Population genetics reveals origin and number of founders in a biological invasion

Propagule pressure is considered the main determinant of success of biological invasions: when a large number of individuals are introduced into an area, the species is more likely to establish and become invasive. Nevertheless, precise data on propagule pressure exist only for a small sample of invasive species, usually voluntarily introduced. We studied the invasion of the American bullfrog, Rana catesbeiana, into Europe, a species that is considered a major cause of decline for native amphibians. For this major invader with scarce historical data, we used population genetics data (a partial sequence of the mitochondrial cytochrome b gene) to infer the invasion history and to estimate the number of founders of non‐native populations. Based on differences between populations, at least six independent introductions from the native range occurred in Europe, followed by secondary translocations. Genetic diversity was strongly reduced in non‐native populations, indicating a very strong bottleneck during colonization. We used simulations to estimate the precise number of founders and found that most non‐native populations derive from less than six females. This capability of invasion from a very small number of propagules challenges usual management strategies; species with such ability should be identified at an early stage of introduction.