30
R.E Dame et al.
the Chesapeake and Delaware Bays, the populations of Crassostrea virginica
declined rapidly and the ecosystems changed to planktonic food web dominance (Newell 1988; Rothschild et al. 1994). In the San Francisco Bay, native
species of benthic suspension feeders were replaced by several waves of
trophically similar introduced species (Cohen and Carlton 1998). In
Marennes-Oleron over-cultivation and disease caused a crash in oyster stocks
that was eventually corrected through recultivation of introduced species
(HeralI993). Finally, in the Wadden Sea, there has been a change in dominance by a triad of oysters, mussels and worms, to mussels and cockles after
oysters were over-harvested and subject to disease and the worm reefs were
destroyed by fishing gear (Reise 1982; Reise et al. 1989). In those systems
where the suspension-feeding benthos was lost or replaced by other species,
at least two out of three possible external events occurred: large-scale climatic
events, over-cultivation/over-harvesting, accompanied by disease, and nutrient loading.
Herman et al. (1999) have speculated that dense suspension-feeding
benthic populations may induce strong positive feedback linkages that may
invoke alternate stable states or phases. Utilizing Ulanowicz's (1997) terminology, the coupling of the benthic suspension-feeder component to the
phytoplankton component via the positive effect of suspension-feeder
grazing on the suspension feeders and suspension-feeder nutrient excretion
on the phytoplankton is termed a direct relationship. If there are more than
two components, the relationships between components are said to be indirect. When components are linked together by positive processes, they form
a positive feedback loop that is said to be "autocatalytic" or "self-enhancing"
(Ulanowicz 1997). In addition to exhibiting acceleration in some parameters,
the positive feedback system can induce competition between different properties of components. In this case, there are at least two distinct groups of
phytoplankton grazers present, the benthic suspension feeders and pelagic
grazers. If something happens to the system that allows the pelagic grazers to
provide greater enhancement to the system, then they either will grow to
overshadow the benthic suspension feeders or will displace them altogether.
Further, positive feedback systems also are typically fragile and exhibit
threshold effects (DeAngelis et al.1986). This shift in dominance and collapse
of a strong positive feedback mechanism may explain why natural benthic
suspension-feeding systems are so slow to return after fishing (Herman et al.
1999) has destroyed them. From the available evidence, it is clear that the
system transforming benthic suspension feeders do not destroy their ecosystems because negative controls by internal and external factors also influence their systems. It appears that poor human management and strong
climatic events are necessary for a system-level shift in the dominant component to occur.
R.E Dame et al.
the Chesapeake and Delaware Bays, the populations of Crassostrea virginica
declined rapidly and the ecosystems changed to planktonic food web dominance (Newell 1988; Rothschild et al. 1994). In the San Francisco Bay, native
species of benthic suspension feeders were replaced by several waves of
trophically similar introduced species (Cohen and Carlton 1998). In
Marennes-Oleron over-cultivation and disease caused a crash in oyster stocks
that was eventually corrected through recultivation of introduced species
(HeralI993). Finally, in the Wadden Sea, there has been a change in dominance by a triad of oysters, mussels and worms, to mussels and cockles after
oysters were over-harvested and subject to disease and the worm reefs were
destroyed by fishing gear (Reise 1982; Reise et al. 1989). In those systems
where the suspension-feeding benthos was lost or replaced by other species,
at least two out of three possible external events occurred: large-scale climatic
events, over-cultivation/over-harvesting, accompanied by disease, and nutrient loading.
Herman et al. (1999) have speculated that dense suspension-feeding
benthic populations may induce strong positive feedback linkages that may
invoke alternate stable states or phases. Utilizing Ulanowicz's (1997) terminology, the coupling of the benthic suspension-feeder component to the
phytoplankton component via the positive effect of suspension-feeder
grazing on the suspension feeders and suspension-feeder nutrient excretion
on the phytoplankton is termed a direct relationship. If there are more than
two components, the relationships between components are said to be indirect. When components are linked together by positive processes, they form
a positive feedback loop that is said to be "autocatalytic" or "self-enhancing"
(Ulanowicz 1997). In addition to exhibiting acceleration in some parameters,
the positive feedback system can induce competition between different properties of components. In this case, there are at least two distinct groups of
phytoplankton grazers present, the benthic suspension feeders and pelagic
grazers. If something happens to the system that allows the pelagic grazers to
provide greater enhancement to the system, then they either will grow to
overshadow the benthic suspension feeders or will displace them altogether.
Further, positive feedback systems also are typically fragile and exhibit
threshold effects (DeAngelis et al.1986). This shift in dominance and collapse
of a strong positive feedback mechanism may explain why natural benthic
suspension-feeding systems are so slow to return after fishing (Herman et al.
1999) has destroyed them. From the available evidence, it is clear that the
system transforming benthic suspension feeders do not destroy their ecosystems because negative controls by internal and external factors also influence their systems. It appears that poor human management and strong
climatic events are necessary for a system-level shift in the dominant component to occur.