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. A free Plain Language Summary can be found within the Supporting Information of this article. ; This work was conducted as part of the 'Completing the dryland puzzle: creating a predictive framework for biological soil crust function and response to climate change' Working Group supported by the John Wesley Powell Center for Analysis and Synthesis, funded by the U.S. Geological Survey. We thank the University of Colorado Boulder Undergraduate Research Opportunities Program (UROP), which helped support undergraduate researchers who assisted with data entry for this project. Particularly, we thank CU Boulder undergraduate researcher assistants Emma Brokyl, Julius Gayo and Whitney Gabbert for their invaluable work on database compilation and organization for this project. C.A.H. was supported by the Department of Ecology and Evolutionary Biology at the University of Colorado Boulder and a National Science Foundation Graduate Research Fellowship (award DGE‐1144083). V.B.C. was supported by grants from the National Science Foundation (award DEB‐1844531) and the DePaul University College of Science and Health. E.H.S. was supported by Consejo Nacional de Ciencia y Tecnología (project SEP‐CONACYT 251388). J.B. was supported by the USGS Ecosystem and Land Change Sciences program. Any use of trade, firm or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.