Bottlenecks in large populations: the effect of immigration on population viability

We model a large population that is subject to successive short bottlenecks, in order to investigate the impact of different extents of immigration on the change in genetic load and on viability. A first simple genetic model uncovers the opposite effects of immigration on fitness according to the type of deleterious mutations considered: immigration increases fitness if the genetic load is comprised of mildly deleterious mutations, whereas it decreases fitness if it is comprised of lethals. When considering both types of mutations and adding explicit stochastic demographic considerations, in which bottlenecks are engendered by random catastrophes, the global impact of immigration on viability is dependent upon a balance between its opposite effects on the two components of the genetic load and on demographic stochasticity. In this context, immigration tends to increase the probability of extinction if occurring preferentially when population density is high, while it decreases extinction if occurring preferentially towards low-density populations.