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.

Keywords: consumer–resource interactions; ecological networks; ecosystem engineering; facilitation; foundation species; non-trophic interactions.

Figure 1.

Conceptual representation of performed removal procedures. (a,b) To test the hypothesis that observed differences in food web structure between habitats resulted from non-trophic facilitation by a habitat modifier, we first removed its non-trophic effect by removing species that depend obligatorily on its non-trophic facilitation (e.g. as attachment substrate). (a–c) Second, to test whether food webs differed due to the trophic effects of the habitat modifier, we deleted it and species exclusively feeding on it from the food web. (a–d) Third, we compared the trophic and non-trophic effects by habitat modifiers to those of the most highly connected species.

Figure 2.

Salt marsh and seagrass system food webs in the absence and presence of primary (cordgrass/seagrass) and secondary (mussels/crabs) habitat modifiers. Bare sites are typified by relatively simple food webs (a,d). Food webs have higher species richness and link density in colonizing habitats with primary habitat modifiers (b,e) and these effects are further enhanced by secondary habitat modifiers in established habitats (c,f). Node colour changes from red (basal species) to yellow with increasing trophic level.

Figure 3.

Species richness (a,d), link density (b,e) and connectance (c,f) as conditional on primary and secondary habitat modifiers. For each habitat, the most left bar indicates the natural situation, and subsequent bars depict the outcomes of the removal of species dependent on habitat modification (i.e. removal of non-trophic effects). Letters indicate post hoc grouping; error bars represent s.e.m. Species richness and link density are significantly higher in modified habitat, whereas connectance is generally lower. Removal of species dependent on habitat modification increased similarity to unmodified, bare habitat.

Figure 4.

The relative effect of secondary and primary habitat modifiers, and the most highly connected species on species richness (a,d), link density (b,e), and conductance (c,f). Relative effects are calculated as (1 − value calculated for each modified food web matrix) / (value calculated for the original food web matrix). Letters indicate statistical grouping; error bars represent s.e.m. The analyses show that habitat modification was more important for diversity and of more or similar importance for both link density and conductance measures of complexity.