306
S.P. Thrush and R.B. Whitlach
mental disturbances in soft sediments have resulted in the development of an
assemblage that is distinctly different from that in the adjacent undisturbed
sediment. This implies that for small-scale experiments the local pool of colonists and their mobility control the recolonisation process. It also emphazises the importance of site history in affecting the recovery trajectory.
Unfortunately, most recovery experiments are conducted in the intertidal
and very shallow sublittoral where large and long-lived epifauna are often
rare. Organisms capable of creating biogenic reefs over soft sediments are
likely to be particularly important because they influence sediment stability
and facilitate the development of structurally complex benthic communities.
Yet these organisms often have a low potential for fast recolonisation. Removal of these organisms, for example in heavily fished areas, may radically
influence the benthic community composition achieved as a successional
endpoint (e.g., Dayton et al. 1970, 1995; Reise 1982; Riesen and Reise 1982;
Cranfield et al. 1999).
14.5 Critical Scales of Disturbance and Recovery Dynamics
Experimentally disturbed patches and many larger-scale disturbances
recover to contain benthic assemblages very similar to those in adjacent undisturbed sediments because of the mobility of macro benthic species. However, it is important to qualify this statement because larger burrowing or
biogenic reef-forming organisms that are not very mobile are usually rare,
making their influence on statistical comparisons of abundance and community composition weak. Even so, it is clear that mobility is a fundamental
process influencing macrobenthic succession. This process can potentially
provide a framework to improve our ability to predict rates of recovery and
differences in community composition over a range of spatial scales and
intensities of disturbances.
To identify when local vs. broader-scale processes drive recovery dynamics,
we use a framework developed by Horne and Schneider (1994, 1997) and
Schneider et al. (1997). Essentially the aim is to identify critical thresholds in
the relative importance of competing processes, i. e. thresholds are reached
when the ratio of two processes (with the same units of measurement) equals
one. Here, we conceptualise how the potential source of colonists available to
colonise a disturbed patch on a sand flat may vary depending on the scale or
intensity of disturbance (Fig. 14.1). Our hypothetical sand flat is located in an
embayment that is typical of many with a well-mixed water mass driven by
tidal flows and wind-waves. Disturbed patches within the sand flat will be
colonised predominantly via passive and active movement of organisms in
the adjacent sediments, the dominant process here is sediment bed-load
S.P. Thrush and R.B. Whitlach
mental disturbances in soft sediments have resulted in the development of an
assemblage that is distinctly different from that in the adjacent undisturbed
sediment. This implies that for small-scale experiments the local pool of colonists and their mobility control the recolonisation process. It also emphazises the importance of site history in affecting the recovery trajectory.
Unfortunately, most recovery experiments are conducted in the intertidal
and very shallow sublittoral where large and long-lived epifauna are often
rare. Organisms capable of creating biogenic reefs over soft sediments are
likely to be particularly important because they influence sediment stability
and facilitate the development of structurally complex benthic communities.
Yet these organisms often have a low potential for fast recolonisation. Removal of these organisms, for example in heavily fished areas, may radically
influence the benthic community composition achieved as a successional
endpoint (e.g., Dayton et al. 1970, 1995; Reise 1982; Riesen and Reise 1982;
Cranfield et al. 1999).
14.5 Critical Scales of Disturbance and Recovery Dynamics
Experimentally disturbed patches and many larger-scale disturbances
recover to contain benthic assemblages very similar to those in adjacent undisturbed sediments because of the mobility of macro benthic species. However, it is important to qualify this statement because larger burrowing or
biogenic reef-forming organisms that are not very mobile are usually rare,
making their influence on statistical comparisons of abundance and community composition weak. Even so, it is clear that mobility is a fundamental
process influencing macrobenthic succession. This process can potentially
provide a framework to improve our ability to predict rates of recovery and
differences in community composition over a range of spatial scales and
intensities of disturbances.
To identify when local vs. broader-scale processes drive recovery dynamics,
we use a framework developed by Horne and Schneider (1994, 1997) and
Schneider et al. (1997). Essentially the aim is to identify critical thresholds in
the relative importance of competing processes, i. e. thresholds are reached
when the ratio of two processes (with the same units of measurement) equals
one. Here, we conceptualise how the potential source of colonists available to
colonise a disturbed patch on a sand flat may vary depending on the scale or
intensity of disturbance (Fig. 14.1). Our hypothetical sand flat is located in an
embayment that is typical of many with a well-mixed water mass driven by
tidal flows and wind-waves. Disturbed patches within the sand flat will be
colonised predominantly via passive and active movement of organisms in
the adjacent sediments, the dominant process here is sediment bed-load