Figure 3. - Theoretical changes in biomass and production of
vertebrate zooplanktivores, large herbivores, and phytoplankton in
relation to piscivore biomass (from Carpenter et aL, 1985).
The concept of cascading trophic interactions
links the principles of limnology to those of the
fishery biologists. Potentially, variation of primary
productivity is mechanistically linked to variation in
piscivore populations. If piscivore populations can
be controlled, it should therefore be possible to
control the cascade of trophic interactions that regulate
algal dynamics, through programmes of stocking and
harvesting (Carpenter et al., 1985). For these authors,
“altering food webs by altering consumer populations
may be a promising management tool”.
In natural environments, sequences of cascading
trophic interactions will be propagated from stochastic
fluctuations in piscivore year-class strength and
mortality. Fish stocks, reproduction rates and mortality
rates exhibit enormous variance, and lags in ecosystem
response occur because generation times differ among
trophic levels. Examples of lags may be found in the
development of the predator, when it acts first as a
zooplanktivore (during its juvenile stage) and then as
piscivore. As a planktivore, it drives the ecosystem
toward small zooplankton and higher phytoplankton
abundance which is the reverse of when the fish grows
and becomes a predator of zooplanktivorous fish. If
potentially a system may be managed by increasing
or decreasing the intensity of piscivore predation,
the system responses are non-equilibrium, transient
phenomena that are difficult to detect using long-term
averages. Nevertheless, in a few cases, following for
instance the introduction of a new predator in an
aquatic ecosystem, examples of trophic cascades have
been documented.