250
<) Stability Properties In the Waddell Sea
tlats, lower parts at the edge of tidal channels, or in the subtidal), extremely different life histories (Ruth 1991). Essink et al. (1989) showed that M. edulis does adapt
its feeding apparatus in response to high (Wadden Sea) or low (North Sea) concentrations of suspended matter. An example observed in ELA W A T is the
spawning behaviour of Macoma balthica (Chap. 5.1) with a minor spawning in
spring and a second, much stronger spawning in summer 1995 (Fig. 5.1.6). After
the following ice winter, the larval abundance of M. balthica was extremely low.
This spawning behaviour by M. balthica had not been recorded before (Gunther et
al. 1998); M. balthica usually shows a strong recruitment after ice winters.
Phenotypic plasticity is the mechanism at the individual level which leads at the
population level to resistance or low sensitivity with respect to environmental
changes. Another example of this mechanism recorded by ELA W AT are the "red"
and "green" colour forms of Carcinus maenas (Wolf 1997, Chap. 5.6).
"Resilience by migration" and metapopulations - After the ice winter in 1995/96,
another important stability mechanism of macrobenthic species was observed
when the adults of Lanice conchilega were killed not only in the intertidal but also
in the subtidal. Therefore, the larval supply in the following summer must have
originated in remote regions. This "resilience by migration" is also characteristic
of other macrobenthic species which are sensitive to certain types of disturbances
(e.g., Mytilus edulis). This stability mechanism would break down if the last areas
where a species occurs were so close to each other that a disturbance event could
affect all of them. In this case, a species could disappear from the Wadden Sea, as
has been reported for the oyster (Ostrea edulis; Reise 1982, 1991, 1994; Michaelis
& Reise 1994).
Yet it is not clear whether the populations of macrobenthic species occurring in
different regions (different parts of the Wadden Sea, North Sea, English Channel,
French or British coast) should be interpreted as parts of a metapopulation (Hanski
& Gilpin 1997; Reich & Grimm 1996). In metapopulations, local extinctions
which may be due to disturbance events can be remedied by recolonizations from
subpopulations that still exist. In this "classical" notion of metapopulations (Levins
1969) all local populations are prone to extinction, whereas on a regional scale the
persistence of the whole metapopulation is possible if certain conditions are fulfilled.
Another notion of metapopulations is the "mainland-island" scenario, in which
some populations are so safe (because of being very large or living on protected
sites; e.g., populations of Lanicc cOf/chilega off the French coast) that they are the
ultimate source of the temporary existence of populations which are smaller or
which are more exposed to disturbances. Safe populations would constitute the
"mainland" upon which the "island" population depends. Unfortunately, little is
known about the origin of larvae in the Wadden Sea except that many of them will
originate from the North Sea. Therefore it is not yet possible to fully discuss
whether the concept of metapopulation applies to macrobenthic species on spatial
scales extending beyond the Wadden Sea.
However, at the spatial scale of the Wadden Sea there is a stability mechanism
which resembles the "mainland-island" scenario: environmental tluctuations and
disturbance events like storms or drift ice mainly affect organisms in the intertidal.
In the subtidal, living conditions for most macrobenthic species are much more
<) Stability Properties In the Waddell Sea
tlats, lower parts at the edge of tidal channels, or in the subtidal), extremely different life histories (Ruth 1991). Essink et al. (1989) showed that M. edulis does adapt
its feeding apparatus in response to high (Wadden Sea) or low (North Sea) concentrations of suspended matter. An example observed in ELA W A T is the
spawning behaviour of Macoma balthica (Chap. 5.1) with a minor spawning in
spring and a second, much stronger spawning in summer 1995 (Fig. 5.1.6). After
the following ice winter, the larval abundance of M. balthica was extremely low.
This spawning behaviour by M. balthica had not been recorded before (Gunther et
al. 1998); M. balthica usually shows a strong recruitment after ice winters.
Phenotypic plasticity is the mechanism at the individual level which leads at the
population level to resistance or low sensitivity with respect to environmental
changes. Another example of this mechanism recorded by ELA W AT are the "red"
and "green" colour forms of Carcinus maenas (Wolf 1997, Chap. 5.6).
"Resilience by migration" and metapopulations - After the ice winter in 1995/96,
another important stability mechanism of macrobenthic species was observed
when the adults of Lanice conchilega were killed not only in the intertidal but also
in the subtidal. Therefore, the larval supply in the following summer must have
originated in remote regions. This "resilience by migration" is also characteristic
of other macrobenthic species which are sensitive to certain types of disturbances
(e.g., Mytilus edulis). This stability mechanism would break down if the last areas
where a species occurs were so close to each other that a disturbance event could
affect all of them. In this case, a species could disappear from the Wadden Sea, as
has been reported for the oyster (Ostrea edulis; Reise 1982, 1991, 1994; Michaelis
& Reise 1994).
Yet it is not clear whether the populations of macrobenthic species occurring in
different regions (different parts of the Wadden Sea, North Sea, English Channel,
French or British coast) should be interpreted as parts of a metapopulation (Hanski
& Gilpin 1997; Reich & Grimm 1996). In metapopulations, local extinctions
which may be due to disturbance events can be remedied by recolonizations from
subpopulations that still exist. In this "classical" notion of metapopulations (Levins
1969) all local populations are prone to extinction, whereas on a regional scale the
persistence of the whole metapopulation is possible if certain conditions are fulfilled.
Another notion of metapopulations is the "mainland-island" scenario, in which
some populations are so safe (because of being very large or living on protected
sites; e.g., populations of Lanicc cOf/chilega off the French coast) that they are the
ultimate source of the temporary existence of populations which are smaller or
which are more exposed to disturbances. Safe populations would constitute the
"mainland" upon which the "island" population depends. Unfortunately, little is
known about the origin of larvae in the Wadden Sea except that many of them will
originate from the North Sea. Therefore it is not yet possible to fully discuss
whether the concept of metapopulation applies to macrobenthic species on spatial
scales extending beyond the Wadden Sea.
However, at the spatial scale of the Wadden Sea there is a stability mechanism
which resembles the "mainland-island" scenario: environmental tluctuations and
disturbance events like storms or drift ice mainly affect organisms in the intertidal.
In the subtidal, living conditions for most macrobenthic species are much more