A quiet revolution – the science of complex systems
Glossary
ecosystem. A term used to encompass all the organisms in a community together with the associated physical environmental factors with which they interact (eg, a rockpool ecosystem, a forest ecosystem).
epidemiologists. Researchers who study diseases or conditions in human populations and the factors that influence their incidence and prevalence.
equilibrium. When a reaction and its reverse occur at equal rates, they effectively cancel one another, so there is no net change.
feedback. The process whereby the output of a system affects the input. Positive feedback reinforces or increases something; negative feedback acts to keep a process within certain limits. Positive feedback can work in systems by amplifying a very small effect, changing the previous equilibrium.
metabolic pathways. A group or series of chemical reactions occurring within a cell, catalysed by enzymes. Pathways can breakdown compounds to yield energy, or involve the step by step modification of an initial compound to create a new product.
model. Solving complex problems associated with real situations is often made easier by setting up a model of the situation – a mathematical description of the problem. To set up a model, a problem is simplified and only those aspects that can be represented mathematically are included.
After the problem is solved mathematically, tentative solutions are translated back to the real situation, as possible real solutions. At this stage the inadequacy of the simple model may be revealed, and some parts of the process may need to be changed. More information on models and modelling can be found at What is modelling? (Nova: Science in the news, Australian Academy of Science).
non-linear. For non-linear systems, a small perturbation may cause a large effect, a proportional effect, or even no effect at all – the behaviour of the system is not simply the sum of its parts. In linear systems, effect is directly proportional to the cause. Many systems are best represented by non-linear equations that are difficult to solve, but can give rise to interesting phenomena.
scale-free networks. A network pattern commonly seen in living systems that has some nodes with many links, many nodes with a few links, and the remaining nodes lying somewhere in between. In this system, known as a scale-free network, there is no clear average number of links per node. Scale-free networks are resilient structures because the random removal of any particular node is unlikely to stop the network from functioning. On the other hand, if a node with many links was targeted and removed it could create a large system-wide disturbance. For more information, see Scale-Free Networks (Computerworld, USA).
In some networks the nodes are connected randomly, in others each node has a fixed number of links to adjacent nodes, giving rise to a regular pattern. But most systems observed in nature fall somewhere between these two extremes.
Posted October 2006.