Evolutionary Explanation for Invasive Abilities of Centaurea stoebe (spotted knapweed) in Introduced Areas
Grand Valley University
Since every invasion and every exotic species presents a new challenge with novel invasion strategies, the invasion of Centaurea stoebe (synonymously referred to as C. maculosa) into North America will be used as a case study to deeply examine invasion strategies. Centaurea stoebe is a short-lived (approximately 3 years) perennial member of the aster family (Broz et al., 2007). It occurs in two different cytotypes, or with two different chromosomal factors. In its home range, it exists in both diploid and tetraploid cytotypes, but only the tetraploid version is present in introduced ranges in North America (See Figure 1; Treier et al., 2009). C. stoebe, commonly called spotted knapweed, is native to Eurasia and was introduced to North America in the late 1800s or early 1900s in several distinct introduction events (Hahn and Muller-Scharer, 2013; Marrs, Sforza, & Hufbauer, 2008). According to the U.S. Department of Agriculture and the Natural Resources Conservation Service (2001), C. stoebe is now found in all but four of the continental United States. The detrimental effects of its invasion are most clearly seen in the northwestern United States (See Figure 1). In Montana alone, C. stoebe covers nearly 4.5 million acres (Broz et al., 2007). Bais et al. (2003) introduce spotted knapweed as one of the most economically damaging invasive species in North America. It can take over in a variety of conditions and is especially common in disturbed areas in pastures and prairies. In its native range in Eurasia, it is not problematic, but in North America it has formed monocultures and replaced many native plants. On top of all this, it degrades the infrastructure of communities by increasing runoff that leads to erosion and causing decreases in forage for wildlife (Broz & Vivanco, 2009b). It is important to understand the mechanisms of invasion for specific invasive species, so that conservation efforts can be most tailored and effective. There are two major hypotheses in the literature as to why Centaurea stoebe is a successful invader. One theory assumes that there is pre-existing genetic variation and adaptation in C. stoebe that provide them with success in novel environments. The other postulates that rapid evolutionary change occurs in the invasive species in its new habitat (Muller- Scharer, Schaffner & Steinger, 2004).