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The population sizes of both predators and their natural prey are often subject to fluctuations. If the number of predators increases, the number of prey will decline. In certain situations, however, two predator-prey systems can become interlinked, causing chaos in the food network. This phenomenon has now been demonstrated for the first time by Elisa Benincà and Jef Huisman of the University of Amsterdam (UvA).

The population sizes of both predators and their natural prey are often subject to fluctuations. If the number of predators increases, the number of prey will decline. In certain situations, however, two predator-prey systems can become interlinked, causing chaos in the food network. This phenomenon has now been demonstrated for the first time by Elisa Benincà and Jef Huisman of the University of Amsterdam (UvA). Their discovery will be highlighted in today's edition of Nature and tomorrow's edition of Science. Next month their full study will be published in Ecology Letters.

The number of predators and prey in a natural system can fluctuate dramatically. Food animal populations tend to grow rapidly when the number of predators is low, with numbers dropping dramatically as soon as the number of predators rises. New research has shown that interaction between two predator-prey systems can result in anti-phase oscillations. As a result, the two predator species will continually fluctuate in terms of population size; after a period of expansion by the first group, the other species will grow, followed by a resurgence of the first group, etc.

Elisa Benincà and Jef Huisman of the University of Amsterdam's Institute for Biodiversity and Ecosystem Dynamics (IBED) studied the complex ups and downs of plankton populations in a food network in the Baltic Sea. One of their previous studies - published in Nature in 2008 - demonstrated that population numbers of the various plankton types in this food network fluctuated in a chaotic and thus unpredictable manner. But what was causing these chaotic changes?

Small plankton versus large plankton

For their new study, the researchers applied advanced statistical methods in order to conduct detailed analyses of fluctuations in the plankton types in their food network. This food network consists of a mixture of phytoplankton and zooplankton species of various sizes. The microscopic phytoplankton is the prey, while the zooplankton species act as predators. The researchers discovered that small zooplankton species, such as rotifers, feed on the smallest phytoplankton. Larger zooplankton species, such as copepods, eat the large phytoplankton species.

Each time the amount of small phytoplankton surged, rotifer numbers would rise and small phytoplankton would be eaten. This shifted the balance of competition to the advantage of the large phytoplankton, which became more numerous. However, the growth of the large phytoplankton caused an increase in the number of copepods, which ate the large phytoplankton. This created an opportunity for the small phytoplankton species, allowing them to regain a dominant position. As a result of this phenomenon, the food chain fluctuated between two predator-prey systems, causing anti-phase oscillations. Due to the non-linear growth of the various species within the food network, these anti-phase oscillations could be clearly identified as the chaotic ups and downs of the various species' population numbers.

Linkage between predator-prey systems results in chaos

The UvA scientists' research provides the first experimental evidence that linkage between two predator-prey systems can result in chaos. This throws new light on the complexity of interactions between species in a food network. The study was funded by the Earth and Life Sciences Foundation, a division of the Netherlands Organisation for Scientific Research (NWO).

Publication details

Elisa Benincà, Klaus D. Jöhnk, Reinhard Heerkloss en Jef Huisman. Coupled predator-prey oscillations in a chaotic food web. Ecology Letters 12 (dec. 2009, pp. 1367-1378).