Succession-driven facilitation of macrofaunal communities in sublittoral blue mussel habitats

Whereas it is well known that ecosystem engineers can have positive effects on biodiversity, fundamental gaps still remain in our understanding of the underlying mechanisms in many marine systems. Individual- and population-level traits of engineering species are likely to determine their ability to modify the environment. Blue mussels function as important ecosystem engineer species in the Baltic Sea where they facilitate the existence of distinct associated communities. The population structure of these blue mussel beds fluctuates widely, but ecosystem effects of these fluctuations are poorly known. In this study, we experimentally and descriptively measured the effect of mussel succession stage with differing biomass, density and size structure on macrofaunal diversity and body size at 5 m depth. We found that mussel succession stage is an important determinant of faunal abundance, species richness and species assemblage. Our results demonstrate that mussel succession stage (size structure, density and biomass) highly affect the associated macrofaunal communities and the strength of facilitation in these systems. We also found that the body size of crustaceans and clams was positively correlated with mussel body size, increasing with a growing mussel body size. We suggest that it is of high importance to consider individual traits of ecosystem engineers and the types of environments in which they play critical roles in order to increase the value of the ecosystem engineering concept in ecology and conservation biology.