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Abstract
It has long been postulated that biological diversity is important for ecosystem functioning. Marine ecosystems support a significant proportion of the Earth's biodiversity and yet we have a limited understanding of the functional importance of marine biodiversity. Knowledge of the specificity of species associations is fundamental for predicting how changes to assemblages of species may affect local biodiversity and subsequently ecosystem functioning. Such research is fundamental in light of increasing anthropogenic activities threatening coastal systems. Shallow marine environments are greatly disturbed and heavily invaded by exotic species, posing a considerable threat to the diversity of species that rely on native animals for habitat. In this thesis, I explored species associations between sessile and mobile animals and attempted to identify some of the key factors influencing the diversity of mobile species. I also considered the effects of host assemblages harboring introduced sessile species on associated mobile diversity.
Using a combination of field surveys, manipulative experiments, studies on single focal species of habitat-forming animals, and a global scale meta-analysis, I found that the diversity and composition of mobile species associated with hosts of sessile animals are difficult to predict from traits of their host assemblages. While the specific composition of sessile assemblages and certain individual sessile species clearly influenced the composition and diversity of associated mobile species, other factors which I expected to be good predictors of associated diversity were unreliable. For instance, the extent of habitat provided by the host, species and ordinal diversity, and the identity and composition of functional types of host assemblages all failed to reliably predict diversity of associated species. These results suggest that, in contrast to terrestrial systems, mobile species are more likely to be habitat generalists in that they are not influenced by many differing features of their host assemblage. Additionally, factors which I did not measure (e.g., biofilms, host microbial make-up and chemical properties) may be more important than the identity of sessile macrofaunal hosts in determining the distribution of associated mobile species. My results suggest that many introduced sessile species, in this system, are functionally redundant.