Adaptability As A Key Function Of Ecosystem Dynamics

Having introduced general aspects of disturbance ecology, we can now start to integrate the complexification and the disturbance-induced dynamics of ecosystems. The respective approach is based upon the concepts of the "Resilience Alliance" (see e.g., Holling, 1986, 2004; Gundersson and Holling, 2002; Elmquist et al., 2003; Walker and Meyers 2004; Walker et al., 2004), but they have been restricted to ecosystem dynamics and combined with the sequence of growth forms after Jorgensen et al. (2000) (see also Ulanowicz, 1986a,b, 1997; Fath et al., 2004). Under these prerequisites, we can distinguish the following principle steps of ecosystem development:

- Start of the succession (pioneer stage, boundary growth after Jorgensen et al., exploitation function after Holling, 1986): In this initial state, an input of low-entropy material into the system starts the sere. The developmental potential depends on the genetic information that is available in the seed bank or by lateral inputs. Due to a minor connectivity between the elements, self-regulation is low, leakyness is high, and the sum of potential developmental opportunities (developmental uncertainty) is high. The system provides a very high adaptability and flexibility.

- Fast growth (pioneer stage, structural growth after Jorgensen et al., exploitation function after Holling): Pioneer stages can also be characterized by a high and rapid increase of biomass, correlated with an increase of the numbers and sizes of the ecosystem components. To provide the growing number of participants, the energy throughflow increases as well as exergy degradation, which is necessary for the maintenance of the components. Connectivity is low, and therefore external inputs can modify the system easily; the adaptability is high.

- Fast development (middle succession, network growth after Jorgensen et al., conservation function after Holling): After a first structure has been established, the successful actors start funneling energy and matter into their own physiology. Due to the mutual adaptation of the winning community, the connectivity of the system increases by additional structural, energetic, and material interrelations and cycling mechanisms. The single species become more and more dependent on each other, uncertainty decreases, and the role of self-regulating processes grows, reinforcing the prevailing structure. Adaptability is reduced.

- Maturity (information growth after Jorgensen et al., conservation function after Holling): In this stage, a qualitative growth in system behavior takes place, changing from exploitative patterns to more conservative patterns with high efficiencies of energy and matter processing. Species that easily adapt to external variability (r-selected species) have been replaced by the variability controlling K-strategists; the niche structure is enhanced widely, and loss is reduced. The information content of the system increases continuously. A majority of the captured exergy is used for the maintenance of the system; thus, there is only a small energetic surplus, which can be used for adaptation processes. Sensitivities versus external perturbations have become high, while the system's buffer capacities are much smaller compared with the former stages of the development. These items result in a rise of the system's vulnerability and a decrease of resilience (see Table 7.4). Adaptability has reached minimum values.

- Breakdown (release function after Holling, creative destruction after Schumpeter, 1942): Due to the "brittleness" of the mature stages (Holling, 1986), their structure may break down very rapidly due to minor changes of the exterior conditions. Accumulated resources are released, internal control and organization mechanisms are broken, and positive feedbacks provoke the decay of the mature system. Uncertainty rises enormously, hierarchies are broken, and chaotic behavior can occur (Figure 7.3). There are only extremely weak interactions between the system components, nutrients are lost and cycling webs are disconnected. Adaptability and resilience have been exceeded.

- Reorganization: During this short period the structural and functional resources can be arranged to favor in new directions, new species can occur and become successful, and—in spite of the inherited memory (e.g., seed bank of the old system and neighboring influences)—unpredictable developmental traits are possible. There are weak controls, and innovation, novelty, and change can lead to an optimized adaptation on a higher level. - Reset: A new ecosystem succession starts.

The described sequence has been illustrated in Figure 7.6 as a function of the system's internal connectedness and the stored exergy. Starting with the exploitation function, there is a slow development. The trajectory demonstrates a steady increase in mutual interactions as well as an increase in the stored exergy. As has been described above, this energetic fraction can be distinguished into a material fraction (e.g., biomass, symbolizing the growth conception of Ulanowicz, 1986a,b) and the specific exergy that refers to a complexification of the system's structure (development after Ulanowicz). In spite of multiple variability (e.g., annual cycles), the long-term development shows a steady increase up to the mature state. Here the maximum connectivity can be found, which on the one hand is a product of the system's orientation, but which also is correlated with the risk of missing adaptability, which has been nominated as over-connectedness by some authors. After the fast releasing event, the short-term conditions determine the further trajectory of the system. It might turn into a similar trajectory or find a very different pathway.

This figure looks very similar to the well-known four-box model of the Resilience Alliance, which has been depicted in Figure 7.7. The difference between these approaches lies in the definition of the y-axis. While for interdisciplinary approaches and analyses of human-environmental system the special definition of "potential" in the adaptive cycle metaphor seems to be advantageous; from our thermodynamic viewpoint, the key variable

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