3.3 | Invasion history
The first part of the ABC analysis found that introduced populations in Canada and Chile most likely originated, at least partially, from bridgehead introductions from the previously introduced population in France (Figure 5) (880 cumulative Random Forest (RF) votes), rather than directly from the native range (120 cumulative RF votes) (Supporting Information 1).
When analyzing the introduced French population alone in the second part, the first step found that this introduced population could not be unambiguously assigned to a single origin, as all three regions of the native range received a substantial amount of support (East : 449 RF votes, Central : 341 RF votes and Louisiana/Mississippi : 210 RF votes). However, the most likely single introduction event scenario (287 RF votes) was slightly outvoted when compared against a two-population admixture scenario (314 RF votes, second step), which was itself outvoted by scenarios simulating the French origin through admixture of all three native regions (754 cumulative RF votes, third step). The fourth step of the second part (native range was further divided) found that Georgia and South Carolina (197 RF votes) obtained the highest support for an origin of the French population. However, Louisiana (167 RF votes), and Alabama and Mississippi (161 RF votes) also received a substantial number of votes, while other source populations obtained a significant number of RF votes (i.e.,Maryland and New York: 94 RF votes; Tennessee and North Carolina: 83 RF votes; Florida: 74 RF votes). Overall, these findings suggest the occurrence of multiple introduction events out of the native range. However, at both large (step1) and finer scale (step4), no scenario received a majority vote, preventing a definitive determination of the source population of the introduced population in France.
The third part aimed at analyzing the origins of the Canadian introduced population, using the French introduced population as a potential source. ABC analyses revealed that the most probable scenario for the origin of the Canadian population was an introduction from a French bridgehead and its admixture with a separate introduction event from the native range (463 RF votes), rather than entirely from the native range (240 RF votes) or French bridgehead (297 RF votes). When the native range was further divided, ABC analyses found that the additional introduction event that admixed with the French bridgehead in Canada likely originated from the surrounding regions (i.e., Maryland and New York: 132 RF votes; Ohio, Kentucky, Indiana and Illinois: 122 RF votes; Wisconsin: 120 RF votes). A similar invasion history was identified for Chile, as a bridgehead from France combined with an additional introduction event from the native range was found most likely (542 RF votes), rather than entirely from the native range (138 RF votes) or French bridgehead (320 RF votes). When the native range was further divided, ABC revealed that the additional introduction event likely originated from Wisconsin (182 RF votes).
Overall, the parameter estimation consistently suggested the origin of the introduced French population 200 to 271 years ago, which resulted from the arrival of 54 to 62 migrants (Figure 5). This population was suggested to have experienced a bottleneck for a duration of 8 to 8.5 years. The introduced population in Canada was estimated to originate 101 years ago, and experienced a short bottleneck of 3.5 years. This population arose from an initial propagule of between 47 to 59 migrants, 64% of them coming from a bridgehead from France and 36% from the surrounding localities in the native range. The introduction to Chile was estimated to have occurred around 93 years ago, following a short bottleneck of 3.5 years. This population resulted from the arrival of 46 to 51 migrants, half of them coming from France (52%) and the other from an additional introduction event from the native range, probably Wisconsin. All other posterior probabilities, RF votes and posterior parameter estimates are available in the Supplementary Information.