dictionary {
* file  "volmod1.dct"
* example model for "simula" STATA command  (4/11/91)
* This is a kind of a VOLTERRA model - without delays
* Ref. J. Maynard Smith, 1974. Models in ecology. Cambridge University Press.

str70 _model   "Model statements"

}

"* Prey Predator Volterra model                                  "

"* STATE equations ------------                                  "
"S X=X+dt*(Bx-Dx)                Prey population (n)             "
"S Y=Y+dt*(-Dy+By)               Predator population (n)         "

"* RATE equations ------------                                   "
"R Bx=Kbx*(Cc-X)                 Birth rate of prey   (n/year)   "
"R Dx=Kmx*X*Y                    Death rate of prey   (n/year)   "
"R Dy=Kmy*Y                      Death rate of predator (n/year) "
"R By=Kby*Y*(X-Y)                Birth rate of predator (n/year) "

"* AUXILIARY equations --------                                  "
"A Cc=500+K1*rain/temp           Environm. carrying capacity (n) "
"* e.g. related to the ecosystem grass production                "

"E temp                          Aver. annual temperature (C)    "
"E rain                          Aver. annual rainfall (mm)      "

"I X=1000                        Prey initial density            "
"I Y=100                         Predator initial density        "
"P Kbx=0.05                      Fractional birth rate of X      "
"P Kby=0.006                     Birth coefficient for Y         "
"P Kmx=0.001                     Predation coefficient           "
"P Kmy=0.05                      Fractional death rate for Y     "
"P K1=80                         Effect of the rain/temp ratio   "