1. IFEVA-Departamento de Recursos Naturales y Ambiente, Facultad de Agronomia, Universidad de Buenos Aires, Av. San Martin 4453, 1417 Buenos Aires, Argentina
2. Department of Biology and NERC Centre for Population Biology, Imperial College at Silwood Park, Ascot, Berkshire SL5 7PY, UK
3. The Zoological Society of London, Institute of Zoology, Regent's Park, London NW1 4RY, UK

Correspondence to: Marina Omacini¹Christine B. Müller^2,3 Correspondence and requests for materials should be addressed to M.O. (e-mail: Email: omacini@ifeva.edu.ar) or C.B.M. (e-mail: Email: christine.mueller@ioz.ac.uk).

Abstract

Symbiotic microorganisms that live intimately associated with terrestrial plants affect both the quantity and quality of resources^{1, 2}, and thus the energy supply to consumer populations at higher levels in the food chain. Empirical evidence on resource limitation of food webs points to primary productivity as a major determinant of consumer abundance and trophic structure^{3, 4, 5, 6}. Prey quality plays a critical role in community regulation^{7, 8}. Plants infected by endophytic fungi are known to be chemically protected against herbivore consumption^{9, 10, 11}. However, the influence of this microbe–plant association on multi-trophic interactions remains largely unexplored. Here we present the effects of fungal endophytes on insect food webs that reflect limited energy transfer to consumers as a result of low plant quality, rather than low productivity. Herbivore–parasite webs on endophyte-free grasses show enhanced insect abundance at alternate trophic levels, higher rates of parasitism, and increased dominance by a few trophic links. These results mirror predicted effects of increased productivity on food-web dynamics¹². Thus 'hidden' microbial symbionts can have community-wide impacts on the pattern and strength of resource–consumer interactions.