How habitat-modifying organisms structure the food web of two coastal ecosystems

Abstract

The diversity and structure of ecosystems has been found to depend both on trophic interactions in food webs and on other species interactions such as habitat modification and mutualism that form non-trophic interaction networks. However, quantification of the dependencies between these two main interaction networks has remained elusive. In this study, we assessed how habitat-modifying organisms affect basic food web properties by conducting in-depth empirical investigations of two ecosystems: North American temperate fringing marshes and West African tropical seagrass meadows. Results reveal that habitat-modifying species, through non-trophic facilitation rather than their trophic role, enhance species richness across multiple trophic levels, increase the number of interactions per species (link density), but decrease the realized fraction of all possible links within the food web (connectance). Compared to the trophic role of the most highly connected species, we found this non-trophic effects to be more important for species richness and of more or similar importance for link density and connectance. Our findings demonstrate that food webs can be fundamentally shaped by interactions outside the trophic network, yet intrinsic to the species participating in it. Better integration of non-trophic interactions in food web analyses may therefore strongly contribute to their explanatory and predictive capacity.

Foundation species enhance food web complexity through non-trophic facilitation

“…These results suggest that trophic facilitation on its own is not a likely explanation for the observed enhancements of food web complexity by foundation species. Instead, non-trophic facilitation would seem to be the main driver, corroborating a large body of earlier work showing that habitat modification and stress amelioration by foundation species is critical to their enhancement of species richness [13, 20, 23, 24, 30, 32]. Thus, our empirical work provides compelling cross-ecosystem evidence for the hypothesis that non-trophic facilitation by foundation species, rather than their trophic role, can be an important driver of food web structure [12, 13, 15–17].…”

“…link density) by affecting species and links through non-trophic interactions [14–17]. Although this hypothesis was recently supported by two empirical studies of coastal ecosystems [13, 20], it remained unclear whether these findings could be generalized to other ecosystems. By comparing foundation species-dominated habitats with nearby bare habitats where these foundation species ware absent, we demonstrate that their presence consistently increased food web size and complexity across seven terrestrial, freshwater and coastal ecosystems.…”

“…In addition, species may also be benefit from foundation species because the foundation species concentrates critical resources (e.g. water, detritus), or mitigates physical stress resulting from currents, waves, wind, sediment instability, drought or high temperatures [13, 20, 30, 40]. Furthermore, indirect facilitation through trophic pathways is also possible.…”

Food or furniture: Separating trophic and non‐trophic effects of Spanish moss to explain its high invertebrate diversity

“…Trees, freshwater macrophytes, seagrasses, reefforming bivalves, and corals are all examples of such foundation species which create habitat for other species with their own body tissue (Jeppesen et al 1992, Ellison et al 2005, Coker et al 2014, Christianen et al 2016, van der Zee et al 2016, Ali and Yan 2017. A major factor thought to underlie foundation species' enhancements of associated communities is their positive effect through their ability to modify their habitat (Govenar 2010).…”

“…Next to these facilitative non-trophic pathways generated by increased habitat structure, foundation species may also increase biodiversity through their trophic role by acting as a food source (Strong et al 1984). Although most studies on foundation species and species richness contribute their facilitative effects to their habitat-modifying properties Callaway 1994, Bruno et al 2003), only recently a number of studies have focused on their role in the food web (Miller et al 2015, van der Zee et al 2016. These studies suggest that the direct trophic role of foundation species as a food source is of minor importance, compared to their non-trophic habitat-structuring role.…”