Do Ecological Principles Encompass Other Proposed Ecological Theories Hysteresis In Nature

Numerous examples in nature show that that are combinations of environmental factors (external constraints) that may give rise to two equally viable community structures, i.e. they may provide the same degree of support (possibilities to grow) to different sets of internal constraints. In such cases, a hysteresis relationship exists between the dominant external constraining factors and the community structure (internal constraints relieving the external constraints). For instance, in freshwater shallow lakes ecosystems there are references to such type of scenarios:

(1) For concentrations between approximately 50 ^g P/l and 120-140 ^g P/l a plankton community structure dominated by zooplankton and carnivorous fish has the same probability to occur than structure dominated by planktivorous fish and phytoplank-ton (de Bernardi and Giussani, 1995). (2) For concentrations between approximately 100 and 250 ^g P/l shallow lakes can be dominated either by submerged vegetation or by phytoplankton (Scheffer, 1998).

In both cases is the system history that will determine which one of the two possible community structures will actually occur. Once the community installed, within the indicated ranges, a shift in its structure will only take place in case the community (the internal constraints) is changed by external factors (forcing functions). In the first case, this might mean that the planktivorous fish are physically removed and replaced by more carnivorous fish, and in the second case that phytoplankton is removed and submerged vegetation planted. In fact, such interventions are called bio-manipulation and the experience has shown that it only works in the indicated ranges of nutrients concentrations. It can furthermore be shown in the two referred cases that the relief—indicated as the growth measured by eco-exergy, Jorgensen et al., 2000—is the same for the two possible community structures within the indicated ranges (Figure 8.16) (Jorgensen and de Bernardi, 1997). The hysteresis occurrence can thus be explained in the light of the maximum eco-exergy principle.

Range where biomanipu-

Range where biomanipu-

Nutrient concentration

Figure 8.16 The hysteresis relation between nutrient level and eutrophication measured by the phytoplankton concentration is shown. The possible effect of bio-manipulation is shown. An effect of bio-manipulation can hardly be expected above a certain concentration of nutrients, as indicated on the diagram. The bio-manipulation can only give the expected results in the range where two different structures are possible. The range for a change from zooplankton-carnivorous fish control to planktivorous-phytoplankton control is approximately 50-120/140 ^g P/l and for a change from dominance by submerged vegetation-phytoplankton from approximately 100 ^g P/l to approximately 250 ^g P/l.

Nutrient concentration

Figure 8.16 The hysteresis relation between nutrient level and eutrophication measured by the phytoplankton concentration is shown. The possible effect of bio-manipulation is shown. An effect of bio-manipulation can hardly be expected above a certain concentration of nutrients, as indicated on the diagram. The bio-manipulation can only give the expected results in the range where two different structures are possible. The range for a change from zooplankton-carnivorous fish control to planktivorous-phytoplankton control is approximately 50-120/140 ^g P/l and for a change from dominance by submerged vegetation-phytoplankton from approximately 100 ^g P/l to approximately 250 ^g P/l.

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