Institute for Biodiversity and Ecosystem Dynamics
Coupled oscillations of predators and prey cause chaos in food webs
Right: copepod (Eurytemora affinis)
In 1665, confined to his home by a minor illness, the famous Dutch physicist Christiaan Huygens discovered an ‘odd kind of sympathy' between two pendulum clocks mounted next to each other on the same beam. The two pendula swayed back and forth with exactly the same frequency, but in opposite directions. Such movements are called ‘anti-phase' oscillations. When Huygens briefly pushed one pendulum to disturb its swing, the anti-phase oscillations of the two pendula were again quickly restored. In nature, a predator and its prey species may display oscillations in their population abundances. Prey populations boom when predator abundance is low, but bust later on, when their own large numbers have increased the predator population. A new study now demonstrates that coupling of two of such predator-prey systems can produce similar anti-phase oscillations as in Huygens's clockwork.
Elisa Benincà and Jef Huisman, both from the Institute for Biodiversity and Ecosystem Dynamics (IBED) of the University of Amsterdam, studied the complex ups and downs of plankton species in an aquatic food web isolated from the Baltic Sea. Earlier they had demonstrated that the species abundances in this food web, maintained under constant laboratory conditions for more than eight years, fluctuated up and down in a chaotic fashion. This previous work, published in Nature of 2008, upset the traditional idea of the "balance of nature", and showed that long-term prediction of species abundances in food webs is fundamentally impossible. However, what underlying mechanism was driving the chaotic fluctuations in their aquatic food web?
In a new study, which will be published in the December issue of Ecology Letters, they applied advanced statistical techniques to analyze the species fluctuations in detail. Their food web consisted of several phytoplankton and zooplankton species of different microscopic sizes. Phytoplankton species resemble the grass and trees of our macroscopic world, using sunlight and mineral nutrients for growth. They found that small zooplankton, known as rotifers, fed on the smallest phytoplankton. Larger zooplankton species, such as copepods, fed on the larger phytoplankton. This resulted in two predator-prey systems, coupled through competition for nutrients and light between the small and large phytoplankton species.
Whenever the small phytoplankton were abundant, the rotifers increased and feasted on their small prey. This swung the competitive balance to the larger phytoplankton species, replacing their smaller competitors. However, the rise of the larger phytoplankton favored, in turn, the increase of copepods. The copepods consumed the larger phytoplankton, and swung the competitive dominance back to the smaller phytoplankton species. In this way, the food web alternated back and forth between two predator-prey systems. This produced anti-phase oscillations between the different species, as in Huygens's clockwork. However, the nonlinear growth of the different species resulted in chaotic alternations in species abundances rather than in the strict regularity of Huygens's swaying pendula.
This work presents the first experimental demonstration of coupled predator-prey oscillations in a chaotic food web, and sheds new light on the intriguing complexity of species interactions in food webs.
The research was financed by the Earth and Life Sciences Foundation, which is subsidized by the Netherlands Organization for Scientific Research (NWO).