Global change has led to general concern about how the modification of biodiversity patterns across the planet is modifying the functioning of natural ecosystem. Some general patterns have emerged with regards to the roles of species identity, selection and complementarity effects, for the sustenance of ecosystem properties and processes. The current thesis aimed at gaining an insight into less well understood aspects of the relationship between diversity and ecosystem processes. It centres on the effects of environmental context, species additions to communities, and the comparative roles of species evenness and richness. Emphasis is given to the role of habitat modifying species in these relationships, i.e. ecosystem engineers. An attempt is made to understand what are the key characteristics of these species that shape the
relationship between single species, communities and ecosystem processes.
Using the introduction of Ruditapes philippinarum (the Manila clam) in western European waters as a case-study, the work here developed allowed for a thorough exploration of the effects of bioturbators on the properties of seabed sedimentary environments. These effects were compared across four study sites, providing a good illustration of context-dependency
for invasion impacts on diversity and process relationships. These effects were found to vary in relation to community composition and the relevance of the type of habitat modification analysed in relation to habitat characteristics, which was not yet fully acknowledged.
As the thesis focused on bioturbation, the methods hereto employed for the determination of this type of ecosystem engineering were approached from a critical perspective. In this sense, novel methods were developed that are expected to improve the comparability of bioturbation studies in the future, allowing us to have a more clear understanding of this type of habitat modification for the determination of ecosystem processes.
The effects of ecosystem engineer richness and evenness on sediment m1xmg as an ecosystem process are compared, demonstrating that process rates can be modified long before a species is lost from an ecosystem. Additionally, species-level context effects were found to modify diversity and process relationships on a local scale - such effects have not
been often explored in the li terature and may add to our understanding of the links between the functional role of single species and their environment.
Overall, the findings here present strongly support the view that a single pattern for diversity and process relationships may remain elusive for some time to come, and that future research will have to take account of environmental variation and the response of single species to it.