230
9 StabilIty Properties In the Wadden Sea
to constancy because neither refers to disturbances. Instead, they take ecological
systems and disturbances affecting the system as a unity. Persistence, however, is
- in contrast to constancy - a holistic and qualitative concept: holistic, because it
refers to entire systems by definition, whereas constancy refers to the dynamics of
certain variables of interest; and qualitative, because the focus is no longer on
dynamics with its quantitative details, but on the qualitative question of whether
the whole set of variables of interest used to characterize the system remains
within certain boundaries. In the following, we will also extend the notion of persistence to the abiotic components of the Wadden Sea.
Stability mechanisms
"Stability mechanisms" are processes in ecological systems which are responsible
for certain stability properties. Thus the ultimate goal of any attempt to assess
stability properties is to understand the underlying processes.
Stability measures
To make stability concepts operational, i.e. to make them applicable for practical
purposes, stability properties have to be quantified. To this end, depending on the
particular ecological situation studied, certain "stability measures" are defined
(Connell & Sousa 1983; Williamson 1987; Pimm 1991; Grimm 1996; Gigon &
Grimm 1997). However, quantifying stability properties is subject to certain restrictions. One important condition is that different ecological situations are compared by means of stability measures, because stability measures are necessarily
comparative. There is no theoretical or practical basis for assigning absolute values
of stability measures to certain ecological situations. Thus, stability measures are
used in combination with comparative approaches (Cole et al. 1991), comparative
experiments or ecological modelling. Consequently, in the following we will
mainly discuss qualitative rather than quantitative assessments of stability properties, because comparative approaches were not applied in ELA W AT (with the
exception of recolonization experiments, Chap. 6).
It should be emphasized here that talking about the "assessment" of a certain
stability property does not necessarily imply that this property indeed exists or
even is very pronounced. Instead, "assessment" primarily means that a system or a
component of a system is checked for the property.
Ecological checklist
Most statements about stability properties in the ecological literature are extremely
vague and therefore utterly useless because they refer to entire ecological systems.
But this is, except for the property of persistence, impossible as a rule because the
stability properties of ecological systems may vary enormously depending on the
point of view chosen. Although a number of species of a certain system may be
resistant to a certain disturbance (e.g., ice winter), this finding does not allow any
conclusions about the resistance of, say, biomass to the same disturbance. Similarly, assessments of the constancy (or variability) of the abundance of macroben-
9 StabilIty Properties In the Wadden Sea
to constancy because neither refers to disturbances. Instead, they take ecological
systems and disturbances affecting the system as a unity. Persistence, however, is
- in contrast to constancy - a holistic and qualitative concept: holistic, because it
refers to entire systems by definition, whereas constancy refers to the dynamics of
certain variables of interest; and qualitative, because the focus is no longer on
dynamics with its quantitative details, but on the qualitative question of whether
the whole set of variables of interest used to characterize the system remains
within certain boundaries. In the following, we will also extend the notion of persistence to the abiotic components of the Wadden Sea.
Stability mechanisms
"Stability mechanisms" are processes in ecological systems which are responsible
for certain stability properties. Thus the ultimate goal of any attempt to assess
stability properties is to understand the underlying processes.
Stability measures
To make stability concepts operational, i.e. to make them applicable for practical
purposes, stability properties have to be quantified. To this end, depending on the
particular ecological situation studied, certain "stability measures" are defined
(Connell & Sousa 1983; Williamson 1987; Pimm 1991; Grimm 1996; Gigon &
Grimm 1997). However, quantifying stability properties is subject to certain restrictions. One important condition is that different ecological situations are compared by means of stability measures, because stability measures are necessarily
comparative. There is no theoretical or practical basis for assigning absolute values
of stability measures to certain ecological situations. Thus, stability measures are
used in combination with comparative approaches (Cole et al. 1991), comparative
experiments or ecological modelling. Consequently, in the following we will
mainly discuss qualitative rather than quantitative assessments of stability properties, because comparative approaches were not applied in ELA W AT (with the
exception of recolonization experiments, Chap. 6).
It should be emphasized here that talking about the "assessment" of a certain
stability property does not necessarily imply that this property indeed exists or
even is very pronounced. Instead, "assessment" primarily means that a system or a
component of a system is checked for the property.
Ecological checklist
Most statements about stability properties in the ecological literature are extremely
vague and therefore utterly useless because they refer to entire ecological systems.
But this is, except for the property of persistence, impossible as a rule because the
stability properties of ecological systems may vary enormously depending on the
point of view chosen. Although a number of species of a certain system may be
resistant to a certain disturbance (e.g., ice winter), this finding does not allow any
conclusions about the resistance of, say, biomass to the same disturbance. Similarly, assessments of the constancy (or variability) of the abundance of macroben-