Fig. 6.

${\beta }_{\mathrm{crit}}$ for (a) $\xi =0.15$, (b) $\xi =1.5$. For values of $\beta$ above ${\beta }_{\mathrm{crit}}$, there are no stable steady states with positive mussel density. The colour indicates ${\beta }_{\mathrm{crit}}$ according to the scalebar, with black indicating that there is no positive steady state. Thus, the black region in these plots corresponds to the unshaded region in Figs. 3 and 4. The key message from these plots is that in both cases the minimum value of ${\beta }_{\mathrm{crit}}$ occurs at $\alpha /\beta =2$ and $\lambda =0$, at which point ${\beta }_{\mathrm{crit}}=2$