10 Assessing Stahility Propcrtic,
263
stages of systems (Chap. 4) and ecological modelling coming along with a theoretical consulting (see Chap. II).
Restrictions for the study of stability properties were set both by the dynamic
nature of the system and by the restricted time frame of the project. A stability
property such as resilience can only be identified when a return to a reference state
or a reference dynamic has occurred. This implies a sufficiently long time frame
for observations and a knowledge of the reference dynamic. The restricted duration of the project and the ice winter 1995/96 interfered with the assessment of a
return to a reference state (see Chaps. 3.5, 6 and 7). The time frame to assess the
regeneration has to be adjusted e.g. to the life cycles of the organisms studied
(Connell & Sousa 1983; Grimm et ai. 1992). Thus the potential of ELA WAT to
analyse stability properties was limited by the duration of the project.
A further difficulty was the assessment of reference states or reference dynamics. The studies in ELA W AT showed a high variability on the scale of tidal cycles,
days and years that made it impossible to define a reference situation in most cases
(see e.g. Chap. 5. I). As said in Chap. 9, the "state" of the Wadden Sea is its dynamics, obstructing the definition of reference situations. Furthermore, given the
dependence on boundary conditions such as climatic variations, the results would
have been different if the studies would have taken place in another set of years
(see Chap. 3.2). This demonstrates that long-term studies are an essential prerequisite for studies of stability properties.
The ecological checklist (see above, Chap. 9) used for the analysis of stability
properties ensured a complete characterisation of the ecological situation for respective stability properties, covering the variable of interest, level of description,
reference dynamic, type of disturbance and temporal and spatial scales. This
method (Grimm 1996; Grimm et al. 1992) helped to give a precise enumeration of
stability properties for defined ecological situations.
Evaluation of the approach
The approach taken in ELA W AT focused on processes that are essential for a
persistence of components of the Wadden Sea ecosystem. The analysis of stability
properties enabled an assessment of the qualitative relevance of components and
processes and was a suitable approach for an ecosystem research of the Wadden
Sea. As Chap. 9 shows, stability properties could be identified for many components of the Wadden Sea, but were dependent on temporal and spatial scales of
observation. It was also shown that the stability mechanisms of many biotic components in the Wadden Sea can only operate in an essentially unchanged setting of
abiotic boundary conditions. This analysis was also essential for the recommendations given in Chap. 12 for the protection of processes in the Wadden Sea.
As the Wadden Sea faces a variety of environmental threats, a variety of research approaches is required (Reise 1995). The two approaches realized in the
basic research projects SWAP and ELA W AT within the "Ecosystem Research
Wadden Sea" (see Chap. 2), focussing more on quantitative or qualitative aspects
respectively, complement each other and allow an integrated view on the Wadden
Sea. For this open ecosystem, knowledge about exchange processes is essential,
but since variations in species compositions, abundances and distribution patterns
263
stages of systems (Chap. 4) and ecological modelling coming along with a theoretical consulting (see Chap. II).
Restrictions for the study of stability properties were set both by the dynamic
nature of the system and by the restricted time frame of the project. A stability
property such as resilience can only be identified when a return to a reference state
or a reference dynamic has occurred. This implies a sufficiently long time frame
for observations and a knowledge of the reference dynamic. The restricted duration of the project and the ice winter 1995/96 interfered with the assessment of a
return to a reference state (see Chaps. 3.5, 6 and 7). The time frame to assess the
regeneration has to be adjusted e.g. to the life cycles of the organisms studied
(Connell & Sousa 1983; Grimm et ai. 1992). Thus the potential of ELA WAT to
analyse stability properties was limited by the duration of the project.
A further difficulty was the assessment of reference states or reference dynamics. The studies in ELA W AT showed a high variability on the scale of tidal cycles,
days and years that made it impossible to define a reference situation in most cases
(see e.g. Chap. 5. I). As said in Chap. 9, the "state" of the Wadden Sea is its dynamics, obstructing the definition of reference situations. Furthermore, given the
dependence on boundary conditions such as climatic variations, the results would
have been different if the studies would have taken place in another set of years
(see Chap. 3.2). This demonstrates that long-term studies are an essential prerequisite for studies of stability properties.
The ecological checklist (see above, Chap. 9) used for the analysis of stability
properties ensured a complete characterisation of the ecological situation for respective stability properties, covering the variable of interest, level of description,
reference dynamic, type of disturbance and temporal and spatial scales. This
method (Grimm 1996; Grimm et al. 1992) helped to give a precise enumeration of
stability properties for defined ecological situations.
Evaluation of the approach
The approach taken in ELA W AT focused on processes that are essential for a
persistence of components of the Wadden Sea ecosystem. The analysis of stability
properties enabled an assessment of the qualitative relevance of components and
processes and was a suitable approach for an ecosystem research of the Wadden
Sea. As Chap. 9 shows, stability properties could be identified for many components of the Wadden Sea, but were dependent on temporal and spatial scales of
observation. It was also shown that the stability mechanisms of many biotic components in the Wadden Sea can only operate in an essentially unchanged setting of
abiotic boundary conditions. This analysis was also essential for the recommendations given in Chap. 12 for the protection of processes in the Wadden Sea.
As the Wadden Sea faces a variety of environmental threats, a variety of research approaches is required (Reise 1995). The two approaches realized in the
basic research projects SWAP and ELA W AT within the "Ecosystem Research
Wadden Sea" (see Chap. 2), focussing more on quantitative or qualitative aspects
respectively, complement each other and allow an integrated view on the Wadden
Sea. For this open ecosystem, knowledge about exchange processes is essential,
but since variations in species compositions, abundances and distribution patterns