasses (de los Santos et al 2016 ) and seaweeds (Mauffrey et al 2020b ). Applying these newly available functional traits to assemblages of marine habitat formers across temporal—or as we did, spatial—gradients allows the characterisation of context-dependent functional structure (e.g., redundancy) as well as insights into potential sensitivity to environmental change and attendant species loss.…”

“…Traits were obtained from a database in development since 2016 (see Mauffrey et al 2020b ). For the creation of this trait database, we conducted frequent summer sampling trips predominantly in south Wales; all but two of the species in this study were sampled for trait screening from the same sites as the survey data used here.…”

“…To examine species’ contributions to functional diversity and redundancy across zones, we also addressed species’ occupancy of parts of the trait space via emergent groups. Because emergent groups are based on measured traits, they should be a more accurate representation of species’ differences than the traditional functional groups (Mauffrey et al 2020a ). In our study communities, emergent groups allowed us to observe redundancy (species within groups share similar trait values) and how parts of the trait space (as occupied by different groups) change across zones.…”

“…We tested increasing numbers of groups and selected the highest number of groups (i.e. 6) where all pairwise contrasts using PERMANOVA were significant at α = 0.05 (Hervé 2020 ; Mauffrey et al 2020a ).…”

“…Steneck and Dethier 1994). Yet the functionality of species and assemblages is determined by multiple, often continuous, traits, challenging coarse a priori grouping (Chapin et al 1996;Fong and Fong 2014;Mauffrey et al 2020a). Here, we use the rocky intertidal as a test case for applying a suite of continuous traits to seaweeds to understand patterns of functional diversity across environmental gradients.…”