308
S.P. Thrush and R.B. Whitlach
disturbed patch are themselves not fully recovered, resulting in a similar
broad-scale degradation. Over this range of spatial scales, the relative importance of these different sources of colonists influencing rates of recovery
and community composition in the formerly disturbed area sets the space
and time scales over which source-sink dynamics can function.
Critical thresholds will vary depending on extrinsic factors (Table 14.1)
and the level of biological resolution. For example, the location of critical
thresholds on the rate of recovery vs. size of the area disturbed curve will vary
with hydrodynamic conditions at the disturbed location, as will the types of
organisms used to quantify recovery. For a population, variation can occur
due to differences in life history characteristics (Levin et al.1987).At the community level, emergent patterns will depend on the size and location of
animals within the sediment, their resistivity to transport, and modes of
reproduction and larval development (Tamaki 1987; Commito et al. 1995;
Shull 1997).
This framework predicts that, for species with restricted mobility, rates of
recovery will be determined by the proximity of individuals to the disturbed
area. Recovery rates will change down the estuary associated with both
habitat change and hydrodynamic conditions. Thus, hydrodynamically
isolated habitats should not contain species with low mobility when the
frequency or spatial extent of disturbance is high. This is consistent with the
observed differences in the abundance of oligochaetes in restored and natural
salt marshes (Levin et al. 1998). At the community level, more biologically
diverse parts of the estuary will take longer to recover, as diverse communities
will be composed of species with a wider range of mobilities and reproductive
strategies. In contrast, simple low diversity communities, in estuaries at least,
are typically dominated by mobile and quick growing species.
This framework emphasises the need to address the source of immigrants
to disturbed patches and the role they play in the recovery processes (Whitlatch et al. in press). However, the critical information concerns scales of
mobility: for planktonic larvae this could be examined using hydrodynamic
models (Keough and Black 1996). It is also important that settlement patterns
are assessed because modification of flow over the benthic boundary layer
due to physical or biogenic structures can result in a water mass immediately
adjacent to the sediment that is not well characterised by modelled bulk flow
patterns. For post-larvae, the situation is likely to be even more complex. We
know little about the scales of post-settlement movement of most macrofauna, yet this is critical to understanding the important small-scale details
and making broad-scale predictions.
This framework can also be used to predict the consequences of chronic
degradation/disturbance to macrobenthic communities. As the intensity
and/or frequency of natural or anthropogenic disturbance increases, we
would expect communities to become increasingly dominated by small,
S.P. Thrush and R.B. Whitlach
disturbed patch are themselves not fully recovered, resulting in a similar
broad-scale degradation. Over this range of spatial scales, the relative importance of these different sources of colonists influencing rates of recovery
and community composition in the formerly disturbed area sets the space
and time scales over which source-sink dynamics can function.
Critical thresholds will vary depending on extrinsic factors (Table 14.1)
and the level of biological resolution. For example, the location of critical
thresholds on the rate of recovery vs. size of the area disturbed curve will vary
with hydrodynamic conditions at the disturbed location, as will the types of
organisms used to quantify recovery. For a population, variation can occur
due to differences in life history characteristics (Levin et al.1987).At the community level, emergent patterns will depend on the size and location of
animals within the sediment, their resistivity to transport, and modes of
reproduction and larval development (Tamaki 1987; Commito et al. 1995;
Shull 1997).
This framework predicts that, for species with restricted mobility, rates of
recovery will be determined by the proximity of individuals to the disturbed
area. Recovery rates will change down the estuary associated with both
habitat change and hydrodynamic conditions. Thus, hydrodynamically
isolated habitats should not contain species with low mobility when the
frequency or spatial extent of disturbance is high. This is consistent with the
observed differences in the abundance of oligochaetes in restored and natural
salt marshes (Levin et al. 1998). At the community level, more biologically
diverse parts of the estuary will take longer to recover, as diverse communities
will be composed of species with a wider range of mobilities and reproductive
strategies. In contrast, simple low diversity communities, in estuaries at least,
are typically dominated by mobile and quick growing species.
This framework emphasises the need to address the source of immigrants
to disturbed patches and the role they play in the recovery processes (Whitlatch et al. in press). However, the critical information concerns scales of
mobility: for planktonic larvae this could be examined using hydrodynamic
models (Keough and Black 1996). It is also important that settlement patterns
are assessed because modification of flow over the benthic boundary layer
due to physical or biogenic structures can result in a water mass immediately
adjacent to the sediment that is not well characterised by modelled bulk flow
patterns. For post-larvae, the situation is likely to be even more complex. We
know little about the scales of post-settlement movement of most macrofauna, yet this is critical to understanding the important small-scale details
and making broad-scale predictions.
This framework can also be used to predict the consequences of chronic
degradation/disturbance to macrobenthic communities. As the intensity
and/or frequency of natural or anthropogenic disturbance increases, we
would expect communities to become increasingly dominated by small,