216
~ Gnd Based Modelling
It also follows from Hertweck's (1995) map of the biofacies in the backbarrier
tidal tlat of Spiekeroog that, to model the spatial and temporal dynamics of the
macrozoobenthos in a spatially explicit manner, the modelled area has at least to
embrace the area of an entire sandtlat, for example the Swinnplate or Groninger
Plate (cf. Chap. 3.1).
Size of the grid cells
The basic elements of grid-based models are natural spatial units ("cells") which
allow space to be divided into a grid of these units. With forests, for example, the
space an adult canopy tree requires would be a natural unit, or with the spread of
rabies among red foxes the mean size of a territory (Grimm et al. 1996; Jeltsch et
al. 1997). Depending on the system studied and on the question, the size of a grid
cell may vary between one square centimetre and several square kilometres
(Grimm & leltsch 1996).
For the Wadden Sea, there is no clear indicator what size grid cell would be appropriate for the benthos. Using the cells as building blocks, the size of the grid
cells has to be small enough to reproduce the typical features of the patches which
can be observed. This ought to be possible with grid cells measuring 50x50m 2 , but
seems more reliable with lOx 10m 2 cells. Unfortunately, spatial autocorrelations
were not available to help define the model resolution.
Neighbourhood interactions
The most important advantage of the grid-based approach is that it allows for an
easy investigation of the regional consequences of local interactions between
neighbouring cells. However, this requires a minimum degree of spatial continuity
for the components which interact locally, achieved for example by sessile life
forms. With the macrozoobenthos, however, only the adults of L. conchilega are
sessile. Not even M. edulis is sessile in the strict sense because most individuals
are only attached to each other and not to the sediment.
In fact, our model takes into account only one single local interaction between
neighbouring cells: the dispersal or expansion of Mytilus into adjacent cells. Without this interaction our model would not need to be spatially explicit but could be
replaced by a simple non-spatial model.
There are of course local interactions between the organisms of the macrozoobenthos, but they mainly seem to be significant on smaller spatial scales. The high
mobility of most benthic organisms indicates the ecological and evolutionary significance of these small-scale interactions (Gunther 1990, 1992), for example with
respect to the ability to react to disturbances (Chap. 9).
But in contrast to many other ecological systems, it seems difficult, if not impossible, to find a simple, direct link between local interactions and large-scale
distribution patterns in the intertidal of the Wadden Sea. Evidently, factors which
do not playa major role in other systems and may therefore be neglected in models
are decisive in the Wadden Sea, i.e. abiotic factors.
~ Gnd Based Modelling
It also follows from Hertweck's (1995) map of the biofacies in the backbarrier
tidal tlat of Spiekeroog that, to model the spatial and temporal dynamics of the
macrozoobenthos in a spatially explicit manner, the modelled area has at least to
embrace the area of an entire sandtlat, for example the Swinnplate or Groninger
Plate (cf. Chap. 3.1).
Size of the grid cells
The basic elements of grid-based models are natural spatial units ("cells") which
allow space to be divided into a grid of these units. With forests, for example, the
space an adult canopy tree requires would be a natural unit, or with the spread of
rabies among red foxes the mean size of a territory (Grimm et al. 1996; Jeltsch et
al. 1997). Depending on the system studied and on the question, the size of a grid
cell may vary between one square centimetre and several square kilometres
(Grimm & leltsch 1996).
For the Wadden Sea, there is no clear indicator what size grid cell would be appropriate for the benthos. Using the cells as building blocks, the size of the grid
cells has to be small enough to reproduce the typical features of the patches which
can be observed. This ought to be possible with grid cells measuring 50x50m 2 , but
seems more reliable with lOx 10m 2 cells. Unfortunately, spatial autocorrelations
were not available to help define the model resolution.
Neighbourhood interactions
The most important advantage of the grid-based approach is that it allows for an
easy investigation of the regional consequences of local interactions between
neighbouring cells. However, this requires a minimum degree of spatial continuity
for the components which interact locally, achieved for example by sessile life
forms. With the macrozoobenthos, however, only the adults of L. conchilega are
sessile. Not even M. edulis is sessile in the strict sense because most individuals
are only attached to each other and not to the sediment.
In fact, our model takes into account only one single local interaction between
neighbouring cells: the dispersal or expansion of Mytilus into adjacent cells. Without this interaction our model would not need to be spatially explicit but could be
replaced by a simple non-spatial model.
There are of course local interactions between the organisms of the macrozoobenthos, but they mainly seem to be significant on smaller spatial scales. The high
mobility of most benthic organisms indicates the ecological and evolutionary significance of these small-scale interactions (Gunther 1990, 1992), for example with
respect to the ability to react to disturbances (Chap. 9).
But in contrast to many other ecological systems, it seems difficult, if not impossible, to find a simple, direct link between local interactions and large-scale
distribution patterns in the intertidal of the Wadden Sea. Evidently, factors which
do not playa major role in other systems and may therefore be neglected in models
are decisive in the Wadden Sea, i.e. abiotic factors.