Outbreaks of the spruce budworm have been responsible for some major deforestations in Canada and the United States. The equation
x' = r(1 - x/K)x - c * x^2 / (a + x^2)
has been used to describe the dynamics of spruce budworm populations, where the variable x denotes the population or density of the insect [16]. One explanation that has been given for the occurrence of outbreaks is based on the multiple bifurcations that occur with this differential equation.
a. If a = 0.01, c = 1, and K = 1, we have a family of differential equations parameterized by r,
x' = rx(1 - x) - x^2 / (0.01 + x^2).
Solve the equation
rx(1 - x) - x^2 / (0.01 + x^2) = 0
and plot the result in the xr-plane for 0 <= r < 1.
b. To find the bifurcation diagram for the spruce budworm equation, reflect the graph obtained in part (a) about the line r = x.
c. Estimate the two bifurcation values from your graph. Explain what happens to the population as r increases. That is, when does an outbreak occur? What happens after an outbreak?