Patterns of success in game bird introductions in the United States.
Better predictions of the success of species' introductions require careful evaluation of the relative importance of at least three kinds of factors: species characteristics, characteristics of the site of introduction, and event-level factors such as the numbers of individuals released. (Henceforth, we call this propagule pressure.) The 1644 introductions of 17 Phasianid species released in various US states during the Foreign Game Investigation Program provides a particularly rich source of data to test these ideas. An examination of these records indicates that 13 of these 17 species always failed, despite generally numerous individual releases and large numbers of individuals in each release. Moreover, ten of these species have been successfully introduced elsewhere. Only four of the 17 species were successful in at least one state. Some 20 sets of releases of three of these four species always failed in some states, again given generally numerous individual releases of large numbers of individuals in each release. Simply, the combination of site and species factors explain the lack of successes. This leaves a combination of 18 states where one or more of the four species succeeded. For these, there are significant differences in the numbers of birds introduced from state to state. But only for two species Alectoris chukar and; Tetraogallus himalayensis are there significant differences that show a greater chance of success when more individuals are introduced. These results support the conclusion that the number of individuals released, meaning propagule pressure, is not as important as characteristics of the species and the location to where its introduction occurred.