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Invasive species

Gretchen Hansen's research on invasive species in lakes

invasive species

Invasive species alter aquatic food webs and represent a major management challenge. New invasive species are discovered in Midwestern lakes each year, and developing appropriate strategies requires an understanding of their impacts, potential for spread, and management alternatives. My research has examined landscape-level distributions of a large number of aquatic invasive species, as well as conducted large scale experimental removal of a single invasive species (rusty crayfish) to better understand their role in aquatic food webs and to identify options for control.


whole-lake removal of invasive rusty crayfish

Pile of rusty crayfish removed from Sparkling Lake, Wisconsin. Photo credit Gretchen Hansen.

Pile of rusty crayfish removed from Sparkling Lake, Wisconsin. Photo credit Gretchen Hansen.

Catch rates of rusty (invasive) and virile (native) crayfish from 1980-2014. Rusty crayfish were experimentally removed via intensive trapping from 2001-2008. Data courtesy of the North Temperate Lakes LTER program.

Catch rates of rusty (invasive) and virile (native) crayfish from 1980-2014. Rusty crayfish were experimentally removed via intensive trapping from 2001-2008. Data courtesy of the North Temperate Lakes LTER program.

Once an invasive species establishes a self-sustaining population, few options exist for mitigating its negative effects.  Eradication or control of invasive species can be successful under certain conditions, but most eradication attempts have occurred on small islands.  Eradication efforts can produce unexpected results in terms of community dynamics and ecosystem properties, particularly when an invader has been established for long time periods, exhibits strong interactions with other species, or alters physical properties of an ecosystem. 

Rusty crayfish are native to the Ohio River Valley, and established populations in Wisconsin Lakes in the 1970s. Rusty crayfish eliminate aquatic plant beds and negatively impact panfish and invertebrate populations, including snails and native crayfish. Rusty crayfish were experimentally removed from Sparkling Lake, a 64 hectare lake in Vilas County, Wisconsin, from 2001-2008 via intensive trapping and changes in sport fish regulations in an attempt to restore the lake to pre-invaded conditions.  The removal was successful in reducing rusty crayfish catch rates by over 99%, and the native virile crayfish increased in abundance 100-fold over this time period.  As of 2015, rusty crayfish in Sparkling lake remain at low densities, and the aquatic ecosystem has exhibited substantial changes as a result. Monitoring of Sparkling Lake pre- and post-removal was conducted by the North Temperate Lakes LTER.

Hansen, G. J. A., C. L. Hein, B. M. Roth, M. J. Vander Zanden, J. W. Gaeta, A. W. Latzka, S. R. Carpenter. 2013. Food web consequences of long-term invasive crayfish control. Canadian Journal of Fisheries and Aquatic Sciences. 70: 1109-1122


Abundance distributions of invasive species

Abundance distributions for six invasive species. EWM=Eurasian Water Milfoil.

Abundance distributions for six invasive species. EWM=Eurasian Water Milfoil.

Invasive species are recognized as a leading driver of environmental change. Their impacts are often linked to their population size, but surprisingly little is known about how frequently they achieve high abundances. A nearly universal pattern in ecology is that species are rare in most locations and abundant in a few, generating right-skewed abundance distributions. Together with many collaborators, we collated abundance data from over 24,000 populations of 17 invasive and 104 native aquatic species to test whether invasive species differ from native counterparts in statistical patterns of abundance across multiple sites. Invasive species on average reached significantly higher densities than native species and exhibited significantly higher variance. However, invasive and native species did not differ in terms of CV, skewness, or kurtosis .  Abundance distributions of all species were highly right skewed, meaning both invasive and native species occurred at low densities in most locations where they were present (6% and 2% of the maximum densities observed within a taxa, respectively). The biological significance of the differences between invasive and native species depends on species-specific relationships between abundance and impact. Recognition of cross-site heterogeneity in population densities brings a new dimension to invasive species management, and may help to refine optimal prevention, containment, control, and eradication strategies.

Hansen, G. J. A., M. J. Vander Zanden,  M. J. Blum, M. K. Clayton, E. K. Hain, J. Hauxwell, M. Izzo, M. S. Kornis, P. B. McIntyre, A. Mikulyuk, E. Nilsson, J. D. Olden, M. Papes, S. Sharma. 2013. Commonly rare and rarely common: comparing population abundance of invasive and native aquatic species. PLoS ONE 8(10): e77415. doi:10.1371/journal.pone.0077415.


sea lamprey control in the great lakes

Bucket o' lampreys from larval assessment.

Bucket o' lampreys from larval assessment.

Sea lamprey are an invasive, parasitic fish species in the Great Lakes that kill large numbers of lake trout and other salmonids. A bi-national, multi-million dollar control program exists to suppress sea lamprey populations and allow higher numbers of lake trout to exist in each of the Great Lakes than would be possible in the absence of such a program.

For my master’s thesis, I conducted an adaptive management experiment across the entire Great Lakes basin to evaluate the performance of an alternative method of assessing sea lamprey populations and ranking streams for treatment with lampricides.  I compared the differences in the total number of sea lampreys that would be killed as a result of each assessment method using population estimates generated from the assessments and independent population estimates from mark-recapture studies on a subset of streams. I found that the alternative assessment method outperformed the status quo method, and the international body charged with coordinating sea lamprey management in the Great Lakes subsequently adopted the alternative assessment method in 2008. 

Electrofishing for sea lamprey larvae in Michigan.

Electrofishing for sea lamprey larvae in Michigan.

 Following the completion of the empirical study, I collaborated with an economist to analyze thissystem using an economic modeling approach and to determine the conditions which would favor one assessment method over another.  I also worked in collaboration with several researchers to develop a management strategy evaluation (MSE) style model of sea lamprey management.  MSE models include components of the entire managed system, and incorporate uncertainty in everything from demographic processes to the enforcement of management actions.  We used this model to test the efficacy of various management actions and to direct future research by identifying key uncertainties.

Another component of my master’s work incorporated historical sea lamprey survey data collected from over 300 Great Lakes streams over 50 years.   I used generalized linear mixed models to analyze patterns in larval sea lamprey growth and recruitment from over 30,000 surveys. I found that consistent patterns exist in sea lamprey recruitment that could be used to more effectively assess streams for treatment. 

Hansen, G. J. A and M. L. Jones. 2008. Evaluating an alternative larval assessment method for Great Lakes sea lampreys: A case study in adaptive management. Canadian Journal of Fisheries and Aquatic Sciences 65: 2471-2484.

Hansen, G. J. A. and M. L. Jones. 2009. Variation in larval sea lamprey demographics among Great Lakes tributaries: A mixed-effects model analysis of historical survey data. Journal of Great Lakes Research 35: 591-602.

Jones, M. L., B. Irwin, G. J. A. Hansen, H. A. Dawson, A. J. Treble, W. Liu, W. Dai, and J. R. Bence. 2009. An operating model for the integrated pest management of Great Lakes sea lampreys. The Open Fish Science Journal 2: 59-73.

Fenichel, E. P., and G. J. A. Hansen. 2010. The opportunity cost of information: An economic framework for understanding the balance between assessment and control in sea lamprey management. Canadian Journal of Fisheries and Aquatic Sciences 67: 209-216.

Banded mystery snail in a northern Wisconsin Lake. Photo credit Gretchen Hansen.