Ecological systems are continually exposed to a wide range of perturbations, for example, changes in climate, invasion or extinction of species, disturbances such as fires or floods, or even anthropogenic alterations. Because these systems are embedded in a dynamic environment, understanding how ecosystems respond to change is of importance to both basic and applied ecology. Of particular interest when studying communities and ecosystems is the response of system-level properties or properties that arise from the aggregation of species. Examples of system properties include species richness, energy flow, total abundance of organisms, nutrient flux, and ecosystem biomass production. These properties are often considered to reflect the operation of the system as a whole since they are usually not solely the consequence of any particular species. While ecosystems can respond to perturbations by exhibiting radical shifts in these properties, sometimes ecological systems are able to maintain system-level equilibrium. This maintenance of system properties despite perturbation is often referred to as homeostasis.
Homeostasis, which literally means 'same standing', refers to the maintenance of system-level properties by means of internal adjustments. Homeostasis was originally a concept applied to the maintenance of the internal physiological conditions of organisms. Organism properties such as oxygen concentration, blood pH, and temperature are critical to the function of the organism and are closely regulated through a series of negative feedback mechanisms to maintain these properties within tightly controlled limits. For example, as internal temperature increases above a set point the hypothalamus in the body senses and responds to the rise in temperature by coordinating an increase in the ability of the body to dissipate heat quickly - mainly through dilation of blood vessels near the skin surface and the increase of evaporative cooling through sweating, panting, or other mechanisms. When body temperature declines below some set point, the hypothalamus coordinates an increase in heat production through shivering and the constriction of blood vessels leading to the skin. Because homeostasis is maintained through these types of internal adjustments, it results in a dynamic steady state for the system-level property of interest. Properties may still fluctuate through time, but the fluctuations are less than might be expected from the size of the force pushing the property away from steady state.
Despite its origin as a physiological term, it is now generally recognized that homeostasis operates in a wide variety of systems, particularly in complex systems. Complex systems are often defined as networks containing many components linked together through nonlinear interactions. While the large number of interacting components and the diversity of ways these components interact make complex systems difficult to study, it also gives them the capacity for self-organization. Self-organization refers to an increase in order (e.g., decrease in entropy, increase in information content) which is generated by the internal dynamics of the system itself, sometimes in response to pressure from the external environment. Complex systems also tend to be open systems, obtaining materials and energy from outside themselves, which allows them to maintain complex structures far from thermodynamic equilibrium. These various traits (multiple components, nonlinear interactions, self-organization, and open systems) result in complex systems being dynamic systems that respond adaptively to changes in the environment by alterations in the interactions between components or by altering the components themselves, thus affecting performance and function of the system as a whole. Finally, because these systems persist through time and adapt as the environment in which they are embedded changes, complex systems are shaped by their history. Ecosystems are widely regarded as examples of complex systems and, like other complex systems, they have been shown to exhibit the ability to maintain homeostasis.
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