Shared Enemies and the Competitive Ability of Native Plants in Populations of an Invasive Plant (Cirsium arvense)

Shared enemies commonly mediate interactions between plant species. Here, I investigated how two guilds of enemies can indirectly affect native plant communities through interactions with an invading plant, Cirsium arvense (Canada thistle). First, I assessed how soil microbes in invaded areas can limit the establishment of native plant species. Microbes from invaded soils reduced germination or growth for three of the eight native plant species surveyed, but did not affect Cirsium, which suggests that soil microbial interactions may benefit Cirsium, potentially facilitating its invasion. Second, I assessed the influence of Cirsium invasion on damage by aboveground insect enemies. Based on plants' association with Cirsium, I observed inconsistent and very minor patterns in insect damage, which suggests that other ecosystem factors influence damaging insects more strongly than Cirsium invasion. Together, these results indicate that soil microbial enemies, but not aboveground insect enemies, may be influential in the invasion of Cirsium arvense.M.Sc