The stochastic fish model was written in NetLogo, an agent-based modeling language which allows users
to view and analyze interactions between agents and their environment in an attempt to model complex
interactions. The fish model allows users to see the effects of different concentrations of estrogen on a
freshwater ecosystem containing algae, fish, and birds. The model attempts to capture the long term effects of
estrogen on male fish, and demonstrates how sensitive a freshwater ecosystem can be to various concentrations
of estrogen. The model was also employed in local public schools in the area to help students visualize
interactions between different organisms and organisms and their environment. The current lake I am
simulating is fictional, however, the code has been developed in such a way that one could in theory substitute
my parameters with parameters they have received from a particular lake they wish to test.
Outline
The fish model follows a simple food pyramid structure. The algae is the food source of the fish, which in
turn are the food source for the birds. If no estrogen is introduced into the environment, the ecosystem is
stable and the model simulates what is essentially the predator-prey interaction. Initially there is a relatively
high amount of fish, and relatively low amounts of birds and algae. This puts a strain on the fish population,
while simultaneously making it easy for the birds to find prey due to the combination of a large food source
and low competition for that food source. Thus this leads to a dip in the fish population and a peak in the bird
population. The dip in the fish population also leads to a peak in the algae population, as the algae can grow
without being consumed as fast due to the lack of fish. This scenario puts a strain on the bird population as
there is now too much competition for a smaller food source, while simultaneously making it easy for the fish
to find food due to the combination of a large food source and low competition for that food source. Thus the
population is back to the initial starting conditions, and the model continues to cycle through these scenarios ad
infinitum. The user can tamper with the ecosystem by adding varying concentrations of estrogen. The estrogen
leads to the feminization of male fish, with higher concentrations of estrogen corresponding to an increased
likely hood of feminization. Feminized male fish cannot reproduce, which leads to more frequent dips in the
fish population and can throw the entire ecosystem out of the equilibrium that was described above.
To run the model simply go to the interface tab, hit setup, and then run.
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