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To test the efficacy of seaweed taxonomic relatedness and morphological/functional- group membership as predictors of host-use by invertebrate epifauna, I sampled invertebrate assem- blages 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 spe- cies as on hosts classified in different kingdoms. The utility of seaweeds as hosts for mobile inverte- brates 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 sea- weed 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.

Efficiency in biomonitoring studies is essential to maximize return (i.e. useful data) for investment (e.g. time, training, personnel). Here, we test several options for reducing data resolution when streamlining monitoring protocols, and use the results as a framework to discuss the costs and benefits of decreasing information when sampling intertidal assemblages. Specifically, we ask; (1) Is it necessary to collect species abundance data, or is species presence–absence information sufficient to differentiate sites? (2) Is it necessary to sample organismal abundance at the species-level or is coarser (higher taxon or functional group) resolution sufficient to resolve patterns of difference in intertidal community structure? and (3) How general are these patterns across different oceanic regions? We answer these questions using data from Northeast Atlantic, Northwest Atlantic, Northeast Pacific, and Southwest Pacific intertidal monitoring studies. Results show that compared to species-level sampling, genus-level sampling requires knowledge of 25% fewer taxa, but results in only a 5% difference in the ability to discern between-sample similarities. Likewise, family-level sampling involves 50% fewer taxa, and is accompanied by only an 8% difference in between-sample similarities. Species lists and functional groups were variable in performance, working well for some regions, and poorly for others. These findings will assist in the selection of monitoring protocols with the potential for increased geographic scope and temporal frequency of sampling, resulting in longer time series of data collection, and a reduction in the required taxonomic skills for individuals involved in scientifically useful biomonitoring programs.

Most investigations of biogenic habitat provision consider the promotion of local biodiversity by single species, yet habitat-forming species are often themselves components of diverse assemblages. Increased prevalence of anthropogenic changes to assemblages of habitat-forming species prompts questions about the importance of facilitator biodiversity to associated organisms. We used observational and short-term (30 days) manipulative studies of an intertidal seaweed system to test for the implications of changes in four components of biodiversity (seaweed species richness, functional group richness, species composition, and functional group composition) on associated small mobile invertebrate epifauna. We found that invertebrate epifauna richness and abundance were not influenced by changes in seaweed biodiversity. Invertebrate assemblage structure was in most cases not influenced by changes in seaweed biodiversity; only when algal assemblages were composed of monocultures of species with ‘foliose’ morphologies did we observe a difference in invertebrate assemblage structure. Correlations between algal functional composition and invertebrate assemblage structure were observed, but there was no correlation between algal species composition and invertebrate assemblage structure. These results suggest that changes in seaweed biodiversity are likely to have implications for invertebrate epifauna only under specific scenarios of algal change.