<).5 The Biotic Ingredients of the Wadden Sea
24<)
resident species (Reise 199 I). Whether this will continue to be the case In the
future cannot be predicted.
Stability mechanisms
Mobility - The following processes are responsible for the high local and temporal
variability of the abundance of the macrobenthic species: the heterogeneous distribution of resources, the heterogeneous input of larvae and juveniles, the immigration and emigration of juveniles and adults, and local mortality.
The input of larvae firstly depends on the regional supply of larvae whose size
and phenology are extremely variable for all species studied under ELA W A T
(Chap. 5.1). This is partly due to abiotic factors, e.g., ice winters. In addition,
meroplanktonic larvae are more or less coupled to the dynamics of their food resources, i.e. the phytoplankton. The actual local input of larvae which are ready to
settle depends on local conditions. Lanice conchilega (Chap. 5.3) and Mytilus
edulis need hard substrate for settlement; most other species settle in areas with
low velocities of the near-bottom flow due to higher rates of sedimentation.
Another source of local variability in abundance is the secondary dispersal of
postlarvae, i.e. young-of-the-year (Gunther 1992). Most juveniles leave the areas
of their initial settlement and search actively or passively for suitable areas
(Chap. 5.3). The adults of most macrobenthic species are also mobile. Exchange
rates of individuals between sediment and water column are extremely high, as has
been demonstrated by the staining technique (Chap. 5.3). Thus, a complete turnover of the individuals at a certain location may occur within a few days or weeks.
Local mortality, finally, depends on age or on predation, for example by epibenthic
predators (e.g., Carcinus maenas, Chap. 5.6).
The reasons for the small-scale patchiness of the distribution patterns are the
same as those which are responsible for the high temporal variability on this spatial
scale: because of their mobility the organisms are able to react quickly to smallscale differences of food resources and to the presence or activity of other organIsms.
Resilience and abiotic boundary conditions - The resilience of the large-scale
"zonation pattern" is mainly due to the resilience of the abiotic boundary conditions, i.e. of hydrography, morphology and sedimentation. Even after extreme
events like ice winters, the original morphology and current regime become
quickly re-established.
Phenotypic plasticity - Organisms are not machines which function to a rigid programme. Instead, they can adjust growth rate, the age of reproduction, the allocation of assimilated energy to growth, egg production, or the production of shells
etc. to varying environmental conditions. According to the theory of life history
evolution (Stearns 1992), this "phenotypic plasticity" may be considered a genetically fixed mechanism to cope with a broad range of environmental conditions,
e.g., to achieve the highest reproductive values possible for any environmental
condition.
An impressive demonstration of the wide range phenotypic plasticity may entail
is Mytilus edulis which shows, depending on habitat type (higher parts of the sand-
24<)
resident species (Reise 199 I). Whether this will continue to be the case In the
future cannot be predicted.
Stability mechanisms
Mobility - The following processes are responsible for the high local and temporal
variability of the abundance of the macrobenthic species: the heterogeneous distribution of resources, the heterogeneous input of larvae and juveniles, the immigration and emigration of juveniles and adults, and local mortality.
The input of larvae firstly depends on the regional supply of larvae whose size
and phenology are extremely variable for all species studied under ELA W A T
(Chap. 5.1). This is partly due to abiotic factors, e.g., ice winters. In addition,
meroplanktonic larvae are more or less coupled to the dynamics of their food resources, i.e. the phytoplankton. The actual local input of larvae which are ready to
settle depends on local conditions. Lanice conchilega (Chap. 5.3) and Mytilus
edulis need hard substrate for settlement; most other species settle in areas with
low velocities of the near-bottom flow due to higher rates of sedimentation.
Another source of local variability in abundance is the secondary dispersal of
postlarvae, i.e. young-of-the-year (Gunther 1992). Most juveniles leave the areas
of their initial settlement and search actively or passively for suitable areas
(Chap. 5.3). The adults of most macrobenthic species are also mobile. Exchange
rates of individuals between sediment and water column are extremely high, as has
been demonstrated by the staining technique (Chap. 5.3). Thus, a complete turnover of the individuals at a certain location may occur within a few days or weeks.
Local mortality, finally, depends on age or on predation, for example by epibenthic
predators (e.g., Carcinus maenas, Chap. 5.6).
The reasons for the small-scale patchiness of the distribution patterns are the
same as those which are responsible for the high temporal variability on this spatial
scale: because of their mobility the organisms are able to react quickly to smallscale differences of food resources and to the presence or activity of other organIsms.
Resilience and abiotic boundary conditions - The resilience of the large-scale
"zonation pattern" is mainly due to the resilience of the abiotic boundary conditions, i.e. of hydrography, morphology and sedimentation. Even after extreme
events like ice winters, the original morphology and current regime become
quickly re-established.
Phenotypic plasticity - Organisms are not machines which function to a rigid programme. Instead, they can adjust growth rate, the age of reproduction, the allocation of assimilated energy to growth, egg production, or the production of shells
etc. to varying environmental conditions. According to the theory of life history
evolution (Stearns 1992), this "phenotypic plasticity" may be considered a genetically fixed mechanism to cope with a broad range of environmental conditions,
e.g., to achieve the highest reproductive values possible for any environmental
condition.
An impressive demonstration of the wide range phenotypic plasticity may entail
is Mytilus edulis which shows, depending on habitat type (higher parts of the sand-