  Dispersal-induced growth (DIG) occurs when two populations with time-varying
growth rates, each of which, when isolated, would become extinct, are able to
persist and grow exponentially when dispersal among the two populations is
present. This work provides a mathematical exploration of this surprising
phenomenon, in the context of a deterministic model with periodic variation of
growth rates, and characterizes the factors which are important in generating
the DIG effect and the corresponding conditions on the parameters involved.
