Habitat-mediated interactions: how marine foundation species structure Australian marine ecosystemsMacroecology

Thomas Benoit¹, Aurélien Boyé¹, Mathieu Chevalier¹, Stéphane Robin², Camille Albouy^3,4, Cédric Bacher¹, Martin Marzloff¹

¹Ifremer, France - ²Sorbonne Université, CNRS, France - ³ETH Zürich, Switzerland - ⁴7u Federal Research Institute WSL, Switzerland

Marine foundation species, such as reef-forming bivalves or canopy-forming algae, play a pivotal role in coastal ecosystems by creating biogenic habitats that structure the seafloor. These unique habitats can modify environmental gradients (e.g. temperature, hydrodynamic) and hence strongly influence ecosystem functioning. As global changes drive coastal habitat degradation and induce both local and global changes in foundation species, understanding the cascading effects of habitat changes on associated species diversity is critical. Yet, while the biodiversity-supporting role of a subset of emblematic foundation species is well-studied at local spatial scales, major knowledge gaps remain about foundation species-biodiversity relationships, namely about: the facilitation, or exclusion, role of rare less-studied foundation species; the dependency of individual taxa to single or multiple habitats, as well as ecological mechanisms underlying habitat-fauna interactions. Using the Reef Life Survey dataset, which includes visual diver-based censuses of marine fauna across biogenic habitats, we first applied a range of complementary network inference approaches to investigate associations between foundation species and benthic taxa (macroinvertebrates and fishes) along Australia’s temperate coastlines. Then, to comprehensively investigate habitat-biodiversity relationships, we reconstruct a multilayer network that combines (i) this bipartite network linking foundation species to their associated fauna, and ii) a size-based trophic interaction network. The latter enables us to explore potential trophic cascades resulting from primary extinctions identified by the bipartite network. Through extinction simulations of foundation species, we identify: (i) the benthic taxa that are most vulnerable to habitat loss as highly dependent on specific foundation species; and (ii) foundation species that are most critical for maintaining biodiversity (either directly or indirectly through trophic cascades) and that constitutes management priorities for temperate reef biodiversity conservation. This work will overall quantify the importance of marine foundation species in structuring coastal biodiversity and shed light on the cascading impacts of future habitat losses or changes to reef communities.