2.I.6B

Mathematical Biology
Part II, 2006

Two interacting populations of prey and predators, with populations u(t),v(t)u(t), v(t) respectively, obey the evolution equations (with all parameters positive)

dudt=u(μ1α1vδu)dvdt=v(μ2+α2u)ϵ\begin{aligned} &\frac{d u}{d t}=u\left(\mu_{1}-\alpha_{1} v-\delta u\right) \\ &\frac{d v}{d t}=v\left(-\mu_{2}+\alpha_{2} u\right)-\epsilon \end{aligned}

Give an explanation in terms of population dynamics of each of the terms in these equations.

Show that if α2μ1>δμ2\alpha_{2} \mu_{1}>\delta \mu_{2} there are two non-trivial fixed points with u,v0u, v \neq 0, provided ϵ\epsilon is sufficiently small. Find the trace and determinant of the Jacobian in terms of u,vu, v and show that, when δ\delta and ϵ\epsilon are very small, the fixed point with uμ1/δu \approx \mu_{1} / \delta, vϵδ/μ1α2v \approx \epsilon \delta / \mu_{1} \alpha_{2} is always unstable.