Host Taxonomic Relatedness and Functional-Group Affiliation As Predictors of Seaweed–Invertebrate Epifaunal Associations

Vol. 387: 125–136, 2009 MARINE ECOLOGY PROGRESS SERIES Published July 28 doi: 10.3354/meps08089 Mar Ecol Prog Ser Host taxonomic relatedness and functional-group affiliation as predictors of seaweed–invertebrate epifaunal associations Colin R. Bates* Bamfield Marine Sciences Centre, 100 Pachena Road, Bamfield, British Columbia V0R 1B0, Canada** and Department of Botany, University of British Columbia, Room 3529, 6270 University Boulevard, Vancouver, British Columbia V6T 1Z4, Canada ABSTRACT: To test the efficacy of seaweed taxonomic relatedness and morphological/functional- group membership as predictors of host-use by invertebrate epifauna, I sampled invertebrate assemblages associated with 1652 individual thalli across 32 species of seaweed. Additionally, I tested whether seaweeds within the same functional group had the potential, in the context of species loss, to functionally replace each other as habitat. In total, I found 54 776 individuals across 98 taxa of invertebrates. Similarity of invertebrate assemblages did not decrease as taxonomic distances between seaweed hosts increased; invertebrate assemblages were as different on sibling algal species as on hosts classified in different kingdoms. The utility of seaweeds as hosts for mobile invertebrates varied across components of invertebrate diversity. Invertebrate assemblage composition was different across most algal functional groups, whereas invertebrate taxon richness was different for ca. 25% of functional groups, and no differences in invertebrate abundance across algal functional groups were observed. There was substantial variation in host-use within seaweed functional groups, suggesting that overall functional group performance indicates little about the performance of the constituent species. Specifically, composition, richness and abundance of invertebrate assemblages were different across most hosts within each algal functional group. Therefore, in the event of seaweed species loss, replacements by members of the same functional group generally appear unlikely. These observations bring into question the use of general frameworks for predicting performance of seaweeds as hosts for associated invertebrates; it appears that invertebrate selection of seaweed hosts is largely dependent upon the identity of the host species. KEY WORDS: Seaweed · Invertebrate · Habitat · Taxonomic relatedness · Functional groups Resale or republication not permitted without written consent of the publisher INTRODUCTION et al. 2005). Myriad invertebrate taxa use seaweeds as habitat (Colman 1939) and, in some cases, as a food For small mobile invertebrates, seaweeds are a source (Hawkins & Hartnoll 1983, Arrontes 1999). Epi- refuge from stressful conditions associated with life on fauna composition, taxon richness, and abundance rocky intertidal shores. Seaweeds provide cool, wet, often differ across species of host seaweed (Colman and protective canopies and interstices (Hayward 1939, Seed & O’Connor 1981, Taylor & Cole 1994). In 1980, 1988) and can ameliorate the influence of rapid the present study, I explore several general frame- changes in temperature, ultraviolet radiation, desicca- works that hold promise for predicting such patterns of tion, and hydrodynamic forces caused by twice-daily invertebrate biodiversity based on seaweed traits. emersion and immersion by seawater. At high tides, Behavioral choices by epifauna determine inverte- seaweeds can also offer shelter from predation by fish brate distributions across seaweeds, each seeking and (Holmlund et al. 1990, Martin-Smith 1993, Norderhaug remaining upon their preferred host (Hayward 1988). **Email: [email protected] © Inter-Research 2009 · www.int-res.com **Preferred address for mail correspondence 126 Mar Ecol Prog Ser 387: 125–136, 2009 Criteria for epifauna host choice include factors inter- ness as a predictor of associated epifauna is external nal to seaweeds: material properties such as cell struc- morphology, which in seaweeds can vary seemingly ture and cell wall components, energetic storage prod- independently of taxonomic affinity. Indeed, some ucts, and defensive biochemistry (Hay et al. 1987, genera such as Codium and Caulerpa contain a wide Graham & Wilcox 2000, Padilla & Allen 2000, Van array of morphologies (Silva 1992), while convergent Alstyne & Houser 2003). These properties vary across evolution has resulted in strikingly similar forms seaweeds, resulting in differences in nutritive value shared by algae classified as distantly as subkingdom and palatability (Paine & Vadas 1969, Hawkins & Hart- (e.g. Ulva lactuca, a green alga from Subkingdom noll 1983). In addition to negatively acting to directly Viridiplantae and Porphyra spp., red algae from the deter epifauna (Hay et al. 1987), algal defensive bio- Subkingdom Biliphyta) and kingdom (e.g. Analipus chemistry can act positively to promote epifauna by japonicus, a brown alga [Kingdom Chromista] and conferring associational defenses: e.g. unpalatable Cumagloia andersonii, a red alga [Kingdom Plantae]). algae are avoided by omnivorous fishes, and epifauna This similarity in morphology across taxonomic group- therefore escape predation (Hay et al. 1990). Along ings has been formally recognized since the early with these internal seaweed features, epifauna also 1980s and led to the proposal of morphologically based select seaweed hosts based on external features. ‘functional form’ groupings under which predictions of Architectural complexity of overall seaweed form has ecological and ecophysiological performance of sea- widely been linked to abundance and richness of weeds could be made (Littler & Littler 1980, Steneck & invertebrates (Dean & Connell 1987, Gee & Warwick Watling 1982, Steneck & Dethier 1994). The use of ter- 1994, Taylor & Cole 1994, Davenport et al. 1996, Hull restrial plant functional groups has also become com- 1997, Chemello & Milazzo 2002). Compared to simple mon in recent times (Lavorel et al. 1997, Diaz & Cabido forms such as unbranched blades and crusts, complex, 2001). In a review of the performance of algal func- highly branching seaweeds can better provide preda- tional groups, Padilla & Allen (2000) were critical of tor-free space for epifauna (Duffy & Hay 1991), stay previous applications of seaweed functional form mod- moist to limit desiccation effects (Ji & Tanaka 2002), els, highlighting a lack of empirical support for most of and provide increased surface area and microhabitat the model predictions despite widespread acceptance variety (Christie et al. 2003, Norderhaug et al. 2005). of their use. They did, however, make a point of At present, scientific perception about the quality of suggesting the promise of functional form groupings different seaweeds as hosts for small mobile epifauna for predicting structure of habitat for associated or- are based on a body of studies that compare perfor- ganisms. This idea remains untested, although see mance of relatively few seaweed species, typically Hacker & Steneck (1990) who discussed several habi- between 2 (Kraufvelin & Salovius 2004, Wikström & tat-providing algal species based on their functional Kautsky 2004) and 7 (Holmlund et al. 1990), but some- group placement. times up to 10 (Taylor & Cole 1994, Taylor & Steinberg Inherent in the concept of using functional group- 2005). However, a key goal of community ecology is to ings to predict ecological or ecophysiological proper- establish general principles that describe how biotic ties is the idea that species grouped together will per- and abiotic factors structure biological assemblages form similarly. This premise has been discussed in (MacArthur 1972). To alleviate the need to compare terms of functional redundancy (Naeem 1998, Fonseca the performance of particular algal species as hosts for & Ganade 2001, Rosenfeld 2002, Loreau 2004). This epifauna, a more general predictive framework is concept is relevant to discussions about the conse- desirable. The objective of the present study is to com- quences of changes in biodiversity because loss of par- pare 2 such general frameworks: seaweed taxonomic ticular species could be compensated for by function- relatedness and seaweed functional form groupings. ally equivalent coexisting species. This buffering Ecologists are increasingly recognizing that taxo- capacity of species rich assemblages has been viewed nomic relationships between species can inform pre- as a form of biological insurance (Thebault & Loreau dictions about species performance in an ecological 2006); however, to date, direct tests of the functional context (Webb et al. 2002). Species that are closely equivalence of seaweeds as hosts for invertebrates are related are more likely to share traits (i.e. similarity by limited to comparisons of only 2 seaweed taxa, e.g. common descent) and thus perform similarly in ecolog- Cladophora glomerata and Fucus vesiculosus (Krauf- ical scenarios. Taxonomic relatedness may inform pre- velin & Salovius 2004). dictions of seaweed-host performance because inter- To test the efficacy of seaweed taxonomic related- nal features of seaweeds (material properties, nutritive ness and functional group affinity as predictors of sea- value, and defensive chemistry) tend to be conserved weed–invertebrate epifauna associations, I asked 3 within taxonomic lineages. However, a property that questions: (1) is similarity of invertebrate assemblages could confound the use of seaweed taxonomic related- positively correlated with algal host taxonomic relat- Bates:

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