8.1 Introduction
209
8.1.1
Theoretical Background
In descriptions of macrozoobenthic communities, reference is often made to "distribution patterns" and "patches". The former term refers to the spatial distribution
of species and communities, and does not necessarily imply what is usually considered as a "pattern", i.e. any structure in space or time that is beyond random
variation. A "patch" may be defined as "an area which, on a specified scale of
observation, has some level of consistency in the density of individuals" (Hall et
a!. 1994, p. 333). According to this definition, the spatial (and temporal) scale of
observation has to be specified in any reference to patches or "patchy" distributions.
The notion behind the concept of "patch-dynamics" (Picket & White 1985;
Paine & Levin 1981) is that natural disturbance events create patches in the landscape. Within these patches, the ecological "cards" are reshuffled. For instance, the
disturbance may cause dominant species (e.g. mussels on rocky shores, certain tree
species in forests) to disappear and thereby create space for earlier successional
stages. Depending on the type, frequency, extent and intensity of natural disturbances, different kinds of landscape and ecological systems may emerge, e.g. a
patch-work of different successional stages.
The significance of the concept of patch-dynamics is that it explains the natural
structural diversity (or the lack thereof) of ecosystems. This structural diversity
may be a precondition for species richness or stability properties. Heterogeneous
systems are not uniformly affected by catastrophic disturbance events which otherwise would have the potential to affect the entire system (Kolasa & Picket 1991).
The concept of patch-dynamics has frequently been discussed as an approach to
study the Wadden Sea (Jax et a!. 1993) or, in general, benthic species in softbottom communities (Hall et a!. 1994).
Remmert (1991) formulated a concept related to patch-dynamics: the theory of
mosaic-cycles (lax 1994). He assumes that in many systems local succession is
cyclic. After a disturbance event which clears the local community, a pioneer
community is first established, followed by further successional stages up to the
(local) climax community (e.g. dominant canopy trees; Wissel 1992). If the climax
community breaks down due to senescence or disturbance events, the pioneer
community follows, etc. Locally, the successional cycles may be out of phase, but
regionally an equilibrium distribution of successional stages may emerge.
Based on our experience with the grid-based model presented in this chapter,
we will at the end of this chapter discuss whether the concept of patch-dynamics
and the theory of mosaic-cycles are suitable for application to the macrozoobenthos of the Wadden Sea.
8.1.2
Empirical Background
Besides the general knowledge about macrozoobenthos in the Wadden Sea, two
findings obtained by ELA W AT were used as the biological background to the
grid-based models developed in this chapter. Firstly, studies of Lanice conchilega
(Chap. 5.3) show that after an ice winter in which almost all adult Lanice were
209
8.1.1
Theoretical Background
In descriptions of macrozoobenthic communities, reference is often made to "distribution patterns" and "patches". The former term refers to the spatial distribution
of species and communities, and does not necessarily imply what is usually considered as a "pattern", i.e. any structure in space or time that is beyond random
variation. A "patch" may be defined as "an area which, on a specified scale of
observation, has some level of consistency in the density of individuals" (Hall et
a!. 1994, p. 333). According to this definition, the spatial (and temporal) scale of
observation has to be specified in any reference to patches or "patchy" distributions.
The notion behind the concept of "patch-dynamics" (Picket & White 1985;
Paine & Levin 1981) is that natural disturbance events create patches in the landscape. Within these patches, the ecological "cards" are reshuffled. For instance, the
disturbance may cause dominant species (e.g. mussels on rocky shores, certain tree
species in forests) to disappear and thereby create space for earlier successional
stages. Depending on the type, frequency, extent and intensity of natural disturbances, different kinds of landscape and ecological systems may emerge, e.g. a
patch-work of different successional stages.
The significance of the concept of patch-dynamics is that it explains the natural
structural diversity (or the lack thereof) of ecosystems. This structural diversity
may be a precondition for species richness or stability properties. Heterogeneous
systems are not uniformly affected by catastrophic disturbance events which otherwise would have the potential to affect the entire system (Kolasa & Picket 1991).
The concept of patch-dynamics has frequently been discussed as an approach to
study the Wadden Sea (Jax et a!. 1993) or, in general, benthic species in softbottom communities (Hall et a!. 1994).
Remmert (1991) formulated a concept related to patch-dynamics: the theory of
mosaic-cycles (lax 1994). He assumes that in many systems local succession is
cyclic. After a disturbance event which clears the local community, a pioneer
community is first established, followed by further successional stages up to the
(local) climax community (e.g. dominant canopy trees; Wissel 1992). If the climax
community breaks down due to senescence or disturbance events, the pioneer
community follows, etc. Locally, the successional cycles may be out of phase, but
regionally an equilibrium distribution of successional stages may emerge.
Based on our experience with the grid-based model presented in this chapter,
we will at the end of this chapter discuss whether the concept of patch-dynamics
and the theory of mosaic-cycles are suitable for application to the macrozoobenthos of the Wadden Sea.
8.1.2
Empirical Background
Besides the general knowledge about macrozoobenthos in the Wadden Sea, two
findings obtained by ELA W AT were used as the biological background to the
grid-based models developed in this chapter. Firstly, studies of Lanice conchilega
(Chap. 5.3) show that after an ice winter in which almost all adult Lanice were