Figure 9.

With all other parameters being equal, we change the drift coefficient and measure the time between peak infections in colony 1 and colony 2. We can see for low $\beta$ (panel (a)) that increasing the drift coefficient (i.e., decreasing the distance between colonies) exponentially decreases the time it take an infection to reach colony 2. When $\beta$ is high (panel (b)), we see that there is less importance on drift, it only need be present.