1. Environment Department, University of York, Heslington, York YO10 5DD, UK
2. Faculty of Veterinary Medicine, Theoretical Epidemiology, Utrecht University, 3508 TD Utrecht, The Netherlands
3. Spatial Ecology Department, Centre for Estuarine and Marine Ecology, Netherlands Institute of Ecology (NIOO-KNAW), 4400 AC Yerseke, The Netherlands
4. Department ICT, Wageningen University and Research Centre, 6700 AB Wageningen, The Netherlands
5. Alterra, Soil Science Centre, Wageningen University and Research Centre, 6700 AA Wageningen, The Netherlands
6. Nature Conservation and Plant Ecology Group, Wageningen University, 6708 PB Wageningen, The Netherlands
7. Department of Environmental Sciences, Copernicus Institute for Sustainable Development and Innovation, Utrecht University, 3508 TC Utrecht, The Netherlands
8. Present address: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK.

Correspondence to: Anje-Margriet Neutel^1,8 Correspondence and requests for materials should be addressed to A.-M.N. (Email: anjute@bas.ac.uk).

Abstract

Understanding how complex food webs assemble through time is fundamental both for ecological theory and for the development of sustainable strategies of ecosystem conservation and restoration. The build-up of complexity in communities is theoretically difficult, because in random-pattern models complexity leads to instability¹. There is growing evidence, however, that nonrandom patterns in the strengths of the interactions between predators and prey strongly enhance system stability^{2, 3, 4}. Here we show how such patterns explain stability in naturally assembling communities. We present two series of below-ground food webs along natural productivity gradients in vegetation successions^{5, 6}. The complexity of the food webs increased along the gradients. The stability of the food webs was captured by measuring the weight of feedback loops⁷ of three interacting 'species' locked in omnivory. Low predator–prey biomass ratios in these omnivorous loops were shown to have a crucial role in preserving stability as productivity and complexity increased during succession. Our results show the build-up of food-web complexity in natural productivity gradients and pin down the feedback loops that govern the stability of whole webs. They show that it is the heaviest three-link feedback loop in a network of predator–prey effects that limits its stability. Because the weight of these feedback loops is kept relatively low by the biomass build-up in the successional process, complexity does not lead to instability.

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