The Characteristics Of Disturbance

In such mature states, if certain thresholds are exceeded, fast dynamics can easily become destructive. If there is a change of the exterior conditions, or if strong physical processes become predominant, then the inherent brittleness (Holling, 1986) enhances the risk of gradient degradation, thus the flow schemes are interrupted, and energy, information, and nutrients are lost. Hierarchies break down, the attractors are modified, and the system experiences a reset to a new starting point.

Ecologists have studied these events with emphasis on the processes of disturbances. Picket and White (1985) have used a structural approach to define these events: "any relatively discrete event in space and time that disrupts ecosystem, community, or population structure and changes resources, substrates, or the physical environment is called disturbance." Certainly, functional features are also exposed to respective changes, ecosystem processes, and interactions are also disrupted. Chronic stress or background environmental variabilities are not included within this definition, although these relations can also cause significant ecosystem changes. If a disturbance exceeds certain threshold values, then flips and bifurcations can occur, which provoke irreversible changes of the system's trajectory. Therefore, understanding ecosystems requires an understanding of their disturbance history.

A focal problem of any disturbance definition is how to indicate the "normal state" of an ecosystem (White and Jentsch 2001) because most biological communities "are always recovering from the last disturbance" (Reice, 1994). For our orientor-based viewpoint it might be appropriate to distinguish the temporal phases during which orientor dynamics are executed from phases of decreasing complexifications caused by exceeding threshold values.

Some basic terms from disturbance ecology are introduced in Figure 7.4. Disturbances exhibit certain magnitudes (sizes, forces, and intensities of the events, as variables of the source components), specificities (spectrum of disturbed elements), and severities (the impacts of the events on system properties). They can be characterized by various temporal indicators, such as their spatio-temporal scales, their duration, abruptness, recurrence interval, frequency, or return times. In the literature, exogeneous disturbances resulting from processes outside the system are distinguished from endogeneous disturbances. The latter result from internal ecosystem processes, e.g., as a product of successional development.

Disturbance can have various effects on structural biodiversity. It is clear that high magnitudes can easily reduce diversity enormously, while minor inputs might have no effects at all. Connell and Slayter (1977) have found that the highest species numbers are produced by intermediate disturbances, because such situations provide suitable living conditions for the highest number of species with relation to their tolerance versus the prevailing disturbances (Sousa, 1984). Furthermore, disturbance is a primary cause of spatial heterogeneity in ecosystems, thus it also determines the potential for biodiversity (Jentsch et al., 2002). This concept has been widely discussed within the pattern process hypotheses of patch dynamics (Remmert, 1991). Other ideas concerning the crucial role of disturbance have been formulated, e.g., by Drury and Nisbet (1973) and Sousa (1984). Natural disturbances are an inherent part of the internal dynamics of ecosystems (O'Neill et al., 1986) and can set the timing of successional cycles. Natural disturbances thus seem to be crucial for the long-term ecosystem resilience and integrity.

Taking into account these high dynamic disturbance features, correlating them with the orientor principles (which also are based on changes), focusing on long-term dynamics, and adopting Heraclitus' knowledge from 500 BC ("nothing is permanent but change!"), it becomes rather difficult to find good arguments for an introduction of the stability principle. This conception has been the dominant target of environmental management in the last decades (Svirezhev, 2000), and it was strongly interrelated with the idea of a "balance of nature" or a "natural equilibrium" (Barkmann et al., 2001).

Stability has been described by several measures and concepts, such as resistance (the system is not affected by a disturbance), resilience (the systems is able to return to a referential state), or buffer capacity, which measures the overall sensitivities of system variables related to a certain environmental input. Indicators for the stability of ecosystems are for instance the structural effects of the input (recoverability to what extent—e.g., represented by the percentage of quantified structural elements—do the state variables of a system recover after an input?), the return times of certain variables (how long does it take until the referential state is reached again?), or the variance of their time series values after a disturbance (how big are the amplitudes of the indicator variable and how does that size develop?). All of these measures have to be understood in a multivariate manner; due to indirect effects, disturbances always affect many different state variables.

Our foregoing theoretical conceptions show both, that (a) the basic feature of natural systems is a thermodynamic disequilibrium and that (b) ecosystems are following dynamic orientor trajectories for most time of their existence. Steady state thus is only a short-term interval where the developmental dynamics are artificially frozen into a small-scale average value. Therefore, more progressive indicators of ecosystem dynamics should not be reduced to small temporal resolutions that exclude the long-term development of the system. They should much more be oriented toward the long-term orientor dynamics of ecosystem variables and try to represent the respective potential to continue

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