2.1 Systems analysis in environmental science
In recent decades, many valuable insights have been gained as a result of studying the environment using the methodology of systems analysis, which has been developed in order to investigate complexity in the real world (Smithson et al 2008 p. 9). Systems analysis emphasises the importance of understanding the structure of, and the relationships between and within, different parts of the environment. Indeed, the environment in its entirety may be regarded as a single system consisting of smaller, interconnected sub-systems. One particular way in which a systems analysis approach can be useful in understanding the environment is that it draws attention to the ways in which different parts of a system adjust to each other and to external factors. Systems analysis allows scientists to focus on the parts of the environment in which such adjustments occur. Indeed, as a result of the adoption of systems analysis by environmental scientists, the study of the environment has become a dynamic subject that is sharply focused on the knowledge and understanding of environmental adjustments and transformations (processes). At the same time, by focusing also on entire systems, rather than simply on their component parts, systems analysis is a holistic - rather than a reductionist - approach to the study of the environment.
In order to understand such an approach to studying the environment, it is necessary first to consider some brief definitions of systems and some key terms associated with a systems analysis approach. What is a system? A system may be defined as 'a set of interconnected parts which function together as a complex whole' (Smithson et al 2008 p. 9). In other words, a system is a group of components that work together to perform a function. A commonly-cited example of a system is a computer; yet, many types of system occur and systems may be found at all spatial scales. Thus a single river may be regarded as an environmental system with components (such as tributaries, a main channel and a delta) that work together to perform a function (the transport of water, sediments and nutrients). Materials and energy move through systems via a series of flows, cycles and transformations; in the case of a river system, water, sediments, nutrients, living organisms, dissolved gases, pollutants and energy all pass through the system. Indeed, the components of a system may be regarded as stores, in which materials and energy come to rest for a certain period of time, and in which materials and energy may be modified or transformed. The term process is used to describe any transformation - physical, chemical or biological - that occurs in the environment; and the study of environmental systems often focuses on those processes, since it is often the case that changes in environmental systems are the most interesting and relevant aspects when it comes to understanding the environment. In fact, it may be more accurate to say that environmental science often focuses on process-response systems, in which case both the relevant processes and their environmental consequences are studied together in an integrated manner.
System boundaries and scale
For any system, it is important to define clear boundaries. System boundaries define the limits within which the components interact and thereby define the scale of the system and the ways in which different systems are interrelated. Environmental systems are physical systems with physical boundaries, and environmental conditions may change markedly across those boundaries. Some environmental system boundaries are relatively easy to define, such as the interface between the ocean and the atmosphere, or the watershed surrounding a river catchment. However, other environmental system boundaries are much less straightforward to define, such as the upper limit of the atmosphere (which is defined in a relatively arbitrary manner), or the limits of vegetation communities (which may undergo very gradual transitions). As well as defining the scale of a particular system, its boundaries also determine what type of system it is. Thus systems may be closed systems in which materials do not pass across the system boundary, or they may be open systems in which the relatively free exchange of materials occurs across the boundary. Open systems therefore have inputs and outputs; and the output of one system may form the input to another system (such systems are known as cascading systems). In relation to the earth system, the global water cycle is an example of a closed system because a finite amount of water is maintained within the environment and is transported internally but does not cross the system boundary (with the exception of any water that may have been delivered to the earth as a result of comet impacts). Most environmental systems are open systems, and are interconnected, with the result that changes in one component of one system may ultimately affect all of the other systems in some way.
System scale is another important consideration in defining and understanding environmental systems. Systems exist at all spatial scales, including the microscopic scale (such as a single bacterium) and the planetary scale. For instance, all of the living material on earth comprises a single system (the biosphere), but ecological systems may also be defined at successively smaller scales (such as individual forests) - even at the level of single organisms (such as an individual tree). In some cases, the distinction between open and closed systems is purely a matter of scale; for instance, the global water cycle is typically regarded as a closed system, whilst the individual river catchments within it are obviously open systems. A further consideration is the fact that, because systems may be defined at different spatial scales, they may overlap. Systems may exist entirely within other systems (in a nested hierarchy), as in the case of an individual tree, which is part of a forest, which is part of the biosphere. For convenience, it is often helpful to define the sub-divisions of a system as follows:
- system - which refers to the entire environmental system (such as a river drainage basin)
- sub-system - which refers to major sub-divisions within the system (such as a floodplain)
- system component (or system element) - which refers to a part of the system or sub-system which has specific properties (such as the sediment load carried by a river)
For any environmental system, state one or more of its sub-systems and system components.
Feedback loops and equilibrium
Since most environmental systems are open and interconnected, the changes in any process-response system have effects on many - if not all - others. Such 'knock-on' effects are known as feedback loops. The term 'feedback' refers to the effect that occurs when the output of a system becomes an input to the same system (often for the purpose of control or self-correction (Smithson et al 2008 p. 12). Feedback loops may be positive or negative: positive feedback occurs when the effects of an original change are amplified or accelerated to produce a 'snowballing' effect; in contrast, negative feedback occurs when the effects of an initial change are 'damped out' by subsequent changes, with the result that the system reverts to its original condition. Many examples of each type of feedback loop are found in the environment. For instance, a positive feedback loop occurs when sea ice melts during the polar spring. As ocean and air temperatures increase, the sea ice begins to melt, with the result that the bright white, highly reflective surface of the ice is progressively replaced by open water, which is darker in colour and has a lower reflectivity (albedo). That lower reflectivity has the knock-on effect of increasing the amount of solar radiation that is absorbed at the surface, which in turn raises ocean and air temperatures further, leading to more rapid melting of the remaining sea ice. Hence this positive feedback loop amplifies and accelerates the original perturbation (the initial melting of sea ice).
In contrast, a negative feedback loop in the environment occurs as a result of the interaction between predators and their prey. In ideal conditions, one might expect the numbers of predators and prey animals to be approximately balanced, since the numbers of the former are usually dependent upon the numbers of the latter. If the number of prey animals temporarily increases (perhaps due to an unusually successful breeding season), then a short-term surplus of food becomes available for the predators whose numbers may subsequently increase in response. But the greater number of predators will begin to have an impact upon the size of the prey population, which may reduce substantially, leaving the predators short of food. In ideal conditions, this negative feedback loop ensures that the number of prey animals remains relatively stable around a certain optimum number, since any change (increase or decrease) in prey numbers leads to a response in predator numbers which has the knock-on effect of damping out the original perturbation (the initial rise or fall in the number of prey animals). The existence of feedback loops raises an important and interesting question in environmental science: to what extent are environmental conditions maintained in a state of equilibrium? In other words, do environmental systems tend to exist in stable states (which may be disrupted by human activity, but to which environmental processes will tend to restore those systems)? Or, alternatively, are environmental systems inherently unstable (chaotic), with no particular equilibrium state to which they tend to revert? Or, in yet another possibility, are environmental systems semi-chaotic - displaying stability under certain conditions, but becoming unstable if certain thresholds are exceeded? These are very complex and difficult questions, and they present significant challenges to environmental scientists.