= Solution
Since $\gamma=1-\delta/r_2$, increasing hunting effort moves leftward on these diagrams. The first stability threshold corresponds to
$$
\boxed{\delta_\mathrm{local}=r_2(1-\alpha_{21})}.
$$
Above this removal rate, a sufficiently small grey population can be eliminated while reds recover. Until the second threshold, however, the system is <bistable>: an initially large grey population can still win, so the history and initial abundances matter. The second threshold is
$$
\boxed{\delta_\mathrm{global}=r_2\left(1-\frac1{\alpha_{12}}\right)}.
$$
Above it, any positive red population recovers and the grey population tends to zero, regardless of its initial abundance in this deterministic model. This threshold is smaller than $r_2$, so competition from surviving reds can make eradication possible with less removal than would be necessary to suppress grey growth without competitors. If $\delta>r_2$, part (a)(iii) supplies the simpler competitor-independent extinction bound. These are model predictions about long-term dynamics; neither finite-time extinction nor uncertainty in ecological parameters is encoded in the equations.
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