X
2 ELA W AT: (ioals and Conceptual Framework
Disturbances can be distinguished by their cause, frequency, intensity and time
of occurrence (Probert 1984; Grubb & Hopkins 1986; Skilleter 1995). Certain
species or areas are affected depending on the type of disturbance. Natural disturbances, for example, can occur in the course of biotic interactions and enhance
small-scale heterogeneity (Johnson 1970, 1972; Thistle 1981). Other natural disturbances can result from climatic events in the Wadden Sea, e.g. from storm and
ice winter disturbances resulting in a temporary loss of species or habitats, or increasing the recruitment of some species (Chap. 5.3; Ziegelmeier 1964; Beukema
et al. 1988). The ice winter of 1995/96, which occurred during the field studies for
ELA W A T, allowed an analysis of such an event (Chaps. 6.4 and 7).
In the opinion of Pickett & White (1985), these natural disturbances are by
definition part of a system and playa part in the theories discussed below. According to several other authors, natural abiotic variations should not be seen as
disturbances, as they are part of the environmental factors related to the development of the biological components of a system (Waide 1995).
In addition to natural disturbances, anthropogenic disturbances have increased
over the past few decades. The oldest anthropogenic disturbance in the Wadden
Sea is the construction of dikes, which dates back as far as the II th century
(Homeier 1979; Meier 1994). During the course of coastal engineering, the size of
the tidal basins and consequently the hydrography of the Wadden Sea has changed
over the centuries (Homeier & Luck 1969). The loss of mudflats described in
Chap. 3.4 is ascribed to this development. Fisheries and tourism, as well as the
input of nutrients and pollutants, have had substantial effects on the Wadden Sea.
However, these anthropogenic disturbances were not dealt with by ELA W AT, but
by the applied research projects mentioned above. Only by simulating the increase
of macroalgal mats (which were probably caused by eutrophication) in experimental organic enrichments and defaunation experiments, were the effects of these
disturbances on the sediment chemistry and regeneration of the biota studied in
ELAWAT.
During the synthesis it became clear that concentrating on individual disturbance events can distract from their overall ecological role (see below). To assess
the stability properties of individual components of the Wadden Sea, it was essential to consider the entire disturbance regime. A disturbance regime characterizes
the spatial and temporal attributes of disturbance events such as storms or ice
winters on a longer time scale (50 years or more). What matters is not only the
mean frequency and intensity of disturbances, but the precise frequency distribution and intensity, the size of the affected areas and the duration of disturbancefree periods.
The ecological role of disturbances
Disturbances play an important role as events in the ecology of many ecosystems
(Dayton 1984; Sousa 1984). They can initiate transitions between phases or cycles
of a system and can promote diversity in some cases ("creative disturbances",
Connell 1978). Small-scale disturbances (e.g. wave surf on a rocky shore, predation) can create space for recolonization, leading to a mosaic of different successional stages (Paine & Levin 1981). This "patch-dynamic" concept was developed
2 ELA W AT: (ioals and Conceptual Framework
Disturbances can be distinguished by their cause, frequency, intensity and time
of occurrence (Probert 1984; Grubb & Hopkins 1986; Skilleter 1995). Certain
species or areas are affected depending on the type of disturbance. Natural disturbances, for example, can occur in the course of biotic interactions and enhance
small-scale heterogeneity (Johnson 1970, 1972; Thistle 1981). Other natural disturbances can result from climatic events in the Wadden Sea, e.g. from storm and
ice winter disturbances resulting in a temporary loss of species or habitats, or increasing the recruitment of some species (Chap. 5.3; Ziegelmeier 1964; Beukema
et al. 1988). The ice winter of 1995/96, which occurred during the field studies for
ELA W A T, allowed an analysis of such an event (Chaps. 6.4 and 7).
In the opinion of Pickett & White (1985), these natural disturbances are by
definition part of a system and playa part in the theories discussed below. According to several other authors, natural abiotic variations should not be seen as
disturbances, as they are part of the environmental factors related to the development of the biological components of a system (Waide 1995).
In addition to natural disturbances, anthropogenic disturbances have increased
over the past few decades. The oldest anthropogenic disturbance in the Wadden
Sea is the construction of dikes, which dates back as far as the II th century
(Homeier 1979; Meier 1994). During the course of coastal engineering, the size of
the tidal basins and consequently the hydrography of the Wadden Sea has changed
over the centuries (Homeier & Luck 1969). The loss of mudflats described in
Chap. 3.4 is ascribed to this development. Fisheries and tourism, as well as the
input of nutrients and pollutants, have had substantial effects on the Wadden Sea.
However, these anthropogenic disturbances were not dealt with by ELA W AT, but
by the applied research projects mentioned above. Only by simulating the increase
of macroalgal mats (which were probably caused by eutrophication) in experimental organic enrichments and defaunation experiments, were the effects of these
disturbances on the sediment chemistry and regeneration of the biota studied in
ELAWAT.
During the synthesis it became clear that concentrating on individual disturbance events can distract from their overall ecological role (see below). To assess
the stability properties of individual components of the Wadden Sea, it was essential to consider the entire disturbance regime. A disturbance regime characterizes
the spatial and temporal attributes of disturbance events such as storms or ice
winters on a longer time scale (50 years or more). What matters is not only the
mean frequency and intensity of disturbances, but the precise frequency distribution and intensity, the size of the affected areas and the duration of disturbancefree periods.
The ecological role of disturbances
Disturbances play an important role as events in the ecology of many ecosystems
(Dayton 1984; Sousa 1984). They can initiate transitions between phases or cycles
of a system and can promote diversity in some cases ("creative disturbances",
Connell 1978). Small-scale disturbances (e.g. wave surf on a rocky shore, predation) can create space for recolonization, leading to a mosaic of different successional stages (Paine & Levin 1981). This "patch-dynamic" concept was developed