Figure 2.

Stability modeled using the parameters $N=1$, $\beta =0.8$, $\sigma =0.3$, $\mu =0.4$, $\eta =0.9$, and (a) $\gamma =0.85$ and $\upsilon =0.7$ (upper right), (b) $\gamma =0.75$ and $\upsilon =0.7$ (upper left), (c) $\gamma =\beta \upsilon N\frac{\eta }{\mu (\mu +\upsilon )}-\frac{{\sigma }^2}{2}\frac{{\upsilon }^2{\eta }^2}{{\mu }^2(\mu +\upsilon )}{N}^2-\mu$ and $\upsilon =0.85$ (lower left) and (d) $\gamma =0.55$ and $\upsilon =0.85$ (lower right). The initial condition is $(N,0,0,0)=(1,0,0,0)$ for all of them