').2 The Conceptual Framework: A Glossary
231
thic species clearly depend on the area to which the assessment refers, e.g. one, ten
or ten thousand square metres.
To make the assessment of a stability property unambiguous, the following six
different aspects have to be considered and communicated. These six aspects were
compiled by Grimm et a1. (1992) in an ecological checklist: variable of interest
(e.g., abundance, number of species, biomass), level of description (e.g., individual, population, community), reference dynamics (e.g., equilibrium, cycles, certain range of variation), type of disturbance considered, and temporal and spatial
scale considered. It would, of course, be too time-consuming to specify the whole
checklist each time a statement about a stability property is made. Therefore in the
following we will only specify the checklist in those cases where the aspects are
not clear from the context.
In ELA W AT, an impressively broad spectrum of variables of interest has been
studied in the various abiotic and biotic components of the Wadden Sea on a broad
spectrum of temporal and spatial scales. However, these rather broad spectra,
which are characteristic of a joint research project, are merely a condition which is
necessary but not sufficient to develop an integrated view of the Wadden Sea,
because to this end we would have to understand the interactions between the different components of the Wadden Sea. But not even a joint research project like
ELA W AT can investigate more than a few of these interactions. Therefore, in the
following we will often have to rely on assumptions about the interactions. This
means that the integrated view of the Wadden Sea and, in particular, of its stability
properties, will be partly based on assumptions and cannot be completely founded
on empirical investigations.
Natural disturbances and disturbance regime
A "disturbance" is "any relatively discrete event in time that disrupts an ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment" (White & Pickett 1985, p. 7). This definition
combines the negative aspect of disturbances with possible ecological significance.
In addition, the character of being an event is emphasized, thus distinguishing
disturbances from continuous impacts (e.g., stress; Jax et a1. 1993). The distinction
between event and continuous impact is not absolute but depends on the time scale
considered.
The ecological significance of disturbances, i.e. the way in which they shape
ecological systems, usually becomes evident if entire "disturbance regimes" are
considered instead of individual disturbance events. "Disturbance regimes" encompass all temporal and spatial attributes of disturbances in certain systems
(White & Pickett 1985).
One decisive question in this context is whether a disturbance regime is constant, i.e. stationary in all its attributes, or whether it changes slowly, i.e. by following a trend in some or all of its attributes. If the regime is constant, it makes
sense to speak of "reference dynamics" which may then serve as a baseline for a
reference dynamics of the biotic components of the Wadden Sea. Reference dynamics are needed if the resilience of populations or communities are to be assessed with respect to certain disturbance events. However, if the disturbance regime follows a trend, any attempt to define reference dynamics is questionable.
231
thic species clearly depend on the area to which the assessment refers, e.g. one, ten
or ten thousand square metres.
To make the assessment of a stability property unambiguous, the following six
different aspects have to be considered and communicated. These six aspects were
compiled by Grimm et a1. (1992) in an ecological checklist: variable of interest
(e.g., abundance, number of species, biomass), level of description (e.g., individual, population, community), reference dynamics (e.g., equilibrium, cycles, certain range of variation), type of disturbance considered, and temporal and spatial
scale considered. It would, of course, be too time-consuming to specify the whole
checklist each time a statement about a stability property is made. Therefore in the
following we will only specify the checklist in those cases where the aspects are
not clear from the context.
In ELA W AT, an impressively broad spectrum of variables of interest has been
studied in the various abiotic and biotic components of the Wadden Sea on a broad
spectrum of temporal and spatial scales. However, these rather broad spectra,
which are characteristic of a joint research project, are merely a condition which is
necessary but not sufficient to develop an integrated view of the Wadden Sea,
because to this end we would have to understand the interactions between the different components of the Wadden Sea. But not even a joint research project like
ELA W AT can investigate more than a few of these interactions. Therefore, in the
following we will often have to rely on assumptions about the interactions. This
means that the integrated view of the Wadden Sea and, in particular, of its stability
properties, will be partly based on assumptions and cannot be completely founded
on empirical investigations.
Natural disturbances and disturbance regime
A "disturbance" is "any relatively discrete event in time that disrupts an ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment" (White & Pickett 1985, p. 7). This definition
combines the negative aspect of disturbances with possible ecological significance.
In addition, the character of being an event is emphasized, thus distinguishing
disturbances from continuous impacts (e.g., stress; Jax et a1. 1993). The distinction
between event and continuous impact is not absolute but depends on the time scale
considered.
The ecological significance of disturbances, i.e. the way in which they shape
ecological systems, usually becomes evident if entire "disturbance regimes" are
considered instead of individual disturbance events. "Disturbance regimes" encompass all temporal and spatial attributes of disturbances in certain systems
(White & Pickett 1985).
One decisive question in this context is whether a disturbance regime is constant, i.e. stationary in all its attributes, or whether it changes slowly, i.e. by following a trend in some or all of its attributes. If the regime is constant, it makes
sense to speak of "reference dynamics" which may then serve as a baseline for a
reference dynamics of the biotic components of the Wadden Sea. Reference dynamics are needed if the resilience of populations or communities are to be assessed with respect to certain disturbance events. However, if the disturbance regime follows a trend, any attempt to define reference dynamics is questionable.