2 ELA W AT: Goals and Conceptual Framework
9
on rocky shores with a dominance of sessile species. Johnson (1970, 1972) and
van Blaricom (1982) described similar mechanisms in soft-bottom communities.
Yet, the extent to which the patch-dynamic concept is transferable to soft-sediment
communities is still under debate (Frid & Townsend 1989; Downes 1990).
A concept related but not identical (Jax et a1. 1994b) to the patch-dynamic concept is the mosaic-cycle theory (Remmert 1991). This theory states that a more or
less cyclic sequence of communities exists at a certain site (e.g. of the size of a
tree-habitat) striving for a climax. The climax exists only for a certain time and
then collapses, due to either internal processes (the death of individuals) or external disturbances. Thus a more or less regular cycle occurs on each site, from a
pioneer state through intermediate successional stages to a climax state. All these
sites or patches of an area form a mosaic of successional stages of differing age, so
that a static or cyclic balance appears at the landscape level (Wissel 1992).
Successions in marine benthic communities can be seen in the light of this mosaic-cycle theory (Reise 1991). The application of this theory to the benthic communities of the Wadden Sea is discussed in Chap. 8.
Stability concepts: an approach
"Elasticity" is one of several different stability properties (Orians 1975; Boesch &
Rosenberg 1981; Connell & Sousa 1983; Grimm et a1. 1992) and defines the return time following a disturbance, i.e. the time before a system returns or comes
close to its former state after a temporary disturbance. However, as stated in the
project proposal for ELA W AT, elasticity was used here as a synonym for resilience and defined as the property of a system to restore its structure and system
properties after a disturbance (this confusion on stability concepts is widespread in
the ecological literature and was unavoidable in the beginning of ELA W AT; see
also Grimm & Wissel 1997). This definition of resilience referring to the entire
system was refined in the course of the project to a definition taking specific ecological situations into account, i.e. precise variables, levels of description, disturbances, reference dynamics as well as spatial and temporal scales (Grimm et al.
1992).
It also became evident that the space and time frame necessary for the investigations of disturbances and the assessment of stability properties (Connell &
Sousa 1983) could not be fulfilled by the restricted time frame and defined study
area of ELA WA T. Thus the synthesis of stability properties given in Chap. 9 had
to exceed the results of ELA W AT by incorporating further knowledge about the
Wadden Sea currently available. In Chaps. 10 and II we critically discuss how
these temporal and spatial restrictions limited the potential of ELA W AT to
achieve a fundamental understanding of essential processes in the Wadden Sea and
to gain an integrated view of this ecosystem.
Emergent properties: is the whole more than sum of its parts?
Stability properties are emergent properties, i.e. they can only be observed at a
higher level of the ecological hierarchy. Resilience, for example, has no direct
significance for a single individual, but is relevant to its population. The existence
9
on rocky shores with a dominance of sessile species. Johnson (1970, 1972) and
van Blaricom (1982) described similar mechanisms in soft-bottom communities.
Yet, the extent to which the patch-dynamic concept is transferable to soft-sediment
communities is still under debate (Frid & Townsend 1989; Downes 1990).
A concept related but not identical (Jax et a1. 1994b) to the patch-dynamic concept is the mosaic-cycle theory (Remmert 1991). This theory states that a more or
less cyclic sequence of communities exists at a certain site (e.g. of the size of a
tree-habitat) striving for a climax. The climax exists only for a certain time and
then collapses, due to either internal processes (the death of individuals) or external disturbances. Thus a more or less regular cycle occurs on each site, from a
pioneer state through intermediate successional stages to a climax state. All these
sites or patches of an area form a mosaic of successional stages of differing age, so
that a static or cyclic balance appears at the landscape level (Wissel 1992).
Successions in marine benthic communities can be seen in the light of this mosaic-cycle theory (Reise 1991). The application of this theory to the benthic communities of the Wadden Sea is discussed in Chap. 8.
Stability concepts: an approach
"Elasticity" is one of several different stability properties (Orians 1975; Boesch &
Rosenberg 1981; Connell & Sousa 1983; Grimm et a1. 1992) and defines the return time following a disturbance, i.e. the time before a system returns or comes
close to its former state after a temporary disturbance. However, as stated in the
project proposal for ELA W AT, elasticity was used here as a synonym for resilience and defined as the property of a system to restore its structure and system
properties after a disturbance (this confusion on stability concepts is widespread in
the ecological literature and was unavoidable in the beginning of ELA W AT; see
also Grimm & Wissel 1997). This definition of resilience referring to the entire
system was refined in the course of the project to a definition taking specific ecological situations into account, i.e. precise variables, levels of description, disturbances, reference dynamics as well as spatial and temporal scales (Grimm et al.
1992).
It also became evident that the space and time frame necessary for the investigations of disturbances and the assessment of stability properties (Connell &
Sousa 1983) could not be fulfilled by the restricted time frame and defined study
area of ELA WA T. Thus the synthesis of stability properties given in Chap. 9 had
to exceed the results of ELA W AT by incorporating further knowledge about the
Wadden Sea currently available. In Chaps. 10 and II we critically discuss how
these temporal and spatial restrictions limited the potential of ELA W AT to
achieve a fundamental understanding of essential processes in the Wadden Sea and
to gain an integrated view of this ecosystem.
Emergent properties: is the whole more than sum of its parts?
Stability properties are emergent properties, i.e. they can only be observed at a
higher level of the ecological hierarchy. Resilience, for example, has no direct
significance for a single individual, but is relevant to its population. The existence