MTH307: Problem set 5
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MTH307: Problem set 5
1. (adopted from 2000 exam) In a certain population, the intraspeciic competition limiting its growth is due to two diferent biological mechanisms: density-dependent mortality, and density-dependent birth rate. The latter has a delayed efect due to the gestation period. To take into account both efects, the following variant of the Verhulst logistic equation with an explicit time delay has been suggested:
dN(t)/dt = rN(t)(1 - aN(t) - bN(t - T)) (1)
(a) Explain the meaning of the terms and factors in this evolution equation. Which of the two terms, aN(t) and bN(t - T) describes density-dependent mortal- ity, and which describes density dependent birth rate? What is the biological meaning of the time delay T in your interpretation?
(b) Find the nonzero population density N(t) = K corresponding to an equilibrium in equation (1).
(c) Given that N(t) = K + x(t), derive an approximate linear equation involving dx(t)/dt, x(t) and x(t-T) which describes the behaviour of the population close to the equilibrium K. Show that a solution to this equation can be obtained as in the form x(t) = Re (z0 eλt,, where z0 2 C is an arbitrary constant and λ 2 C is a complex number satisfying equation f(λ) = 0 in which
f(λ) = λ + r .
(d) Explain why equilibrium K is stable if T = 0. Show that if parameter T is continously increased from 0, the instability of the equilibrium is irst observed when the following two conditions are simultaneously satisied:
cos ωT = -a/b
sin ωT = (1 + a/b)ω/r
where ω is the imaginary part of λ . Thus, or otherwise, ind the minimal value of the explicit time delay T for which the equilibrium will become unstable, as a function of parameters r , a and b. Assuming that parameters r and a are ixed, ind for what values of b there exist such a time delay T for which the instability will occur. Give an interpretation of this result in biological terms.
2. (adopted from 2001 exam) Evolution of a population of herbivores from year to year is described by a time-delayed variant of the Rickers model,
Nt+1 = RNt (1 - aNt-T ) (2)
(a) Given that the survival of the population depends on an annual plant, explain why it is more realistic to set T = 1. Explain the meaning of other parameters in this model.
(b) Find the nonzero population density Nt = K corresponding to an equilibrium in equation (2). Investigate its dependence on the parameter T.
(c) Given that Nt = K + ht , derive an approximate linear equation involving ht+1 , ht and ht-T which describes the behaviour of the population close to the equilib- rium K. Write down corresponding characteristic equation for the perturbation multiplier μ .
(d) Let T = 1. Using the stability triangle, or otherwise, show that the equilib- rium is monotonically stable for R more than but close to 1, and as R grows, it becomes oscillatory stable, and then oscillatory unstable, and ind the corre- sponding critical values of R. Find the period of the oscillations at R = 2.
2023-11-05