Landscapes of facilitation: the effects of positive interactions on community structure

Positive interactions between species play an important yet underappreciated role in shaping community composition. Habitat-forming species can facilitate associated organisms directly, or indirectly via facilitation cascades where facilitative interactions between multiple habitat-forming species result in large indirect effects on biodiversity and species abundances. The strong influence of habitat-forming species on biodiversity make them compelling targets for conservation and restoration efforts. However, in order to use habitat-forming species to their full potential it is crucial to ascertain how and where facilitative interactions occur. Despite a growing body of literature, the mechanisms that underpin facilitation cascades remain largely unexplored. Additionally, a considerable data gap exists regarding those factors that influence the spatial extent and strength of facilitation cascades.

In this thesis I addressed these knowledge gaps by examining the facilitative interactions exerted by habitat-forming blue mussels Mytilus edulis L. in The Netherlands and mangroves Avicennia marina in south eastern Australia. Specifically, I investigated (1) the spatial scales across which facilitation cascades affect biodiversity, (2) the effects of inter- and intraspecific variation in species traits on facilitation, and (3) the effects of environmental changes on the strength of facilitation cascades.

My experiment provided evidence for simultaneous large- (cross-habitat) and small- (nested) scale effects of blue mussels on invertebrate communities. At large spatial scales, dampening of wave energy by the habitat-former facilitated cockle banks, which in turn facilitated infauna. At a small spatial scale, the mussels trapped Fucus with its byssus threads, which in turn enhanced epifauna. Both inter- and intraspecific variation in traits were found to affect facilitative interactions at the small spatial scale. While Pacific oysters and blue mussels appear structurally and functionally similar, I demonstrated large differences in the facilitation cascades they support due to fundamental differences in their method of aggregation. Without byssus threads, oyster reefs were unable to capture Fucus and instead served as a substrate for development of green algae of the genus Ulva. Subsequently I showed the complexity of Fucus to support diverse epifaunal communities. Furthermore, habitat heterogeneity generated by self-organization in blue mussels resulted in enhanced biodiversity on the mussel reef through microhabitat creation. Within temperate Australian mangroves, variation in morphological traits of the secondary habitat-forming macroalga Hormosira banksii affected community structure, presumably by determining the type and amount of niches available for invertebrates.

Finally, I revealed that the strength of facilitation cascades in mangroves varied little across environmental gradients. This result was counter to predictions of the Stress Gradient Hypothesis, and presumably reflects the primary role of the mangroves in reducing environmental stresses experienced by inhabitants.

Overall, this thesis has increased the understanding of how facilitation cascades can shape biodiversity across a range of spatial scales and ecosystems, and the sensitivity of positive interactions to trait variation. These results have ramifications for management initiatives that seek to enhance biodiversity through conservation or restoration of habitat-forming species, and their positive interactions.