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 instability (1). There is growing evidence, however, that nonrandom patterns in the strengths of the interactions between predators and prey strongly enhance system stability (2-4). 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 foodwebs increased along the gradients. The stability of the food webs was captured by measuring the weight of feedback loops (7) 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.
Ecologists have long studied processes of community assembly and have revealed general principles governing community diversity, dynamics and functioning in vegetation successions (8), that is, on a single trophic level. The understanding of communities at multitrophic levels, however, is much less advanced. In processes of ecological succession, increasing productivity could give longer food chains (9-11), although opposite trends have also been found (12), and it has been argued that stability constraints may limit food-chain length (13). Moreover, the assembly of complex food webs is not self-evident, because random community models suggest that complexity promotes instability (7). However, there has been increasing evidence that the nonrandom patterning of strong and weak links in food webs greatly enhances the stability of these networks (2-4). It has been suggested that such stabilizing patterns can be caused by the decrease in biomass over increasing trophic levels, which means that long feedback loops in the food webs contain relatively weak links (7). In a comparison between steady states of food-web models varying in complexity, and with hypothetical biomass pyramids, it was shown that complexity does not enhance instability (7).
So far, there have been no studies that show and quantify the buildup of complexity of food webs in long-term successional processes. The question still stands: do food webs become more complex as ecosystems develop and, if so, how can these systems exist--what keeps them stable, despite their increasing complexity?
Here we present a series of food webs sampled over two successional gradients along which natural productivity varies. We show that the food webs increase in complexity and that it is not their complexity as such, but the organization of link strengths over feedback...