A population genetic signature of human releases in an invasive ladybeetle

Biological invasions have been accelerated by a variety of human activities. Propagule pressure, the number of introduced individuals and independent introductions, is likely to be influenced by these human activities and may be an important factor for successful range expansion in new environments. We tested whether the current distribution of the predatory ladybeetle Coccinella septempunctata in the introduced range (USA) is the result of multiple historical human introductions or natural range expansion from the first established populations in the USA. To test this hypothesis, we compared historical records of propagule size, propagule number, specific introduction locations, and the date of each introduction, with estimates of genetic variation in mitochondrial DNA (COI). Our results indicated that genetic diversity in the introduced range was positively correlated with historical records of propagule size and number, and negatively correlated with distance to nearest introduction ...