The return of the trophic chain: Fundamental vs. realized interactions in a simple arthropod food web

The mathematical theory describing small assemblages of interacting species (community modules or motifs) has significantly improved our understanding of the emergent properties of ecological communities. It is not clear whether all interactions accounted for in such models will be realized in real communities.

Here, we use community modules to experimentally explore whether the number of trophic links among species scales with community complexity (i.e. by adding species known to feed on each other from pairwise trials) in a simple mite community present in avocado orchards ( Persea americana ). By varying the presence of each of two predators ( Euseius stipulatus and Neoseiulus californicus ), one herbivore as shared prey ( Oligonychus perseae ) and pollen of Carpobrotus edulis as an alternative food resource, we mimicked communities with simple trophic chains, intraguild predation and/or apparent competition. We then assessed predation rates and the conversion of food into offspring in those communities.

We found that increasing the number of potential interactions did not result in more complex realized community modules. Instead, all species effectively fed upon a single food item, hence all community modules actually corresponded to one or two linear trophic chains.

Therefore, trophic links assumed to occur when species are assembled in pairs do not necessarily occur when other components of the community are present. Consequently, food web structure may be much less complex than predicted by theory.