Biogeographic Variability in the Value of Mussel Beds as Ecosystem Engineers on South African Rocky Shores

Ecological engineers have important effects on biodiversity because they often increase habitat complexity and moderate environmental conditions, implying that their influence on associated fauna will vary across gradients of environmental stress. To test this, we quantified the positive effects of mussel beds on associated benthic communities around the entire South African coastline (~3500 km). We hypothesised that molluscan assemblages would show stronger affinities to the presence of mussel beds with increasing levels of heat-stress. Biomimetic loggers used to characterise thermal properties within and outside mussel beds found that solitary mussels experienced significantly greater daily maximum temperatures than mussels within beds across all locations. However, the magnitude of such differences did not appear to vary with latitude or time of year but rather was strongly influenced by biogeographic region. Differences in the abundance, diversity and community structure of molluscs within and outside mussel beds showed similar biogeographic variability, with differences in total molluscan abundances being most pronounced along the cool temperate west coast during summer and least pronounced along the warm temperate south coast during winter. Greater affinity of molluscan assemblages for mussel beds within cooler biogeographic regions suggests that evolutionary history and/or other abiotic factors may be the primary cause for the stronger influence of mussel beds on the west coast. This highlights the complex, context-dependant nature of ecosystem engineering and the varying degrees to which associated organisms affiliate with these biogenic structures. Such findings have important implications for the use of ecosystem engineers as umbrella species in ecological conservation.