Primitive Equation Modelling ofPlankton Ecosystems
399
that is achieved will it be possible to predict changes due to artificial disturbance
by man, whether through pollution, fisheries, or climate change.
The particular sequence of plankton abundance encountered in any year reflects
the result of competition between many species for the limited resources available.
The pre-requisite for simulating observed changes is to demonstrate that our modelling technique can realistically simulate the demographic consequences of competition. In particular we need to show how competitive advantage translates into
competitive exclusion and extinction, or co-existence with seasonally changing
dominance, depending on the ambient environment. We start this investigation at a
generic level, with numeric al experiments based on simple scenarios. These experiments use variants of the WB model for which we have acquired a deep understanding ofhow the virtual ecosystem works. The numerical experiments described
below were performed by members of my research group.
In the WB model, all the phytoplankton have identical equations and parameter
values, so none has a competitive advantage. The same applies to the zooplankton.
The numerical experiments extended the WB model by introducing competing
populations ofphytoplankton ofthe same function group (i.e. the same equations),
while the zooplankton were kept as one population which grazed on all phytoplankton without discrimination. The competing populations of phytoplankton
were defined by differences in the values of one or more parameters in their biological equations. In the first experiment, intra-population variation was defined by
only one parameter, which was inherited through lineages. The second experiment
was also based on the same parameter, which was allowed to vary by mutation. In
the third experiment, there was no mutation, but intra-population variation was
defined by a large number ofbiological parameters.
18.9.1 Competitive advantage
Competitive advantage is measured by the relative demographic response of
populations with different parameter values when they are exposed to the same
environment, which changed diumally and seasonally according to the ambient climate. Because phytoplankton reproduce many times during the growing season,
there is scope for large demographic differences to established within one year.
However, in our simulations it is rare for uncompetitive populations to be driven to
extinction within one year. So competitive advantage is best assessed on an annual
basis, if the environment performs a stationary annual cycle (as in our experiments
at the Azores); or over a longer period, ifthe ambient climate undergoes a multiyear periodicity (as in the second experiment). The point being made is that the
ranking of competitive advantage is an emergent property of competition in a virtual plankton ecosystem. It depends on a combination of the differences in biological properties of the competing plankton, and on the environment in which they are
competing. We distinguish between (1) synoptic competitive advantage, the potential for demographic gain at the expense of competitors, and (2) annual competitive
advantage, the demographic benefit ofyear-Iong competition.
399
that is achieved will it be possible to predict changes due to artificial disturbance
by man, whether through pollution, fisheries, or climate change.
The particular sequence of plankton abundance encountered in any year reflects
the result of competition between many species for the limited resources available.
The pre-requisite for simulating observed changes is to demonstrate that our modelling technique can realistically simulate the demographic consequences of competition. In particular we need to show how competitive advantage translates into
competitive exclusion and extinction, or co-existence with seasonally changing
dominance, depending on the ambient environment. We start this investigation at a
generic level, with numeric al experiments based on simple scenarios. These experiments use variants of the WB model for which we have acquired a deep understanding ofhow the virtual ecosystem works. The numerical experiments described
below were performed by members of my research group.
In the WB model, all the phytoplankton have identical equations and parameter
values, so none has a competitive advantage. The same applies to the zooplankton.
The numerical experiments extended the WB model by introducing competing
populations ofphytoplankton ofthe same function group (i.e. the same equations),
while the zooplankton were kept as one population which grazed on all phytoplankton without discrimination. The competing populations of phytoplankton
were defined by differences in the values of one or more parameters in their biological equations. In the first experiment, intra-population variation was defined by
only one parameter, which was inherited through lineages. The second experiment
was also based on the same parameter, which was allowed to vary by mutation. In
the third experiment, there was no mutation, but intra-population variation was
defined by a large number ofbiological parameters.
18.9.1 Competitive advantage
Competitive advantage is measured by the relative demographic response of
populations with different parameter values when they are exposed to the same
environment, which changed diumally and seasonally according to the ambient climate. Because phytoplankton reproduce many times during the growing season,
there is scope for large demographic differences to established within one year.
However, in our simulations it is rare for uncompetitive populations to be driven to
extinction within one year. So competitive advantage is best assessed on an annual
basis, if the environment performs a stationary annual cycle (as in our experiments
at the Azores); or over a longer period, ifthe ambient climate undergoes a multiyear periodicity (as in the second experiment). The point being made is that the
ranking of competitive advantage is an emergent property of competition in a virtual plankton ecosystem. It depends on a combination of the differences in biological properties of the competing plankton, and on the environment in which they are
competing. We distinguish between (1) synoptic competitive advantage, the potential for demographic gain at the expense of competitors, and (2) annual competitive
advantage, the demographic benefit ofyear-Iong competition.