304
S.F. Thrush and R.B. Whitlach
relegate the role of species interactions to fine-scale detail that is not relevant
to any broad-scale comparisons?
Biotic interactions will be particularly important when the mechanisms
underlying species interactions involve habitat modification within the
disturbed patch. Particularly important will be situations where colonists
modify the biogeochemistry or the physical stability of the sediments with the
patch. Modification of the biogeochemical environment will be important
when conditions in the disturbed area are "hostile" (e. g., high concentrations
of sulphides, ammonia or contaminants). Hydrodynamic processes can also
ameliorate the hostile conditions. Hydrodynamic conditions capable of
mobilising sediments can ultimately preclude the development of complex
benthic communities with high densities of large, long-lived species (Hall
1994); however, the hydrodynamic limits to species distributions in softsediment habitats have yet to be determined. It is important to recognise that
some macrofauna can play significant roles in stabilising sediments and
modifying habitats, thus ameliorating the effects of hydrodynamic forces on
the seabed. For example, Thrush et al. (1996a) demonstrated very slow and
scale-dependent rates of recolonisation of defaunated patches of sand flat in
an area previously dominated by tube-building polychaetes. The observed
scale-dependent recovery dynamics were attributed to a decrease in sediment
stability with increasing area without tube-mat. High sediment instability
resulted in high rates of emigration from the experimental plots (Thrush et al.
1996a). Cranfield et al. (1999) provide a good example of extensive biogenic
reefs existing in an area with strong tidal flows and frequent storms. These
important biotic-hydrodynamic interactions are often not considered in
assessments of the sensitivity of benthic habitats to disturbance.
14.4 Searching for Generality Part II
Despite variations in the details of the successional process that have been
revealed by field experiments (e. g., variations in the strength of an opportunistic response, variations in the interactions between colonists), we can
make some generalisations. In Table 14.1 we have categorised processes based
on whether they occur within the disturbed patch (intrinsic) or outside the
patch (extrinsic). Extrinsic factors may playa dominant role in small-scale
experiments because of their short duration and small plot sizes that limit
spatial variation in intrinsic factors. For larger scale disturbances, the
importance of intrinsic factors may well increase (Zajac et al. 1998). The split
into intrinsic and extrinsic processes influencing recovery highlights one of
the problems with the development of numerical models to describe macrobenthic successional dynamics. Although models of various intrinsic pro-
S.F. Thrush and R.B. Whitlach
relegate the role of species interactions to fine-scale detail that is not relevant
to any broad-scale comparisons?
Biotic interactions will be particularly important when the mechanisms
underlying species interactions involve habitat modification within the
disturbed patch. Particularly important will be situations where colonists
modify the biogeochemistry or the physical stability of the sediments with the
patch. Modification of the biogeochemical environment will be important
when conditions in the disturbed area are "hostile" (e. g., high concentrations
of sulphides, ammonia or contaminants). Hydrodynamic processes can also
ameliorate the hostile conditions. Hydrodynamic conditions capable of
mobilising sediments can ultimately preclude the development of complex
benthic communities with high densities of large, long-lived species (Hall
1994); however, the hydrodynamic limits to species distributions in softsediment habitats have yet to be determined. It is important to recognise that
some macrofauna can play significant roles in stabilising sediments and
modifying habitats, thus ameliorating the effects of hydrodynamic forces on
the seabed. For example, Thrush et al. (1996a) demonstrated very slow and
scale-dependent rates of recolonisation of defaunated patches of sand flat in
an area previously dominated by tube-building polychaetes. The observed
scale-dependent recovery dynamics were attributed to a decrease in sediment
stability with increasing area without tube-mat. High sediment instability
resulted in high rates of emigration from the experimental plots (Thrush et al.
1996a). Cranfield et al. (1999) provide a good example of extensive biogenic
reefs existing in an area with strong tidal flows and frequent storms. These
important biotic-hydrodynamic interactions are often not considered in
assessments of the sensitivity of benthic habitats to disturbance.
14.4 Searching for Generality Part II
Despite variations in the details of the successional process that have been
revealed by field experiments (e. g., variations in the strength of an opportunistic response, variations in the interactions between colonists), we can
make some generalisations. In Table 14.1 we have categorised processes based
on whether they occur within the disturbed patch (intrinsic) or outside the
patch (extrinsic). Extrinsic factors may playa dominant role in small-scale
experiments because of their short duration and small plot sizes that limit
spatial variation in intrinsic factors. For larger scale disturbances, the
importance of intrinsic factors may well increase (Zajac et al. 1998). The split
into intrinsic and extrinsic processes influencing recovery highlights one of
the problems with the development of numerical models to describe macrobenthic successional dynamics. Although models of various intrinsic pro-