# Lab 4, due Monday, 8 March 04

General Considerations: Your program code should be placed at \$HOME/CZ3102/lab4/ (and set the permission to world readable). Programming can be done in C, MATLAB, Mathematica, or Java. However, you should avoid writing your own ODE solver. Lab 4 will be easier in the sense that it is less open-ended.

Q1:

In the class we have discussed the ecological model -- the basic variables are g (the amount of grass), r (the number of rabbits), and f (the number of wolves). The Sun provides the energy for grass to grow. The Sun light is seasonal. We assume that the growth rate for grass (if there were no crowding) is

s(t) = s0 + eps * cos(6.28*t/T) The over crowding and finite land is modeled by the logistic equation. In addition, the rabbits eat the grass at the rate of e*r. Thus the differential equation for the growth of grass is

dg/dt = (s(t) - g/G) g - e*r

The rabbits grow with a birth rate b, but limited by the availability of grass. Again this is described by a logistic equation (or similar to it). The wolves eat rabbits. This is proportional to how frequent a wolf find a rabbit, which is r*f. The equation for the number of rabbits is

dr/dt = (b - r/g)r - a*r*f

For wolves, the growth rate is assumed to be proportional to the number of rabbits, because the survival of wolves depends on rabbits. The wolves die of natural cause. The equation is

df/dt = c*r*f - d*f.

In the above system of three differential equations, g, r, f are unknown functions of t. G, e, b, a, c, d, s0, eps, T are model constants. They are all nonnegative.

Solve the above set of differential equations. For this, you need to use an ODE solver, such as the 4-th order Runge-Kutta method. I strongly discourage you to write your own solver; use the MATLAB, or Mathematica, or "Numerical Recipes in C" functions or subroutines.

Discuss the following in your report.

(0) How do you know you have used the ODE solver correctly? That is, inputs are given correctly, the results computed are correct, and you did not make programming errors.

(1) What constants did you use, what initial conditions did you use, for some typical plots of g, r, and f vs. t (not more than three)? Are the choices arbitrarily made? Or you decided by some rational reasoning. If you have modified the model, discuss that.

(2) Discuss the general features of the solution, and try to interpret what you see in the solution.

(3) Is there an equilibrium point or few equilibria points? If so, find their locations in the phase space (g,r,f). Are they stable or unstable? How does this correlate with your plots in (1) or what you said in (2).