24R
9 StahliJty PropertIes in the Wadden Sea
Spatial variability (at a given point in time) also depends on scale. On the scale
of a few centimetres to decimetres the spatial distribution of individuals is extremely variable, which is indicated by the wide range of abundances even in individual samples of a "multicorer" (Chap. 5.5). Depending on the species considered, site and sampling date, distribution patterns may be random but are mostly
clumped or patchy (cf. Thrush et al. 1989).
At larger spatial scales, one finally arrives at the "zonation patterns" described
in Chap. 8 by a model, i.e. typical assemblages of species that occur together in
certain areas and are mostly dominated by a few characteristic species (Dorjes
J 978; Reise J 985; Hertweck J 995). In contrast to rocky shores, where zonation
patterns often follow a linear gradient and therefore the zones are linearly arranged, zonation patterns in the intertidal of the Wadden Sea are complex because
of the complex morphology of the tidal tlats and the marked disturbance regime.
Resilience - The large-scale zonation pattern is apparently resilient because it
becomes re-established within a few years after major disturbance events. For
example, the re-establishment of the typical distribution pattern of the sand mason
(Lan ice con chi/ega) took only three or four years (Hertweck 1995; cf. Chaps. 5.4
and 8). The ELA W A T studies on the Groninger Plate show that after the ice winter
in J 996/97, during which virtually all L. conchilega in the intertidal were killed,
L. conchilega settled in the same areas where this species had lived before. Likewise, Myti/us edulis seems to prefer the same areas of a certain sandflat for settlement, as indicated by the relief casts of Hertweck (1995; Chap. 3.5) and a comparison of the distribution of M. edulis of the past 30 years (Michaelis et al. J 995).
Species composition and mean abundance are also resilient on smaller spatial
scales of up to ten metres. On this scale, elasticity, i.e. the time needed to return to
the pre-disturbance situation, depends on the size of the disturbed areas as well as
on the type, duration and date of the disturbance. This is well-known for softbottom communities (Hall et al. 1994) and has been confirmed by ELA W AT's
recolonization experiments (Chap. 6).
Persistence - In contrast to terrestrial ecosystems, where the risk of population
extinction has become a central issue in ecology and conservation biology in the
last decade, the extinction and persistence of soft-bottom species are usually not
discussed. The reason for this is the high resilience of macrobenthic species which,
due to several mechanisms (see below) guarantees persistence. In the Wadden Sea,
so far two species which were able to build large epibenthic structures are recorded
as having been extinguished by mankind: Ostrea edulis and Sahel/aria spinulosa.
The latter species built sandy reefs in the tidal channels. Ostrea was overfished,
while Sabel/aria reefs were presumably destroyed by bottom trawlers. In turn,
several species exclusively associated with oyster banks or Sahellaria reefs have
also disappeared (Reise 1982,1991,1994; Michaelis & Reise 1994).
Besides these historical extinctions, the diversity or, to be more precise, species
richness of the macrobenthos is almost constant because of the persistence of the
individual species. Changes of species richness and composition would only occur
as a result of further anthropogenic intluences or drastic alterations of the abiotic
boundary conditions. Part of the anthropogenic influences are alien invasions,
which so far have become established in the Wadden Sea without ousting any
9 StahliJty PropertIes in the Wadden Sea
Spatial variability (at a given point in time) also depends on scale. On the scale
of a few centimetres to decimetres the spatial distribution of individuals is extremely variable, which is indicated by the wide range of abundances even in individual samples of a "multicorer" (Chap. 5.5). Depending on the species considered, site and sampling date, distribution patterns may be random but are mostly
clumped or patchy (cf. Thrush et al. 1989).
At larger spatial scales, one finally arrives at the "zonation patterns" described
in Chap. 8 by a model, i.e. typical assemblages of species that occur together in
certain areas and are mostly dominated by a few characteristic species (Dorjes
J 978; Reise J 985; Hertweck J 995). In contrast to rocky shores, where zonation
patterns often follow a linear gradient and therefore the zones are linearly arranged, zonation patterns in the intertidal of the Wadden Sea are complex because
of the complex morphology of the tidal tlats and the marked disturbance regime.
Resilience - The large-scale zonation pattern is apparently resilient because it
becomes re-established within a few years after major disturbance events. For
example, the re-establishment of the typical distribution pattern of the sand mason
(Lan ice con chi/ega) took only three or four years (Hertweck 1995; cf. Chaps. 5.4
and 8). The ELA W A T studies on the Groninger Plate show that after the ice winter
in J 996/97, during which virtually all L. conchilega in the intertidal were killed,
L. conchilega settled in the same areas where this species had lived before. Likewise, Myti/us edulis seems to prefer the same areas of a certain sandflat for settlement, as indicated by the relief casts of Hertweck (1995; Chap. 3.5) and a comparison of the distribution of M. edulis of the past 30 years (Michaelis et al. J 995).
Species composition and mean abundance are also resilient on smaller spatial
scales of up to ten metres. On this scale, elasticity, i.e. the time needed to return to
the pre-disturbance situation, depends on the size of the disturbed areas as well as
on the type, duration and date of the disturbance. This is well-known for softbottom communities (Hall et al. 1994) and has been confirmed by ELA W AT's
recolonization experiments (Chap. 6).
Persistence - In contrast to terrestrial ecosystems, where the risk of population
extinction has become a central issue in ecology and conservation biology in the
last decade, the extinction and persistence of soft-bottom species are usually not
discussed. The reason for this is the high resilience of macrobenthic species which,
due to several mechanisms (see below) guarantees persistence. In the Wadden Sea,
so far two species which were able to build large epibenthic structures are recorded
as having been extinguished by mankind: Ostrea edulis and Sahel/aria spinulosa.
The latter species built sandy reefs in the tidal channels. Ostrea was overfished,
while Sabel/aria reefs were presumably destroyed by bottom trawlers. In turn,
several species exclusively associated with oyster banks or Sahellaria reefs have
also disappeared (Reise 1982,1991,1994; Michaelis & Reise 1994).
Besides these historical extinctions, the diversity or, to be more precise, species
richness of the macrobenthos is almost constant because of the persistence of the
individual species. Changes of species richness and composition would only occur
as a result of further anthropogenic intluences or drastic alterations of the abiotic
boundary conditions. Part of the anthropogenic influences are alien invasions,
which so far have become established in the Wadden Sea without ousting any