1. The likelihood of range changes in the event of climate change makes study of the foundation and establishment of populations of particular relevance and interest.
2. The butterfly Pararge aegeria has expanded its range in Britain over many decades. Since 1985, ten monitored sites in eastern England have been colonized and the pattern of population growth examined.
3. Geometric annual growth rates in the establishing populations were initially, on average, three- to fourfold but subsequently declined. The decline provides possible evidence that density-dependence limits final population densities, but other explanations are possible.