  We consider the optimal spacing between marine reserves for maximising the
viability of a species occupying a reserve network. The closer the networks are
placed together, the higher the probability of colonisation of an empty reserve
by an occupied reserve, thus increasing population viability. However, the
closer the networks are placed together, the higher the probability that a
catastrophe will cause extinction of the species in both reserves, thus
decreasing population viability. Using a simple discrete-time Markov chain
model for the presence or absence of the species in each reserve we determine
the distance between the two reserves which provides the optimal trade-off
between these processes, resulting in maximum viability of the species.
