Centaurea stoebe is a major invader of sensitive dune habitats of the Great Lakes region in North America, home to many unique and increasingly rare species. We analyzed the beginning stages of invasion of Centaurea on a relatively isolated island in northern Lake Michigan and sought to determine whether Centaurea is negatively impacting two species of concern, Cirsium pitcheri and Tanacetum huronense. Using a nearest neighbor analysis of mapped individuals in a long term study plot, we determined Centaurea, Cirsium, and Tanacetum all have aggregated underlying spatial structures. Two-species analyses revealed highly statistically significant segregation between the species but no significant positive or negative association between the invader and either native species. Using abundance data in widespread smaller plots, we found no statistical indication that high abundances of Centaurea were correlated with low abundances of the two native species, despite marked increases in the invader.
In the face of evidence across North America of the detrimental effects of the Centaurea invasion, our results are surprising. The clumped distributions of the invader and the two native species decrease the chance for interspecific interactions. Therefore, detecting negative impacts of an invading plant species in the early stages of an invasion will require a more experimental approach, where native and invasive plants are observed in direct competition.