 One of the central questions of interest within systems theory and science in general is the question of how some given system will develop over a period of time and what is called dynamics. What recurring patterns do we see in the system's development and how can we model these patterns? This is of course the same for the study of ecosystems which can be highly dynamic, exhibiting complex patterns as they change over time. This area of study within systems theory is called system dynamics which interprets the dynamics of complex systems primarily in terms of feedback loops that regulate its long-term behavior in a distributed fashion and feedback loops have since been identified as the central mechanism through which ecosystems regulate themselves. Indeed the processes through which organisms regulate themselves in their environment is identified as one of the defining characteristics of life. It is understood today that all organisms survive by transforming energy and by regulating their internal environment in order to maintain a stable condition conducive to that functionality. This regulation process is central to understanding the dynamics of the organism or ecosystem over time. Linear and nonlinear Linear processes of development are bound within some set of parameters that enable the system to function and to develop in a semi-stable pattern. Linear processes of development are driven by negative feedback loops where two things balance each other out where for every source of energy or resource within the system there is a sink. For every anabolic process there is a catabolic process within that overall system. These negative feedback loops form closed cycles giving us an oscillatory pattern of development. Nonlinear processes of development in contrast are driven by positive feedback where two variables change in the same direction, more begets more. These positive feedback loops drive the system in one direction over time. As positive feedback loops have a compounding effect upon themselves the net result is often a radical process of exponential growth or decay which can drive the system outside of its normal operating parameters. Homeostasis, all living organisms whether unicellular or multicellular or even whole ecosystems exhibit homeostasis to maintain dynamic equilibrium and effectively carry out certain functions a system must detect and respond to perturbations. After the detection of a perturbation a biological system normally responds through negative feedback. This means stabilizing conditions by either reducing or increasing the activity of an organ or system. Examples of homeostasis within the human body include the regulation of temperature, the balance between acidity and alkalinity or release of glucagon when sugar levels are too low. On the micro level of a single organism this process is often done through a centralized regulatory system such as the hypothalamus in the human body. Homeostasis requires a sensor to detect changes in the condition to be regulated an effector mechanism that can vary the condition and a negative feedback connection between the two. Although macro level systems like ecosystems typically do not have centralized control systems they do have distributed mechanisms for regulatory. Ecosystems also manage to keep their physical conditions within some set of limits. For example, biotic and abiotic processes regulate the quantity of water in the soil. Plants best function when there is neither too much nor too little water. Too much water can push out air needed by microorganisms and plant roots. Too little water restricts plant growth. If there is too much water in the soil after heavy rain plants consume large quantities and excess water percolates downward through the soil. If there is too little water during periods of lesser rainfall plants reduce their water consumption and clay and soil organic matter store water for use by plants and soil microorganisms. This is an example of homeostasis on the level of a local ecosystem but the Gaia hypothesis posits the concept of ecosystem homeostasis for the whole macro level biosphere. For example, the global carbon cycle maintains atmospheric oxygen and carbon dioxide at concentrations required by the plants and animals in Earth's biosphere. This is accomplished by a variety of processes including photosynthesis, respiration and the carbonate buffer system in the oceans. This hypothesis sees global ecosystem homeostasis as a consequence of homeostasis in the large number of distributed local but mutually interacting ecosystems across the planet. Ecosystems like the biosphere are then complex systems composed of many distributed feedback loops that regulate their dynamics. As scientists discover more about Earth vast numbers of positive and negative feedback loops are being discovered on an ongoing basis that together maintain a metastable condition sometimes with a very broad range of environmental conditions but this stable state to the ecosystem enables a gradual and prolonged incremental pattern of development. Succession. This stable state of development can be understood as ecological succession where succession is a phenomenon or process by which an ecological community undergoes more or less orderly and predictable changes following an initial disturbance. Succession was one of the first theories advanced in ecology and the study of succession remains at the core of ecological science today. During succession, biotic and abiotic conditions change. Pioneer and intermediate communities alter conditions so much that they promote the growth of new communities that eventually replace them. Succession is the natural evolution or progression of an ecosystem and there are just two types primary or secondary. Primary succession is a natural process where life colonizes new land. Secondary succession is the succession that occurs after the initial process has been disrupted and some plants and animals still exist. As such, it is usually faster than primary succession. Succession shows how an ecosystem stabilizes after a disruption as plants and animals return. During succession, two of the most notable changes are an increase in species diversity and ecosystem stability. Example, one of the clearest documented cases of this process of succession was the study of the island of Krakatoa after its major eruption in 1883. In the years after the eruption, Krakatoa went through a sequence of ecological changes in which successive groups of new plant or animal species followed one another, leading to increasing biodiversity and eventually culminating in a re-established climax community. In the case of Krakatoa, the island reached its climax community with 800 different recorded species in 1983, 100 years after the eruption that cleared all life off the island. Evidence confirms that this number has been homeostatic for some time, with the introduction of new species rapidly leading to the elimination of old ones. The evidence of Krakatoa and other disturbed island ecosystems have mimicked general principles of ecological succession, albeit in a virtually closed system, comprised almost exclusively of endemic species. Climax In 1916, Frederick Clements published a descriptive theory of succession and presented it as a general ecological theory. His theory of succession had a strong influence on ecological thought. Clements' concept is usually termed classical ecological theory. According to classical ecological theory, succession stops when the process has arrived at an equilibrium or steady state with the physical and biotic environment. This end point of succession is called climax. It is self-perpetuating through negative feedback and in equilibrium with the physical habitat. There is mostly no net annual accumulation of organic matter in a climax community. The annual production and use of energy is balanced in such a community. Implicit in this model of classical ecology is that ecological development is seen to be a somewhat linear progression towards some new kind of single climax state with a stable equilibrium. Non-equilibrium Throughout history, ecological succession was seen as having a stable end stage called the climax, sometimes referred to as the potential biodiversity of a site shaped primarily by the local climate. However, this idea has become less prevalent within contemporary ecology being somewhat replaced in favor of non-equilibrium models to how ecosystems function and change. This may be owing to a recognition that most natural ecosystems experience disturbance at a rate that makes a climax community unattainable. Coupled with the recognition that most ecosystems are not isolated islands tightly bound but in fact follow environmental gradients making them open systems with poorly defined boundaries across which flow energy and resources creating out-of-equilibrium dynamics. Added to this, we might cite an increased interest in ecological collapse and rapid discontinuities created by human intervention which are forms of nonlinear change. Positive feedback Positive feedback loops are the drivers of nonlinear change. Positive feedback loops can be understood as broken negative feedback where the counterbalancing force becomes externalized and the system stays developing off in one direction. An example of this might be an invasive species. An invasive species is an organism that is not native to a specific location. It is introduced from some foreign ecology and thus may have no consumer within that environment allowing it to spread out of control. The classical example of this being when rabbits were first introduced to Australia where they ate native plants and overpopulated because there were no natural predators. This disrupts homeostasis because invasive species unbalance the food web. As in this example, many beings have been key drivers in introducing invasive species to different ecologies around the world. But invasive species are just one form of disturbance that can be understood in terms of a broken negative feedback loop. Disturbance In ecology, a disturbance is a temporary change in environmental conditions that causes a pronounced change in an ecosystem. Disturbances often act quickly and with great effect, sometimes resulting in the removal of large amounts of biomass. Major ecological disturbances may include fires, flooding, wind storms, insect outbreaks, earthquakes, tsunami, or the effects of human impact on the environment, such as clear-felling forests. As another example, acid rain is a human-caused environmental effect resulting from chemical pollution that produces the pH level of precipitation. Acid rain is a disturbance by affecting the reproduction of plants and animals. All of these are examples of such phenomena that the system cannot counterbalance. They take it outside of its normal operating parameters and, thus, we call them disturbances. The balance in the system is maintained by feedback loops where, for any source, there is a sink. Those balancing feedback loops are often the product of co-evolution over prolonged periods of time. Elements within the system co-adapt and self-organize to complete some process from source to sink. When we break those source-sink loops, such as the rapid combustion of fossil fuels or fertilizer nitrification, then we get an over-accumulation and this puts stress on the system. When an ecosystem is subject to some sort of stress or perturbation, it responds by moving away from its initial state, moving towards the limits of its homeostatic parameters. The tendency of the system to remain close to its equilibrium state, despite the disturbance, is termed its resistance. On the other hand, the speed with which it returns to its initial state after disturbance is called its resilience. We will be discussing ecosystems' resilience in a future model, so we will move on for now. Punctuated Equilibrium A combination of positive and negative feedback during a system's development gives us a model to its dynamics called punctuated equilibrium. Punctuated equilibrium is a model first derived from evolutionary biology, but complexity theory has abstracted it, making it applicable to all nonlinear systems such as whole ecologies. This model deals with the dynamics of a complex system, suggesting that most nonlinear systems exist in an extended period of stasis, which are later punctuated by sudden shifts of radical change. Ecosystems may be characterized by long periods of stability where negative feedback loops work to maintain an equilibrium, holding them within a well-structured homeostatic state. This is then punctuated by large, though less frequent, shifts driven by a positive feedback loop that drives the system far from equilibrium and out of its current homeostatic regime. Summary In this module we have been talking about ecosystem dynamics and how ecosystems change over time. We identified two qualitatively different patterns of development, linear and nonlinear. We talked about negative feedback loops as the basic mechanism through which biological creatures and whole ecosystems regulate themselves in order to achieve some condition conducive to their stable functioning. We talked about the classical ecological theory of succession as a linear process of development seen to lead a climax community at a stable equilibrium state. We then discussed positive feedback loops as broken negative feedback that leads to a nonlinear process of development where the system can be said to be out of control resulting in disturbance and possible collapse. By then combining these two patterns of development we looked at a model called punctuated equilibrium where the system follows both long-term, incremental linear development and rapid nonlinear punctuations.