Can Food Niche Width (Δ13C) Explain Properties of Detritus-based Food Webs in aquatic and Terrestrial Contexts? An analysis of food web and trophic niche using stable isotopes

A long-standing question in the study of food webs is whether there are similarities in the topological patterns of terrestrial and aquatic webs and how they can be explained. Some food web models have considered the role of the species niche, but field support is limited and, as such, the mechanisms by which consumers regulate web topology remain uncertain. In this study, we explored the influence of species niche width on (i) web structure, particularly the relationship between the number of links and species richness, and (ii) the web robustness to species loss, determined after in-silico removal of rare species. To this purpose, we reconstructed 42 new invertebrate detritus based food webs and measured the trophic niche width as δ13C variation of predators and prey from two aquatic and two terrestrial ecosystems. Our results show that the niche width of predators and prey increased in proportion to the number of species at their respective trophic levels, but the rate of niche enlargement was higher in terrestrial than in aquatic webs. Since niche width was also related to linkage density, the different rates in aquatic and terrestrial systems implied different mathematical relationships between the number of links and species richness. Specifically, the linkage density increased with S in terrestrial food webs, whereas it did not change significantly in aquatic ones, which implies S-scaling connectance. The web’s robustness to rare species loss increased with the connectance and niche width of predators in both types of web. The results provide a pioneering field demonstration of the central role of species niche width, driven by optimal foraging, in determining food web structure, thus suggesting that optimal foraging and food web theories are just the two sides of the same coin.