The Influence of Invasive Centaurea stoebe on Cirsium pitcheri and Tanacetum huronense
Centaurea stoebe is a major invasive species within the United States that has invaded the sensitive dune habitats of the Great Lakes region. This area is home to many unique and increasingly rare species, two of which are Cirsium pitcheri and Tanacetum huronense. We sought to determine whether Centaurea is negatively impacting Cirsium and Tanacetum on the shoreline of Beaver Island in northern Lake Michigan. We mapped the precise locations of these plants on a stretch of shoreline dunes using a GPS. In addition to our GPS-mapped points, we also collected abundance data for sites spread across the island. Using a nearest neighbor analysis, we determined that Centaurea, Cirsium, and Tanacetum all have clumped underlying spatial structures. Using methods described by Dixon (1994) and Coomes et al. (1999) we also performed two-species analyses measuring segregation and association between Centaurea and the native species, finding that there was highly statistically significant segregation between the species, but no significantly positive or negative association. Using a density-dependence size analysis, we determined that the size of reproductive Cirsium and Tanacetum was correlated with the number of neighbors they have in a 25 cm radius. Using our abundance data, we found no statistical indication that high abundances of Centaurea were correlated with low abundances of the two native species. We concluded that Centaurea, Cirsium, and Tanacetum are highly spatially segregated. As a result of this, if solely looking at the spatial structure of this plant community, it is impossible to determine whether proximity to Centaurea negatively impacts these two native species at this stage of the Centaurea invasion. In the future, we suggest performing experimental studies in which Centaurea is planted next to Cirsium and Tanacetum to examine its impact, in addition to continuing to track the Centaurea invasion on Beaver Island.