Natural ecosystems are made up of complex networks of interactions, or potential interactions, between biotic and physical elements which can create a web of feedback loops. Do we need to know all the details of the food web before we can predict the dynamics of a population we are interested in?

1. Laws of planetary motion. Isaac Newton realized that the orbit of each planet could be affected by all the other planets in the solar system but was able to derive accurate laws predicting the orbit of a planet by considering the single interaction between the planet and its star. Are the dynamics of complex ecological systems also be dominated by single interactions? If true it would certainly make the job of the ecologist and resource manager a lot easier.
2. Lack of chaos in nature. If populations exist in complex environments with many feedbacks, and if all these feedbacks affect population dynamics, then populations should exhibit complex, unpredictable, fluctuations known as chaos. But the empirical evidence suggests that natural populations fluctuate in a much more regular manner than expected under the chaos hypothesis.
3. Logic of control systems. First, positive and negative feedback cannot dominate the dynamics of a system simultaneously, for their effects counteract each other; i.e., ⁺feedback causes unstable behavior while ^-feedback is stabilizing. A system cannot be unstable and stable at the same time! Hence, populations must either be dominated by positive or negative feedback at any given time and place. But what if many ^-feedback loops are present? Consider a room in which there are two thermostats, each hooked up to separate heaters. In this system, the temperature of the room is controlled by two ^-feedback mechanisms (thermostats are engineered to provide ^-feedback control of temperature). Now visualize how the temperature of a cold room will rise when the thermostats are set at different temperatures. The room will first heat up quickly under the influence of both heaters. However, when the room reaches the temperature of the first thermostat, its heater will turn off and the temperature dynamics will be driven by the second alone. The room will continue to heat up, but more slowly, until it reaches the higher setting. At this point the temperature of the room will be completely controlled by the higher thermostat, and the other one can be removed or destroyed with no effect on the dynamics of the system. On the other hand, should the higher thermostat malfunction, the room will come immediately under the control of the lower thermostat. Hence, multiple feedback loops can provide a higher degree of stability to a dynamic system because the failure of one loop need not result in a general loss of stability. This leads to the idea of feedback hierarchies in natural ecosystems; i.e., multiple feedback process operating over different population densities come into operation in a stepwise or hierarchal manner (Berryman et al. 1987). More about this in the next topic.
4. Concept of feedback dominance. Out of the complex network of feedback loops acting on a particular population, only one will dominate the dynamics in the neighborhood of equilibrium (Berryman 1993).

• Berryman, A. A. 1993. Food web connectance and feedback dominance, or does everything really depend on everything else? Oikos 68: 183-185.
• Berryman, A. A., N. C. Stenseth and A. S. Isaev. 1987. Natural regulation of herbivorous forest insect populations. Oecologia 71: 174-184.

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