2.I.6 B2 . \mathrm{I} . 6 \mathrm{~B} \quad

Mathematical Biology
Part II, 2007

A field contains XnX_{n} seed-producing poppies in August of year nn. On average each poppy produces γ\gamma seeds, a number that is assumed not to vary from year to year. A fraction σ\sigma of seeds survive the winter and a fraction α\alpha of those germinate in May of year n+1n+1. A fraction β\beta of those that survive the next winter germinate in year n+2n+2. Show that XnX_{n} satisfies the following difference equation:

Xn+1=ασγXn+βσ2(1α)γXn1X_{n+1}=\alpha \sigma \gamma X_{n}+\beta \sigma^{2}(1-\alpha) \gamma X_{n-1}

Write down the general solution of this equation, and show that the poppies in the field will eventually die out if

σγ[(1α)βσ+α]<1\sigma \gamma[(1-\alpha) \beta \sigma+\alpha]<1