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Dean E. Pearson

Research Ecologist

800 East Beckwith Avenue
Missoula, MT 59801
Contact Dean E. Pearson

Current Research

Dr. Pearson's current research focus is applying community ecology to study the problem of biological invasions (introduced pest species). He leads a small team that is tackling the three key components of the invasion problem: 1) understanding the causes of invasion by conducting international research on many plant species in their native and introduced ranges to determine how the introduction of organisms to new ranges creates pest species; 2) studying invader impacts in the introduced range to identifying mitigation targets and strategies; and 3) quantifying the efficacy of management tools (e.g., biological control, herbicides, restoration strategies) and refining their applications for mitigating invader impacts.

Ecology and management of invasive species:

Invasive plant impact rankings for intermountain bluebunch wheatgrass:

Research Interests

Dr. Pearson's primary research interest is to advance community ecology through studies of biological invasions and to use this information to improve invasive species management. Biological invasions serve as grand natural experiments that provide unique opportunities to view the processes that structure ecological communities. Even in the most recently assembled natural communities organisms have had hundreds or thousands of years to interact and sort themselves into the modern assemblages we see. This makes it very challenging to identify the relative roles of competition, predation, parasitism, resource availability, etc. in structuring extant communities. However, with biological invasions, it is possible to observe these processes in real time as a novel organism invades, establishes, and dominates (in the case of strong invaders) a new assemblage. It is also possible to see how the assemblage finally settles into a new equilibrium state as the invader's dominance wanes through the ecological and evolutionary adaptation of the community to its presence. Newly hypothesized understandings of these processes can then be tested in the context of directed community assembly through the applied largescale natural experiments that are attempts to manage the invasion problem, for example, classical biological control - the intentional introduction of exotic organisms to manage invasions. Thus, biological invasions offer an opportunity to advance understandings of community ecology that can in turn be tested by applying these understandings to the management of the invasives problem.

Past Research

Dr. Pearson's prior research focused on wildlife biology, studying wildlife-habitat relationships and techniques for wildlife studies.

Why This Research is Important

Biological invasions present the second greatest threat to natural ecosystems and the services they provide next only to actual land transformation. Exotic species invasions cost the United States 120-140 billion dollars per year in revenue losses and mitigation costs. Management of this problem has been particuarly challenging due to a lack of understanding of the fundamental processes underlying invasion. My research attempts to better understand the causes and consequences of invasion and the efficacy of current managmen tools in order to better guide management approaches toward more effective mitigation. Success in these efforts offers tremendous benefits in terms of US revenues and stability of ecosystem services.


  • University of Montana, Ph.D., Organismal Biology and Ecology, 2005
  • University of Montana, M.A.S., Zoology, 1995
  • University of Montana, B.S., Wildlife Biology, 1992
  • Awards

    Visionary Science Publication Award, 2013
    USDA Forest Service, Rocky Mountain Research Station, January 2013, for the publication: Pearson, D. E. and R. M. Callaway. 2003. Indirect effects of host-specific biological control agents. Trends in Ecology and Evolution 18(9):456-461.
    National Forest System Invasive Species Program Award, 2012
    March 2012, In recognition of his high level of leadership and expertise on invasion biology and invasive species management, linking management concepts to long-term landscape restoration and rehabilitation.
    National Academy of Sciences, Kavli Fellow Recipient, 5 November 2010., 2010
    Deputy Chief s Early Career Scientist Award., 2009
    This honor was awarded in recognition of significant contributions to the fields of biological control, invasive species ecology, and wildlife biology.
    Presidential Early Career Award for Science and Engineering (PECASE)., 2009
    "Selection for this award is based on the combination of innovative research at the frontiers of science and technology and community service demonstrated through scientific leadership and community outreach.
    Deputy Chief s Early Career Scientist Award., 2009
    Received 25 March 2009 in Washington D.C. This honor was awarded in recognition of significant contributions to the fields of biological control, invasive species ecology, and wildlife biology.

    Featured Publications


    Smith, Jennifer N.; Emlen, Douglas J.; Pearson, Dean E., 2016. Linking native and invader traits explains native spider population responses to plant invasion
    Rudgers, Jennifer A.; Fletcher, Rebecca A.; Olivas, Eric; Young, Carolyn A.; Charlton, Nikki D.; Pearson, Dean E.; Maron, John L., 2016. Long‑term ungulate exclusion reduces fungal symbiont prevalence in native grasslands
    Pearson, Dean E.; Ortega, Yvette K.; Runyon, Justin B.; Butler, Jack L., 2016. Secondary invasion: The bane of weed management
    Pearson, Dean E.; Ortega, Yvette K.; Eren, Ozkan; Hierro, Jose L., 2015. Quantifying "apparent" impact and distinguishing impact from invasiveness in multispecies plant invasions
    Ortega, Yvette K.; Greenwood, Leigh F.; Callaway, Ragan M.; Pearson, Dean E., 2014. Different responses of congeneric consumers to an exotic food resource: Who gets the novel resource prize?
    Pearson, Dean E.; Ortega, Yvette K., 2013. Biogeography of plant invasions
    Litt, Andrea R.; Pearson, Dean E., 2013. Non-native plants and wildlife in the Intermountain West
    Zwolak, R.; Pearson, Dean E.; Ortega, Yvette K.; Crone, E. E., 2012. Mechanisms driving postfire abundance of a generalist mammal
    Ortega, Yvette K.; Pearson, Dean E.; Waller, Lauren P.; Sturdevant, Nancy J.; Maron, John L., 2012. Population-level compensation impedes biological control of an invasive forb and indirect release of a native grass
    Maron, John L.; Pearson, Dean E.; Potter, Teal; Ortega, Yvette K., 2012. Seed size and provenance mediate the joint effects of disturbance and seed predation on community assembly
    Pearson, Dean E.; Sutherland, Steve; Butler, Jack L.; Smith, Jane; Sieg, Carolyn H., 2011. I. Plants
    Pearson, Dean E.; Ortega, Yvette K., 2011. Invasive Species Science Update (No. 5)
    Pearson, Dean E.; Kim, M.; Butler, Jack L., 2011. Rocky Mountain Research Station invasive species visionary white paper
    Pearson, Dean E.; Finch, Deborah M., 2011. V. Terrestrial vertebrates
    Maron, John L; Pearson, Dean E.; Fletcher, Robert J. Jr., 2010. Counterintuitive effects of large-scale predator removal on a midlatitude rodent community
    Zwolak, Rafal; Pearson, Dean E.; Ortega, Yvette K.; Crone, Elizabeth E., 2010. Fire and mice: Seed predation moderates fire's influence on conifer recruitment
    Maron, John L.; Pearson, Dean E.; Hovick, Stephen M.; Carson, Walter P., 2010. Funding needed for assessments of weed biological control
    Finch, Deborah M.; Pearson, Dean E.; Wunderle, Joseph; Arendt, Wayne, 2010. Terrestrial animals as invasive species and as species at risk from invasions
    Ortega, Yvette K.; Pearson, Dean E., 2009. Factors influencing plant invasiveness
    Butler, Jack L.; Pearson, Dean E.; Kim, Mee-Sook, 2009. Invasive Species Working Group: Research Summary and Expertise Directory
    Zwolak, Rafal; Foresman, Kerry; Crone, Elizabeth; Pearson, Dean E.; Ortega, Yvette K., 2008. Are mice eating up all the pine seeds?
    Ritter, Sharon; Jones, Greg; Watson, Alan E.; McCaughey, Ward; Harrington, Mick; Zwolak, Rafal; Foresman, Kerry; Crone, Elizabeth; Pearson, Dean E.; Ortega, Yvette K.; Loeffler, Dan, 2008. ECO-Report - Finding common ground: Montana Forest Restoration Committee
    Shick, Katharine R.; Pearson, Dean E.; Ruggiero, Leonard F., 2006. Forest habitat associations of the golden-mantled ground squirrel: Implications for fuels management
    Ortega, Yvette K.; Pearson, Dean E.; McKelvey, Kevin S., 2004. Effects of biological control agents and exotic plant invasion on deer mouse populations
    Pearson, Dean E.; Callaway, Ragan M., 2004. Response to Thomas et al.: Biocontrol and indirect effects
    Pearson, Dean E.; Callaway, Ragan M., 2003. Indirect effects of host-specific biological control agents
    Pearson, Dean E.; Ortega, Yvette K.; Ruggiero, Leonard F., 2003. Trap-induced mass declines in small mammals: Mass as a population index
    Pearson, Dean E.; Ortega, Yvette K.; McKelvey, Kevin S.; Ruggiero, Leonard F., 2001. Small mammal communities and habitat selection in Northern Rocky Mountain bunchgrass: Implications for exotic plant invasions
    Buskirk, Steven W.; Ruggiero, Leonard F.; Aubry, Keith B.; Pearson, Dean E.; Squires, John R.; McKelvey, Kevin S., 2000. Comparative ecology of lynx in North America [Chapter 14]
    Pearson, Dean E.; McKelvey, Kevin S.; Ruggiero, Leonard F., 2000. Non-target effects of an introduced biological control agent on deer mouse ecology
    Canton-Thompson, Janie; Smith, Jane Kapler; Jones, Greg; Brown, Perry J.; Arno, Stephen F.; Daniels, Orville L.; Burk, Dale A.; Hardy, Colin C.; Silvieus, Dave; Pflug, Kristi D.; Winhorst, Bruce; Thompson, Brooke; Ortega, Yvette K.; McKelvey, Kevin S.; Thompson, Tom; Pearson, Dean E.; Stewart, Cathy; Sullivan, Janet, 1999. ECO-Report - 1999 Symposium highlights five years of learning
    Ruggiero, Leonard F.; Pearson, Dean E.; Henry, Stephen E., 1998. Characteristics of American marten den sites in Wyoming
    Herbicide treatment targeting the invasive plant, spotted knapweed, in Montana.
    A rapidly emerging problem is that of secondary invasion – an increase in non-target exotics following efforts to suppress targeted invasive plants. Researchers conducted a global literature review and meta-analysis directed at quantifying the magnitude of secondary invasion effects and identifying possible causes in order to improve management outcomes. 
    Invasive Canada thistle (National Park Service photo)
    Research sets forth a framework for understanding why exotic plants invade and how to fight the invasions.
    Spotted knapweed
    Rocky Mountain Research Station scientists evaluated the effects of a common herbicide treatment on grassland plants in western Montana to determine if and when suppression of spotted knapweed may relieve impacts of this notorious invader.
    We quantified and ranked invasiveness and impact for 48 exotic plant species based on surveys over 20,000 km2 of grasslands in western Montana. These data provide a valuable tool for managers to determine the relative impacts of invaders for prioritizing exotic plants for control in the bluebunch wheatgrass habitat type.
    Invasive plant impacts on native plants can ripple through native food webs from plants to insects all the way to birds. We observed that the invasion of spotted knapweed into grasslands of western Montana affects not only breeding success of songbirds but also the way song is passed between generations. 
    For over 10 years, Rocky Mountain Research Station scientists and their partners have engaged in research to 1) determine the causes underlying plant invasions, 2) identify invader impacts in native systems, and 3) improve the efficacy of invasive plant mitigation efforts.