Functionally distinct foundation species influence functional properties in associated communities

Background/Question/Methods

Foundation species are spatially dominant species that locally ameliorate conditions and create habitat through their body structures. Thanks to these properties, they often host a high diversity of small associated species. While the nature of this facilitation has been investigated from various perspectives, an area that remains intriguing is understanding how different foundation species may affect associated communities under the same environmental conditions. To address this knowledge gap, we used two foundation species that differ functionally (a leafy primary producer and a hard-bodied consumer) and occur in the same intertidal habitats in Atlantic Canada. Both the clonal seaweed Chondrus crispus and blue mussels Mytilus spp. form dense stands that host various associated species. In this study, we ask: “Do communities differ structurally and functionally depending on the foundation species they are associated with”? We hypothesized that functionally different foundation species influence habitat conditions differently and, therefore, host different communities even under the same environmental conditions. We conducted a mensurative experiment in which we identified and counted invertebrate fauna occurring in 100-cm2 patches covered by either C. crispus or Mytilus spp. on the same coast. We compared communities based on both their taxonomic diversity and functional diversity.

Results/Conclusions

We identified 29 invertebrate taxa from 7 phyla. Principal component analysis revealed a marked difference in species composition between both community types. Species diversity indices differed significantly between C. crispus communities (CC) and Mytilus spp. communities (MC). Species richness was 16 % higher, on average, in CC than in MC. The mean total abundance of organisms, though, was 346 % higher in MC than in CC. Interestingly, when we determined the functional traits of the species (trophic level, development type, mobility, and morphology), there was no significant difference in functional richness, suggesting both communities occupy similar ranges of niche space. However, comparing individual traits between communities indicated distinct functional compositions. For example, herbivores, swimmers, organisms with lecithotrophic larvae, and organisms with exoskeletons were predominantly represented in CC, whereas predators, crawlers, organisms with planktotrophic larvae, and soft-bodied organisms were more represented in MC. These are relevant findings because they highlight that functionally distinct foundation species occurring in the same environment may influence the functional properties of the associated communities. Future studies on the underlying mechanisms could thus enrich the ecological theory on the role of foundation species in natural ecosystems.