so the other kind of what I might call
hydron modeling approach so we want to
take a look at today is system dynamics
modeling so SDM is also often used to
understand complex systems but it's a
somewhat different approach in system
dynamics modeling we want to model the
world using what are called stocks or
numerical states of the world for
instance amount of pollution or ammount
of a whale or things along those lines
and we want to talk about changes in
those states which are called flows so
all the world is map abusing these
stocks it slow it down low go there's a
model called systems dynamic model which
is similar in some ways to other system
dynamic modelers out there called for
instance the most popular one is
probably Stella right now these tools
often work by using a graphical drawing
capability where you features for all
the stock and then link it to another
stock via a flow and then describe the
relationship between those flows and
what's interesting is that system
dynamics modeling in net logo the system
dynamics model could be used by itself
to do system dynamics modeling or they
could use it in combination with
agent-based modeling and we'll talk
about two different ways to do that the
first is the wolf she'd predation dr.
model in this case we can have a chat
based models and smiles side-by-side and
then we can kind of compare what they
might say differently about the world
now what thing is that stocks in
systematics models are often done as
real-time values and so you can have
things go to almost zero and come back
in interesting ways whereas HBase
modeling has this notion of discrete
units right and so it doesn't
necessarily have the same results as the
system dynamics models right
another way to think about it is a
hybrid model and so the Tabitha Phil you
grew mo model which is in the net logo
models library's an example of this it
contains both the ABA and the system and
every small working together for
instance the each of these model
controls and this model is a model of
forest growth in the presence of
hurricanes and it's up to different
speeches the Taven eco and the aroma
maudlin plant right and it controls the
location of the trees while the system
magnetic file controls the death and
birth rate of the trees right because if
you think about it like a lot of times
growth in death and birth rate or
something that happened in a more non
discrete fashion whereas something like
location is something that happens to
have much more discrete faction so I'm
gonna pause right now and kind of walk
you through those two models so I've now
opened the whoopsie predation docked hi
great model in that logo right and the
first thing you'll notice when you load
it is that automatically pops up this
system saying either by the way you can
get to the systematics modeler at any
time by just pushing the tools button
and then clicking on system dynamics or
you actually can hit shift command + D
to bring it up as well right but anyways
as I mentioned there are stocks and
there are flows in this particular case
so the two stocks are the Sheep stock
and the woof stock right and the flows
are basically sheep births and deaths ow
and the flow into the Lusaka sheep was
perfect and it were steps out and then
there are also variables which control
these different flows right so there's a
sheep birth rate that it interacts with
the sheep stock to create the amount of
sheep verse and so these are actually
done as equations right and so the Sheep
the birth rate is equal to whatever the
sheep stock is times the sheep birth
rate sorry to see burst is equal to the
sheep stock tanks issued per second
right on the other hand that she does is
equal to the shoe shock times the
predation rate times the wolf stock
right so the more the wood stock the
more Chancellor is too pretty to per day
and the more sheep stock there's no more
chance to intervene right so let's take
that out of the way so now let's look at
the actual
hybrid model no in this case on the
right hand side was called the a great
model as the system dynamics model and
we're gonna plot the wolf in the sheep
population there now the left hand side
is the ancient base model that you see
before
the wool sheep predation models and what
we we can write either one independently
so this is me reading the woof sheet
model need to be smiled by itself right
but you can also run it you can want in
the system may be having smiled
separately you know that system there
that's model because not an agent-based
model doesn't actually have the
graphical output of what's going on in
it right so you can see
but you can see that from the graph
itself what's going on it doesn't have
the agents moving around but what's
interesting to me is we can also set up
a comparison so we can actually set it
so the variables of the two models are
roughly equivalent right and then we can
step them through individually each of
there's different states and they start
at the same level of a fat sheep as you
can see our reading your base model is
starting to differ from the the area
model in fact if you look at the
comparison run for a while you know the
graphs looks somewhat similar maybe but
we have this like and the agent-based
model for instance the populations now
died off and so there's all the sheep
population is exploding where that can
never quite happen in the aggregate
model because you can always go down to
this infinitely small
so that's one use of
systematics model in combination with
major base model to compare and contrast
the results
so here we have the tavaa ghoom tavaa
ghoom
LUMO hybrid model and this particular
context we have both the system dynamics
model interacting with an agent-based
model and if you look at the code
right essentially the code and the
agent-based model is going to control a
lot of where the agents actually go and
everything like that right and so
there's going to place the agents at
different locations but we're going to
do it based upon the variables from the
system dynamics model right which are
the tabie new co stock in the remote
stock right and those control how many
of those things exist and then what's
gonna happen is that we're going to use
the the
assistant IMAX mountain patrol the
actual growth and death of the of the
the plants but then the ancient base
model is going to implement where those
plants actually exist right and so where
they can possibly spread to and things
like that so if you look at the system
dynamics model it has some you know a
basically a similar flow that we saw
before where the tap a new Co is born
and it dies and they grew Mose born and
dies now in this case the tab a new co
actually can grow fast in the new bromo
so at times that debt has they basically
the effect of converting young moonless
docked at Avenue go stock right and so
through these different processes were
simulating the growth and death cycle of
the tavern infinite remote and then with
the age of base model we're simulating
where they are so first so we can set up
again go right you can see that we're
using the hf base model to control the
location of the actual stock but the
system dynamics model is controlling how
much of the stock
then every time it wakes out that's when
our hurricane
and so it's kinda nice because you could
start to simulate things like spatial
patterning which you don't get normally
through a system dynamics model okay
that's it for the to have a new bogey of
grim oh and first it's the romantic spot