Universal rules for food-web stability found

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Mumbai:  Using computer models, scientists from Germany, Austria, and the United States have discovered fundamental rules that determine the stability of ecosystems.

The computations reveal that small ecosystems follow other rules than large ecosystems: differences in the strength of predator-prey links increase the stability of small webs, but destabilise larger webs.

The findings also conclude that food-web stability is enhanced when many diverse predator-prey links connect high and intermediate trophic levels (the position that an organism occupies in a food chain), a paper published in the latest issue of Science said on Thursday.

Scientists from the Max Planck Institute for the Physics of Complex Systems (MPIPKS) in Dresden, Germany, have developed a new method that allows them to efficiently analyse the impact of innumerable parameters on complex systems.

"By using a method called generalised modelling, we examine whether a given food web can, in principle, be stable, that is, whether its species can co-exist in the long term," Thilo Gross from MPIPKS said.

"Complex ecosystems can thus be simulated and analysed under almost any conditions. In this way we can estimate which parameters will keep ecosystems stable and which will upset their balance," he said.

Applying this innovative modeling approach together with colleagues at the International Institute for Applied Systems Analysis (IIASA) in Laxenburg, Austria, and Princeton University, USA, the scientists have succeeded in discovering not just one, but several universal rules in the dynamics of ecosystems.

"Food-web stability is enhanced when species at high trophic levels feed on multiple prey species and species at intermediate trophic levels are fed upon by multiple predator species," says Ulf Dieckmann of IIASA.

The scientists have also identified additional stabilising and destabilising factors within ecosystems. Ecosystems with high densities of predator-prey links are less likely to be stable, while a strong dependence of predation on predator density destabilises the system.

On the other hand, a strong dependence of predation on prey density has a stabilising impact on food webs.

A further important finding is that food webs consisting of only a few species behave qualitatively different from webs consisting of many species. Small ecosystems apparently follow different rules than large ecosystems, said Dieckmann.

"Systems with fewer species are more stable if there are strong interactions between some species, but only weak interactions between others. For food webs with many species, exactly the opposite is true," he added.

Natural ecosystems consist of interwoven food chains, in which individual animal or plant species function as predator or prey.

Potential food webs not only differ by their species composition, but also vary in their stability. Observable food webs are stable food webs, with the relationships between their species remaining constant over relatively long periods of time.

Understanding complex systems such as food webs present major challenges to science. They can either be examined by observing natural environments, or by computer simulations.

To enable computer simulations of such systems, scientists often had to make simplifying assumptions, keeping the number of model parameters as low as possible. Yet, the computational demands of such simulations are high and their relevance is often limited, scientists observed.

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