Across islands and continents,mammals are more successful invaders than birds

Many invasive species cause ecological or economic damage, and the fraction of introduced species that become invasive is an important determinant of the overall costs caused by invaders. According to the widely quoted tens rule, about 10% of all introduced species establish themselves and about 10% of these established species become invasive. Global taxonomic differences in the fraction of species becoming invasive have not been described. In a global analysis of mammal and bird introductions, I show that both mammals and birds have a much higher invasion success than predicted by the tens rule, and that mammals have a significantly higher success than birds. Averaged across islands and continents, 79% of mammals and 50% of birds introduced have established themselves and 63% of mammals and 34% of birds established have become invasive. My analysis also does not support the hypothesis that islands are more susceptible to invaders than continents, as I did not find a significant relationship between invasion success and the size of the island or continent to which the species were introduced. The data set used in this study has a number of limitations, e.g. information on propagule pressure was not available at this global scale, so understanding the mechanisms behind the observed patterns has to be postponed to future studies.