National Science Foundation Award DEB-0918553

Biological invasions are simultaneously one of the world’s largest problems and most fascinating laboratories for ecological and evolutionary investigation.  Intuitively, successful biological invasion may result simply from a taxon being pre-adapted to an environment from which it was previously excluded because it simply couldn’t get there.  In other words, human introductions may simply help many taxa reach new suitable homes.  On the other hand, introduced taxa may well find themselves in habitats that they have not experienced before, and evolutionary processes that facilitate their rapid adjustment to novel environments may promote successful invasions.  Indeed, there is increasing evidence for rapid evolutionary change associated with many biological invasions.  

This project investigates biological invasions of the aquatic plant, variable leaf watermilfoil (Myriophyllum heterophyllum), utilizing a combination of genetic and ecological approaches.  

Flowering M. heterophyllum in a pond in South Carolina, where it is native.

Though native to much of eastern North America, it is not considered native to the northeastern and western US.  It has spread rapidly over the past 30 years in the northeastern US, and has recently been found in numerous western US lakes and ponds, causing concern among lake managers. 


Multiple introductions of geographically distinct genotypes - Using DNA sequences from two genes, our lab has discovered that variable leaf milfoil consists of several geographically distinct populations in its native range.  For example, populations located along the US Atlantic coast are quite distinct from populations located along the US Gulf Coast, Central Plains, Ozark, and Great Lakes regions.  Introduced populations in the northeastern and western US contain genotypes from both the Atlantic Coastal Plain and other regions from the native range, indicating that variable leaf milfoil has been independently introduced on multiple occasions and from multiple distinct geographic sources.  This work was published in Biological Invasions (volume 13: 1687-1709).


Introduced interspecific hybrids are 'more invasive' than introduced parental genotypes - Recently completed Master's research by Hannah Tavalire (currently a PhD student at Oregon State University) demonstrates that hybrid genotypes are more invasive than parental genotypes in the northeastern US.   Below is the abstract from Hannah's thesis manuscript, which is soon to be submitted for peer review. 

Recently, hybridization has been identified as an important precursor to the evolution of invasiveness in biological invasions.  That being said, there are very few empirical studies that compare the relative invasive growth of hybrids to that of other invasive ‘pure’ lineages.  Invasive “variable-leaf watermilfoil” (Myriophyllum heterophyllum) in the northeastern US consists of at least three morphologically cryptic but genetically distinct lineages: an interspecific hybrid (M. heterophyllum x M. laxum) and two historically allopatric lineages of pure M. heterophyllum.  Experimental comparison of growth characteristics of these three lineages thus provides a unique opportunity to test whether an introduced hybrid is more invasive than its pure counterparts.  Here, we compared several growth variables that capture different aspects of ‘nuisance growth,’ including individual plant size, branching rate, plant bed density, and biomass.  When compared to the pure lineages pooled, the hybrid lineage exhibited more nuisance growth in individual plant size, branching rate, and biomass.  In comparisons between the two pure lineages, we see significant differences in individual mass and plant bed densities, suggesting that these are in fact different taxa that also differ in their invasiveness.  In addition, we also used polymorphic AFLP markers to investigate whether gene flow among the introduced lineages has occurred.  We found no evidence for admixture, even where these distinct invasive lineages co-occurred, demonstrating that the variable-leaf watermilfoil invasion consists of a cryptic grouping of genetically and ecologically distinct taxa as opposed to a coalescent complex of admixed lineages.  Overall, our results support the hypothesis that hybridization plays an important role in the evolution of invasiveness of variable-leaf watermilfoil, but that distinct introduced lineages (hybrid and pure) vary in the extent to which different, key growth traits influence how they are perceived as invasive.


Papers resulting from this project -

1) Tavalire, H. F., Bugbee, G., LaRue, E. A., Thum, R. Hybridization and the evolution of invasiveness: Introduced hybrids are more invasive than introduced parental lineages of variable-leaf watermilfoil. Submitted to Evolutionary Applications (November 2011).

2) Thum, R.A., A.T. Mercer, and D. Wcisel. Loopholes in the regulation of invasive species?: Genetic identifications identify the sale of invasive and uncharacterized watermilfoils. In press (August 2011), Biological Invasions: DOI: 10.1007/s10530-011-0130-8.

3) Thum, R.A., M. P. Zuellig, M.E. Moody, C. Vossbrinck, and R.L. Johnson. 2011. Molecular markers reconstruct the invasion history of variable leaf watermilfoil (Myriophyllum heterophyllum) and distinguish it from closely related species. Biological Invasions 13: 1687-1709.

4) LaRue, E.A., C.R. Ruetz, III, M.B. Stacey, and R.A. Thum. 2011. Population genetic structure of the invasive round goby in Lake Michigan: implications for natural versus anthropogenic dispersal after introduction. Hydrobiologia 66: 71-82.

Page last modified March 11, 2014