Wetenschap - 30 november 2010

Seasons add to chaos in nature

Populations in nature wax and wane in a chaotic fashion. And that chaos is only reinforced by the influence of the seasons.

This is the thesis which Italian PhD researcher Elisa Benincà defends in her dissertation with the challenging title of Is there chaos out there? By chaos she means the unpredictability principle built into a regulated system. One of the characteristics of such chaos is that small changes can have big consequences in the long term. Benincà demonstrated the presence of such chaos in a plankton community, and her discovery made it into Nature. In her thesis she describes how she used model studies to support and strengthen her case.
Benincà demonstrated the chaos in populations of plankton in a barrel containing one hundred litres of water and sand from the Baltic Sea. In spite of a constant temperature and regular pattern of day and night, the plankton populations in the barrel never stabilized. Chaos: Benincà had the evidence. But an experiment of this kind is different to the real world. Would the seasons and day-to-day changes in the weather cancel out the chaos?

The chaos in the dynamics of plankton is fairly easy to explain: zooplankton eat phytoplankton. This causes numbers of species to fluctuate. Benincà identified two dominant predator-prey combinations which are interconnected because the prey animals compete among themselves for light and nutrients. The result of these interconnections is chaos. In order to study the influence of environmental factors, Benincà went on to develop a model with six predators and ten prey, living at a temperature that fluctuates with the seasons. 'So I looked at how the dynamics between the predators and the prey change with this seasonal effect.'

Increasing chaos
The result was surprising. Instead of being reduced, the chaos factor was actually increased. Every year, the order in which species numbers peaked and the height of the peak changed. Only the total biomass followed a more or less regular pattern. 'Many ecologists think that the weather has a big influence on populations, but I show that you get chaotic behaviour precisely when there is a fixed seasonal rhythm. So chaos is intrinsic to the dynamics.' Besides the influence of the season, changes in the weather from day to day also appear to be able to amplify chaos.

So does chaos rule? Benincà thinks so. 'But it is not easy to prove, because ecosystems are very complex. I have shown chaos in a relatively simple experimental system. And of course external influences on ecosystems also affect population sizes. But the internal conditions, the interaction between species, are often underestimated. The chaos that is intrinsic to those dynamics is important. It has implications for nature management. Long-term predictions are theoretically impossible. In Benincà's plankton community, there was predictability for a maximum of 15 to 30 days. That underlines the importance of constant monitoring of natural populations, say Benincà, so that predictions can be regularly adjusted.
Elisa Benincà receives her PhD on Monday 6 December from professor Marten Scheffer (Aquatic Ecology) and professor Jef Huisman (Aquatic Microbiology, University of Amsterdam).