Nature prefers chaos on the rocks
Plants and animals in natural ecosystems do not always settle at a stable equilibrium. Researchers from the University of Amsterdam present the first field evidence for chaos among multiple interacting species, based on a long-term study of the ups and downs of barnacles, algae and mussels in a coastal nature reserve in New Zealand. The results were recently published in the journal PNAS and received high appraisal by various international scientists.
Often it is assumed by scientists, conservationists and hunters that changes in the species composition of ecosystems are caused by external disturbances, such as human interventions. Species in undisturbed ecosystems would approach a natural equilibrium, also known as ‘the balance of nature’.
Professor Jef Huisman and Dr Elisa Benincà of the Institute for Biodiversity and Ecosystem Dynamics (IBED) of the UvA describe how barnacles, algae and mussels living on a rocky stretch of coast in New Zealand have fluctuated erratically in numbers during the past 20 years. Although the species replaced each other in a predictable order, quantitatively the abundances of the species in the studied ecosystem were unpredictable.
Huisman explains: ‘During some years barnacles would dominate, while mussels or algae took over during other years. The species followed a cyclic succession pattern, where bare patches of rock were first colonised by barnacles. These were then overgrown by algae, followed by a dense carpet of mussels. After the mussels had killed the underlying barnacles, they detached from the rock, and the succession cycle started all over again. Since the speed of these processes is strongly dependent on the seasonal variability in temperature, the result is not a regular pattern. The succession of species is cyclic, but the number of individuals of each species varied dramatically. No predictable pattern was found in the timing and quantity of barnacles, algae and mussels.’
Illusion of natural harmony
One of the most important conclusions drawn from recent scientific studies is that there is not such a thing as ‘natural harmony’ in an ecosystem. Mathematical models have predicted that interactions between species can lead to erratic fluctuations in species abundances. This has been supported by laboratory experiments with plankton and insects demonstrating chaotic dynamics, and also the wax and wane of some infectious diseases may follow a chaotic pattern. Yet, field evidence of chaos remained rare.
The study of Huisman and Benincà now provides the first evidence for chaotic fluctuations of multiple species in a natural ecosystem. The scientists believe that such chaotic fluctuations occur much more frequently in nature than presently assumed. An important implication of chaos in ecosystems is that changes in species abundances can only be accurately predicted over shorter periods of time, but not in the long-term.
The newly published work underlines the value of long-term scientific studies of natural ecosystems. Data used in the study was collected by the conservationist Dr Bill Ballantine, one of the founding fathers of the marine reserves in New Zealand, who has logged species abundance and meteorological data on a monthly basis during the past 20 years. The researchers of IBED analysed the data together with Professor Stephen Ellner from Cornell University. The study was financed by the program for Sea & Coastal Research (‘Zee- en Kust Onderzoek’) of NWO.
Elisa Benincà, Bill Ballantine, Stephen P. Ellner, Jef Huisman (2015): Species fluctuations sustained by a cyclic succession at the edge of chaos. PNAS, 112(20), 6389-6394, doi:10.1073/pnas.1424353112
In a recent commentary by Ottar Bjørnstad in PNAS, the work of Benincà et al. received high appraisal. Bjørnstad notes that ‘it sheds new light on a number of threads of discussion in contemporary ecology related to variability and predictability in ecology’, and concludes that the study ‘is a beautiful illustration of just how far the discipline has moved’.
Ottar Bjørnstad (2015): Nonlinearity and chaos in ecological dynamics revisited. PNAS, 112(20), 6252–6253, doi: 10.1073/pnas.1507708112