Date of Award



© 2015 Michelle Slater

Document Type


Degree Name

Master of Science in Marine Sciences


Marine Science

First Advisor

Charles Tilburg

Second Advisor

Pamela Morgan

Third Advisor

Steven Travis


Allee effects are important to species invasions because population growth may be limited by low outcrossing potential in newly founded populations leading to depressed levels of fitness. Non-native Phragmites australis (common reed) has been present in the United States for over 200 years, but has been slow to reach some areas, possibly owing to the Allee phenomenon, whereby young or isolated Phragmites patches with initially low genotypic richness are slow to recruit viable seedlings due to inbreeding, which limits the rate of patch expansion. In order to explore the importance of the Allee effect in a recently invaded system, the Saco River estuary of southern Maine, we characterized the genotypic richness of Phragmites patches using molecular markers to determine whether it was correlated with patch vigor. In addition, we evaluated the degree to which genotypic richness was controlled by the hydrodynamics of the estuary, which would be expected to strongly influence waterborne propagule dispersal, as well as a variety of environmental influences, particularly land development. Our models indicate that patch vigor (i.e. expansion) was not influenced by genotypic richness directly, but that the two were driven by somewhat overlapping sets of environmental variables. Stem heights were found to be positively affected by ammonium levels in marsh sediments, although we did not see a correlation with other nutrients as observed in previous studies. Our findings indicate that genotypic richness was positively influenced by specific land-cover types: the presence of nearby open fresh water, barren sand, and low-level human development. Additionally, seed viability appeared both low and random. We conclude that there is evidence for an Allee effect in the Saco River estuary, in the form of a positive relationship between genotypic richness and patch vigor in the form of stem density, in addition to low levels of seed viability in most patches.


Master's thesis

This digital object has been funded in part with Federal funds from the National Science Foundation, Division of Graduate Education, under Award No. #0841361, "The Interactions of Biology, Chemistry and Physics at the Land-Ocean Interface: A Systemic PARTnership Aimed at Connecting University and School (SPARTACUS)", to the University of New England.