Open Collections

Abstract

In many coastal environments, anthropogenic stressors yield changes in seaweed biodiversity. Here, I describe three studies addressing how such floristic changes might affect provision of habitat by seaweeds for small mobile invertebrate epifauna. In chapter 2, I used observational and manipulative (transplant) experiments to test how changes in seaweed biodiversity influenced biodiversity of associated invertebrates. I found that invertebrate epifaunal richness and abundance were not affected by changes in seaweed biodiversity. Invertebrate assemblage structure was, in most cases, not influenced by changes in seaweed composition; only when algal assemblages were composed of monocultures of species with 'foliose' morphologies did I observe a change in invertebrate assemblage structure. Correlations between algal functional composition and invertebrate assemblage structure were observed, but not between algal species composition and invertebrate assemblage structure. These results suggest that changes in seaweed biodiversity will have implications for invertebrate epifauna only under specific scenarios of algal change. In Chapter 3, I tested the performance of host taxonomic relatedness and functional (i.e. morphological) group affiliation as predictors of associated invertebrate epifauna. Neither general framework performed well; invertebrate assemblages found on congeneric host species were as similar as those found on hosts classified in different kingdoms, and taxon richness and abundance of invertebrates varied substantially within seaweed functional groups. Species identity was identified as a key predictor of the performance of seaweeds as hosts for invertebrate epifauna. In chapter 4, I examined the context dependence of these host identity effects by testing how host morphological complexity and maximum wave velocity interacted to determine local invertebrate diversity. Three types of host species were identified: (a) morphologically `simple' thalli that were minimally utilized as habitat under any of the tested wave regimes, (b) thalli that were coarsely branched and were utilized by invertebrates under relatively benign wave conditions but became less utilized under higher wave action, and (c) 'complex' algal hosts that supported diverse invertebrate assemblages under all tested wave conditions. Together, these studies support the view that invertebrates that use seaweeds as habitat are host-generalists, and therefore consequences for invertebrates of changes in seaweed biodiversity are likely to be minimal.