Habitat complexity provided by foundation species drives spatio-temporal diversity patterns and promotes species richness and community stability in aquatic ecosystems

Background/Question/Methods

The role of habitat complexity on biodiversity is a fundamental question in ecology, and one of its cornerstones is that increasing habitat complexity promotes species richness and abundance. This topic became popular in aquatic environments as they are faced with intense pressures, and most studies focused on the complexity provided by a single or few foundation species. Maerl or rhodolith beds are ecologically important marine biogenic habitats founded on a few species of free-living coralline algae that aggregate and form complex structures. Their high biodiversity has been justified by both their high local productivity and the habitat complexity that maerl thalli provide, but the latter hypothesis remained to be directly tested. Using long-term monitoring data from ten maerl beds in Brittany (France), we investigated the links between the structural complexity they provide, the physical environmental conditions they’re exposed to and the benthic macrofaunal communities they harbor. Habitat complexity was quantified holistically at different scales, through both species-specific traits and metrics that would allow for cross-ecosystem comparisons, such as density of structuring elements and fractal dimension. Both macrofaunal communities and variations in physical environmental constraints over 6 years were analyzed.

Results/Conclusions

We quantitatively demonstrate that habitat complexity varies greatly within maerl beds at a regional scale. Depth, hydrodynamics, and sediment characteristics explained part of this variation. This indicates that environmental conditions partly drive habitat complexity provided by foundation species even if intrinsic autecological factors may play a major role. We provide further evidence that habitat complexity promotes species richness, regardless of the environmental forcings. Our results show that habitat complexity plays a more important role in driving regional diversity patterns than the measured environmental constraints. In our study, most beds with higher complexity levels presented similar diversity and assemblages, while beds with relatively lower habitat complexity were the most distinct in terms of community structure. Our work suggests that habitat complexity can promote community stability, with overall lower temporal diversity found in highly complex beds. We demonstrate for the first time the effects of habitat complexity in both local, regional and temporal diversity patterns in habitats under a broad range of environmental conditions and complexity levels. Therefore, our results contribute to a fundamental question of ecological research: we provide evidence for the facilitative universal effect of complexity for biodiversity in aquatic ecosystems and discuss its associated underlying mechanisms.