The music in this lesson was composed by Ross Allchurch.

Feedback

What is feedback? It is a process that is the result of mutual causal interaction: X affects Y and Y affects X. The mutual causal interaction creates a circuit of effects, so that any change in X, causing a change in Y, in turn causes another change in X, and so on – a feedback loop. Feedbacks are either positive or negative. The word positive here means that they reinforce a disturbance, an initial kick. Positive feedbacks are amplifiers. They are forces of divergence, they can make a system expand, but also spin out of control. They can cause development or degradation. Negative feedbacks on the other hand are forces of convergence, of conservation. The word negative here means that they counteract a disturbance. Negative feedbacks can restore the balance. They pull the system back.

Feedbacks affect the way every single variable, every population in an ecosystem for example, responds to both internal and external perturbations. Feedbacks determine how each part of a system responds to such changes, and all the feedbacks together determine the resilience of the whole system. It is only at the system level that you can really speak about stability. Just like when you make music, a single tone can sound very different in isolation, compared to when it is played in combination with other tones. Or, when you play on your own, your sound is very different from when you play in an ensemble - when you make music together the instruments influence each other.

That is what happens with feedbacks in an ecosystem as well, it is all about the combination of feedbacks.

Learn more about how feedbacks work, about positive feedbacks in natural systems, and the relation between feedback and system stability.

Ecological networks

Natural communities do not consist of simple food chains, they are complex networks of interactions. But how do we get our heads around the multitude of feedbacks in an ecological network?

What will happen to a food web when we hunt big predators to extinction? How vulnerable is our food production when honeybees are under threat? And how important is dead organic matter for the stability of an ecosystem? All these are questions about ecological feedbacks. When there is a perturbation - disturbance - to one species in an ecosystem (for example increased mortality of a species due to pollution, reduced fecundity because of habitat fragmentation, introduction of an exotic species, and so forth), all the species in the food web may show some response, because they are connected. How much they will change and in what direction, depends on the feedback structure of the entire system.

To learn more about chains of interactions in complex ecological networks, watch the TED-Ed Lessons about the extinction of big predators, pollination by honeybees, and detritus.

Measuring the feedbacks in real systems

It is only recently, mainly because of increased computer power, that ecologists have started to describe and quantify the feedbacks in observed ecological networks and identify patterns, taking steps in understanding the behaviour and structure of real complex ecosystems.

Here are some examples of papers by the educators: on making predictions for ecosystem management, on the patterns of feedbacks causing stability, and on ecosystem complexity and stability.

Additional References

Basic concept and examples in various realms of life:
Maruyama, M. (1963) The second cybernetics: Deviation-amplifying mutual causal processes. Am. Sci. 51, 164-179.

Mathematical biology textbook on positive feedback:
DeAngelis, D.L., Post, W.M. & Travis, C.C. (1986) Positive Feedback in Natural Systems. Springer-Verlag, Berlin, pp. 221-244.

Mathematical background on the relation between feedback and stability in biological systems:
Levins, R. (1974) The qualitative analysis of partially specified systems Ann. N Y Acad. Sci., 231, 123-138.

Papers on food webs, feedbacks and stability by the educators:
Bodini A. (1998) Representing ecosystem structure through signed digraphs. Model reconstruction, qualitative predictions and management: the case of a freshwater ecosystem Oikos 83, 93- 106.
Neutel, A.M., Heesterbeek, J.A.P. & de Ruiter, P.C. (2002) Stability in real food webs: Weak links in long loops. Science, 296, 1120-1123.
Neutel A. M. & Thorne, M.A.S. (2014). Interaction strengths in balanced carbon cycles and the absence of a relation between ecosystem complexity and stability. Ecology Letters 17, 651-661.

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