Both Japanese knotweed and mugwort are perennial geophytes with long histories of human use, but contrasting primary introduction pathways to North America [12].
The large-scale dispersal of Japanese knotweed and mugwort, invasive species with contrasting introduction histories in the US, could be explained by secondary dispersal from primary introduction sites into adjacent counties.
For instance, a recently developed model of green crab range expansion in the region suggests that primary introduction from the native range is likely a rare event [35], consistent with the most likely invasion scenario detailed above.
Both these studies suggested that South African populations are likely a primary introduction from a different source population in the native range.
Such outbreaks are difficult to detect when the primary introduction of viruses through food occurs simultaneously in several countries or continents (7 – 9 ).
After primary introduction of the virus through food, secondary person-to-person and environmental transmission can rapidly take over, making it hard to trace the disease back to contaminated food.
We collected 1 12 samples per site, for a total of 218 individuals in and around these primary introduction sites, as well as in other locations in those three general regions (Fig. 2; see Supporting Information ).
However, two supercolonies have been identified in the Western Cape, and although Stellenbosch forms part of the large supercolony in South Africa, it is not known whether ants from the small supercolony may also originate from a primary introduction from the native range or from a secondary introduction from other introduced supercolonies not yet identified.
Eurasian wild boars were introduced into the United States several times, but the primary introduction that became established occurred in 1912 when 14 wild boars (11 females and 3 boars) were released in a game preserve in the southwestern part of North Carolina in Hooper Bald (26 ).
It is highly unlikely the Australian supercolony resulted from primary introduction events from the native range as there are no records of L. humile interceptions in Melbourne, Adelaide or Perth that came from its native range, although a single interception in Sydney originated from Argentina (E. L. Suhr and D. J. O'Dowd, unpublished data).
Whether or not a major outbreak can occur after a primary introduction of an infectious agent into a population is determined by the (basic) reproduction number, which gives the expected number of secondary infections caused by one infected individual in a large population in the early stages of an outbreak.
