Date of Award


Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Biological Sciences (MS)

Administrative Home Department

Department of Biological Sciences

Advisor 1

Erika Hersch-Green

Committee Member 1

Casey Huckins

Committee Member 2

Christopher Webster


Populations of invasive species are often subjected to novel selective forces in the form of anthropogenic control agents in their introduced ranges. These control agents, applied unevenly among populations within a species’ new range, can send invasive populations on drastically different evolutionary and ecological trajectories. In these studies, we aimed to see if different histories of chemical herbicide treatment are differentially influencing the genetic diversity, structure, and performance of populations of invasive watermilfoil (Myriophyllum spicatum and M. spicatum x M. sibiricum hybrids) from waterbodies in the state of Michigan. We sampled ten waterbodies with different histories of herbicide treatment in order to examine patterns of genetic variation amongst milfoil populations, to determine the extent of admixture among invasive watermilfoil populations, and to assess whether histories of herbicide application have an impact on the abiotic environment and/or biotic macrophyte community. We also grew invasive watermilfoil plants that were collected from waterbodies with and without histories of repeated exposure to herbicides together in mesocosms to test for tradeoffs in the expression of invasive traits. We found that genetic diversity is greater in populations with no history of herbicide treatment, populations with histories of herbicide treatment have more admixture and evidence of hybridization, and plant communities appear to be differentially shaped by histories of herbicide treatment. We also found that a history of herbicide treatment significantly affected plant survival, net growth, and mean growth rate and that these effects depended upon whether neighboring plants were from herbicide or non-herbicide treatment waterbodies. In general, plants from waterbodies with histories of herbicide treatment were more likely to survive and expressed increased growth relative to plants collected from waterbodies with no history of herbicide treatment. These findings indicate that histories of herbicide application could be selecting for populations comprised of less genetically diverse (but more admixed) individuals with potentially higher fitness for herbicide conditions. Our results suggest that repeated exposure to chemical herbicides could be selecting for increased invasiveness among invasive watermilfoil populations. This could have drastic ecological consequences and implications for the efficacy of long-term management efforts of invasive watermilfoil.