Ecological Feedback Loops
Ecosystem feedback is the effect that change in one part of an ecosystem has on another and how this effect then feeds back to effect the source of the change inducing more or less of it.1 These feedback loops form the basic dynamics for regulating the state of the ecosystem. A negative feedback loop is where the state of one element affects the other in the opposite direction, with the net result of this being a stable system where different forces are counterbalancing each other out creating some equilibrium. As such negative feedback can be identified as providing stability. All ecosystems are composed of many negative feedback loops that keep every part of the system within the boundaries necessary for the whole system to continue functioning. Population regulation is a classical example of negative feedback. Because the resources that sustain populations are limited, no population can exceed the carrying capacity of the ecosystem for long. Negative feedback loops between predators and prey work to keep plant and animal populations within the limits of the carrying capacity of their environment and thus maintain some form of stability.
Positive Feedback Loops
Positive feedback stimulates change and it is responsible for the sudden appearance of rapid changes within ecosystems. Positive feedback is a circular link of effects that are self-reinforcing. When part of the system increases, another part of the system also changes in a way that makes the first part increase even more. Positive feedback is a source of instability and strong force of change as it can drive the system outside of its normal operating parameters. As an example we could cite exponential population growth, when there is a surplus of resources, or lack of predators, this allows a plant or animal population to grow without limit. More population leads to more births, and more births lead to an increasing population creating a compounding effect over time.
Positive & Negative Feedback
Ecosystems and complex systems, in general, have a tension between forces that resist change, the negative feedback, and forces that promote change, the positive feedback. Negative feedback may dominate at some times and positive feedback may dominate at other times, depending on the situation. As a result, ecosystems may stay more or less the same for long periods, but they can also change very suddenly. This change can be like a rapid switch from one state to another, this flipping is known to be a characteristic of nonlinear systems and complex systems in general.
As an example of these two counteracting forces we can look at the succession of an ecosystem from grass to shrub community, beginning with an ecosystem in which the ground is covered with grasses. Shrubs may be present, but they are young and scattered. The ecosystem may stay this way for five to ten years, or possibly longer, because shrub seedlings grow very slowly. They grow slowly because grass roots are located in the topsoil, while most of the shrub roots are lower down. Grasses intercept most of the rainwater before it reaches the roots of the shrubs. Because the grasses limit the supply of water to the shrub seedlings, they maintain the integrity of the ecosystem as a grass ecosystem. At this stage, negative feedback is acting to keep the biological community the same. However, after a number of years, some of the trees and shrubs, which have been growing slowly, are finally tall enough to shade the grasses below them. The grasses then have less sunlight for photosynthesis, and their growth is restricted. This results in more water for the shrubs, which grow faster and shade the grasses even more. This process of positive feedback allows the shrubs to take over in a relatively short period of time. They now dominate the available sunlight and water, and the grasses decrease dramatically.
The term vicious circle refers to a complex chain of events which reinforce themselves through a feedback loop. If the outcome is a negative result this would be termed a vicious cycle. The melting of the polar ice caps is an example of a vicious cycle, as the reflective ice caps melt they reflect less sunlight and heat back to the atmosphere, with more of this heat being trapped by the dark ocean which is now exposed by the loss of ice cap. This retained heat then increases the temperature feeding back to induce the melting of more ice caps creating what we would call a vicious circle. As another example, we might cite the pollution of the lagoons that surround small South Pacific islands. Many South Pacific communities now consume imported packaged and canned foods, disposing of the empty cans and other waste in dumps. Rainwater runoff from the dumps pollutes the lagoons, reducing the quantity of fish and other seafood. With less seafood, people are forced to buy more and more cheap canned food, the pollution becomes worse and the lagoon has fewer fish. This positive feedback loop changes the lagoon ecosystem while also degrading the people’s diet again creating a vicious circle.
The term virtuous circle refers to the opposite phenomena, a chain of events which reinforce themselves through a positive feedback loop creating some favorable outcome. As an example of a virtuous ecological cycle we might cite the Philippine’s fishery after World War II. With the introduction of destructive fishing methods such as dynamite, cyanide, and small-mesh fishing nets a number of interlocking and mutually reinforcing vicious cycles were set in motion to significantly degrade the state of the marine ecosystem surrounding Apo island up to this point. The positive tipping point for Apo Island was the creation of a marine sanctuary, setting in motion a cascade of changes that reversed the vicious cycle with additional virtuous cycles arising in association with the marine sanctuary. The sanctuary served as a nursery, contributing directly to the recovery of fish stocks in the island’s fishing grounds. Success with the sanctuary stimulated the fishermen to set up sustainable management for the fishing grounds. A virtuous cycle of increasing fish stocks, accompanied by growing management experience, pride, and commitment to the sanctuary, was set in motion. As fishing improved around the island, fishermen were no longer compelled to travel far away for their work. Fishing right at home, where they had to live with the consequences of their fishing practices, reinforced their motivation for sustainable fishing, this compounding of positive outcomes is a virtuous cycle.