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The STELLA® program is used to
create a simulation model which describes the dynamics of the frog population
in these ponds. A graphical user interface has been developed for this model.
Our STELLA program models the metapopulation
dynamics of green treefrogs over a period of
time. The metapopulation consists of four
subpopulations, representing four different ponds at the NWRC complex.
The model is age-structured with a weekly time step. In describing the
lifetime of the green treefrogs, we consider four
life stages: Tadpole, Immature Adult Female, Mature Adult Female, and
Bred Adult Female. In other words, Tadpoles metamorphose into Immature
Adult Females, who then immature adults into Mature Adult Females (reproductive
females). After these females have bred, they are considered Bred Adult
Females and stay non-breeding for a specified period of time before their
breeding cycle starts all over again. Since green treefrog
eggs hatch within a week, we disregard the egg stage and assume that newly
hatched eggs immediately become tadpoles. Parameters obtained from literature
and impending experiment are inserted into the model (e.g. survivorship,
birth rate, breeding seasonality). We are using time steps in
weekly units for the transition from one class to another.
In building the green treefrog metapopulation model, our goal was to obtain a better
understanding of the dynamics of the green treefrog
population in an isolated urban area, specifically at the NWRC/NMFSC pond
complex. The initial model results model indicate the following:
1. The population fluctuates widely throughout the
year due to tadpole influx and the seasonality of breeding.
2. The metapopulation
dynamics can be important, especially when migration between ponds allows for
better survivorship in relation to carrying capacity.
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