 As we've previously touched upon, feedback loops are central to understanding the dynamics of complex systems of all kind in that they help us to get some kind of global vision as to how the different elements within the system interact. In this module, we'll be continuing on with our discussion as we talk about the different types of feedback loops, virtuous and vicious cycles, attractors and stability landscapes. Ecosystem feedback is the effect that change in one part of the ecosystem has on another and it forms the basic dynamics for regulating the overall state to 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 kind of equilibrium. 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 plants and animal populations within the limits of the carrying capacity of their environment and thus maintain some form of stability. 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 limits. More population leads to more births and more births leads to an increased population creating a compounding effect over time. 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. Positive feedback may dominate at some time 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 rapidly. This change can be 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 5 to 10 years or possibly longer because shrub seedlings grow very slowly. They grow slowly because grass roots are located in the topsoil whilst 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 state of the ecosystem as primarily a grass community. 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 grass 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 now decrease dramatically and this is the non-linear stage in the process of change. The term vicious cycle refers to a complex chain of events which reinforce themselves through a positive loop. If the outcome is a negative result, this would be termed a vicious cycle. The melting of polar ice caps is an example of a vicious cycle. As the reflective ice sea caps melt, they reflect less sunlight and heat back to the atmosphere. With more of this heat being trapped, the dark ocean which is now exposed by the loss of ice cap retains even more of the sunlight's heat. This retained heat then increases the temperature feeding back to induce the melting of more ice caps, creating what we would call a vicious cycle. As another more concrete 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, depositing of the empty cans and other waste in dumps. Rainwater runoff from the dumps pollutes the lagoons, reducing the quality 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 whilst also degrading the people's diet, again creating a vicious circle. The term virtuous cycle 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 Philippines 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 into 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 reverse 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 the fish stock 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 increased fishing 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, they had to live with the consequences of their fishing practices. This reinforced their motivation for sustainable fishing and this compounding of a positive outcome is a virtuous cycle. Negative feedback loops create what we might call an attractor, that is a relatively stable set of states that the system cycles through. In order to understand an attractor, we need to firstly talk about what is called a state space. The state space of a system is a model that tries to capture all of the different states to that system. If for example we define a range land system by the amount of grass, shrubs and livestock that are present, then the state space is the three-dimensional space of all possible combinations of the amount of these three variables. The state of the system at any time is defined by their current combined values, creating one unique point within the state space that represents the overall makeup to that system at that point in time. A basin of attraction is a region in the state space in which the system tends to remain. For systems that tend towards an equilibrium, the equilibrium state is defined as an attractor. The basin of attraction constitutes all initial conditions that will tend towards the equilibrium state with it being held within that equilibrium by negative feedback of some kind. There may be more than one such basin of attraction for any given system, for example there may be two or more combinations to the amount of grass, shrubs and livestock towards which a range land might tend depending on the starting point. The various basins that a system may occupy and the boundaries that separate them are known as the stability landscape. Both exogenous drivers such as rainfall or sunlight availability and endogenous processes such as plant succession or predator prey cycles can lead to changes in the stability landscape such as changes in the number of basins of attraction, changes in the position of the basins within the landscape, changes in the positions of the thresholds between basins or changes in the depths of basins, where depth is a measurement of how difficult it is to move the system around within the basin, with steep sides implying more and stronger negative feedback loops, where greater perturbations or management efforts are needed to change the state of the system. Restructuring the system changes its position within a basin relative to the edge and defines its precariousness or capacity to move into a new basin. In evolved systems that have been subjected to strong selection processes, the elements to the ecosystem have co-developed and often form strongly interrelated negative feedback loops. For example one axis of the stability landscape for individual human health is temperature. One could imagine three basins of attraction in this landscape, healthy, sick or dead. For good physiological reasons the optimal temperature for the body is very close to the threshold between life and death and thus very precarious. Many millions of years of homeothermal natural selection has ensured that there are strong negative feedbacks in the form of temperature regulation mechanisms making it highly unlikely and difficult for the body to move across the critical temperature threshold. In other words being precariously close to such a threshold has meant the evolution of strong resistance. Evolving towards what is called the edge of chaos corresponding in this case to the edges of the basin of attraction is often a consequence of selection for maximum efficiency. Recently developed industrial systems such as managed fisheries and virtually all agro systems for example have short co-evolutionary histories therefore we cannot rely on such selected relationships of feedback control and the likelihood of crossing thresholds is much higher as evidenced by the many examples of collapsed fisheries and other degraded agricultural and forestry regions. These attractors within the landscape can change over time given internal or external alterations for example many lakes occupy a stability landscape with essentially two basins of attraction one that is initially wide and deep characterized by clear water and a smaller one characterized by turbid water agricultural practices within the larger socio-ecological system through application of fertilizers and manure have gradually increased the phosphorus content of soils in some wetlands this cross scale effort has changed the stability landscape of the lakes in several ways as lake basins fill with sediment a third basin attraction has appeared one in which the lake is dominated by rooted vegetation the first basin clear water and sparse vegetation shrinks and nearly disappears from the stability landscape the second basin turbid water and frequent blooms of toxic algae moves from being small to being wide and deep repellors correspond qualitatively speaking to the opposite behavior of attractors given a fixed point or cyclical trajectory of a dynamical system they are called repellotype trajectories if small perturbations can make the system evolve to trajectories that are far from the original one thus these are unstable regimes characterized by positive feedback if there are several attractors in a phase space then their attraction regime are separated by unstable points sets representing repellors so that all or almost all neighboring phase trajectories are repelled from each other we can say then that the stable equilibria are attractors with negative feedback unstable equilibria are repellors governed by positive feedback where the positive feedback can give us the butterfly effect a non-linear amplification of some small event into a large change process the idea of feedback loops is central to how we currently interpret ecosystems resilience as with the model of feedback loops attractors and the stability landscape we now have some basic models for approaching this subject resilience is the capacity of a system to absorb disturbance and reorganize while undergoing change so as to still retain essentially the same structure and functionality this can be interpreted as the strength and number of the negative feedback loops creating the likelihood of the system staying in the same basin of attraction given some perturbation when we ask what is the maximum amount the system can be changed before losing its capacity to recover we are basically talking about the width of the basin of attraction wide basins mean a greater number of system states can be experienced without crossing a threshold we can talk about resistance as the ease or difficulty of changing the system this is then related to the topology of the basin deep basins of attraction indicate that greater forces or perturbations are required to change the current state of the system away from the attractor within this model we can talk about the precariousness of a system which would correlate to the current trajectory of the system and how close it currently is to the limit or threshold which if breached makes recovery difficult or impossible in this module we've been continuing on with our discussion on feedback loops a topic of central interest in systems ecology and systems theory in general we first gave a brief outline to the two different types talking about negative feedback as a stabilizing mechanism while positive feedback can have a destabilizing effect leading to both rapid compounding beneficial outcomes called virtuous cycles or compounded detrimental outcomes called vicious cycles we then went on to talk about some of the basics to non-linear dynamics introducing the idea of a state space and attractors within that space with these attractors forming some stable equilibrium to the system state we looked at the idea of a stability landscape that can have multiple stable basins of attraction within it and the idea of a repeller that forms an unstable space governed by positive feedback between the attractors finally we talked about how the models of feedback loops and stability landscapes can be used as a basic framework for analyzing the resilience of an ecosystem