Biological invasions present the single greatest threat to North American eastern deciduous forests and the second leading threat to biodiversity across all ecosystems in the United States. The aim of our research is to understand the factors that control invasion patterns, the relative importance of these factors and the direction of relationships between factors controlling invasions. The main focus of our research on invasives, thus far, has been earthworms and buckthorn.

Funded by: Environment and Natural Resources Trust Fund
University of Minnesota


In our gardens, earthworms are great for soils, since they mix and aerate soil and increase nutrients. But hardwood forests in the Great Lakes region developed during the thousands of years since glacial retreat without any earthworms. Natural processes kept the soil loose and uncompacted. Hardwood trees produce tons of nutrient rich leaf litter each year and since the leaf litter is produced faster than it decomposes, a thick forest floor and soil layers develop. Then as litter in the forest floor is decomposed by bacteria and fungi, nutrients are made available for understory plants and tree seedlings. The thick forest floor of earthworm-free hardwood forests turns out to be a central feature of these ecosystems since it is where most nutrient cycling occurs and where virtually all understory plants and tree seedling germinate and grow. When earthworms invade, the hardwood forest ecosystem begins to change rapidly as this forest floor is removed. Many prized spring ephemerals and new tree seedling regeneration eventually disappears.

Some of our studies focus on specific aspects of worm impact on forest ecology, including their effect of plants and forest floor, soil, nutrients, distribution and invasion patterns.


Eisenhauer, N., A. Stefanski, N.A. Fisichelli, K. Rice, R. Rich, P.B. Reich. 2014. Warming shifts 'worming': effects of experimental warming on invasive earthworms in northern North America. Scientific Reports 4: 6890.

Eisenhauer, N., N.A. Fisichell, L.E. Frelich, P.B. Reich. 2012. Interactive effects of global warming and 'global worming' on the initial establishment of native and exotic herbaceous plant species. Oikos, doi: 10.1111/j.1600-0706.2011.19807x

Holdsworth, A., L.E. Frelich, P.B. Reich. 2012. Leaf litter disappearance in earthworm-invaded northern hardwood forests: role of tree species and the chemistry and diversity of litter. Ecosystems DOI: 10.1007/s10021-012-9554-y

Larson, E.R., K.F. Kipfmueller, C.M. Hale, L.E. Frelich, P.B. Reich 2010. Tree rings detect earthworm invasions and their effects in northern Hardwood forests. Biological Invasions DOI 10.1007/s10530-009-9523-3

Hale, C., L.E. Frelich, P.B. Reich, J. Pastor. 2008. Exotic earthworm effects on hardwood forest floor, soils and native plants: a mesocosm study. Oecologia 155(3): 509-518.

Holdsworth, A.R., L.E. Frelich, P.B. Reich. 2007. Regional extent of an ecosystem engineer: earthworm invasion in northern hardwood forests. Ecological Applications 17:1666–1677.

Holdsworth, A.R., L.E. Frelich, P.B. Reich. 2007. Effects of earthworm invasion on plant species richness in northern hardwood forests. Conservation Biology 21(4):997-1008.

Frelich, L.E., C.M. Hale, S. Scheu, A.R. Holdsworth, L. Heneghan, P.J. Bohlen, P.B. Reich. 2006. Earthworm invasion into previously earthworm-free temperate and boreal forests. Biological Invasions 8:1235-1245.

Hale, C.M., L.E. Frelich, and P.B. Reich. 2006. Changes in hardwood forest understory plant communities in response to European earthworm invasions. Ecology 87:1634-1649.

Hale, C.M., L.E. Frelich, P.B. Reich. 2004. Allometric equations for estimation of ash-free dry mass from length measurements for selected European earthworm species (Lumbricidae) in the western Great Lakes region. American Midland Naturalist 151(1):179-185.

Worm Watch

Earthworms in Minnesota


Introduced from Europe as an ornamental, buckthorn is an aggressive invader of oak forest, savannah, prairie and riparian zones. It is one of Minnesota's most invasive species and is on the MN Dept of Agriculture's list of restricted noxious weeds. Buckthorms are shrubs or small trees that outcompete native plants and form a dense layer of vegetation. They leaf out earlier and retain leaves longer than native vegetation creating unfavorable light conditions for natives, which become eliminated in the understory.

Within the B4WARMED project we are asking whether climate warming will accelerate the potential for buckthorn to invade northern forests.

'Healthy Forests to Resist Invasion', a project funded by the Legislative-Citizen Commission on Minnesota Resources (LCCMR), examined in detail, factors that might determine links between forest attributes and buckthorn invasion. Information from this study can aid in the development of land management practices that incorporate the current invasive status of the plant community and the health and integrity of the ecosystem.


Roth, A., T. Whitfeld, A. Lodge, N. Eisenhauer, L. Frelich, P.B. Reich. 2014. Invasive earthworms interact with abiotic conditions to influence the invasion of common buckthorn (Rhamnus cathartica) Oecologia 10.1007/s00442-014-3175-4

Whitfeld, T., A. Lodge, A. Roth, P.B. Reich. 2014. Community phylogenetic diversity and abiotic site characteristics influence abundance of the invasive plant Rhamnus cathartica L. J. Plant Ecology 7: 202-209.

Whitfeld, T., A. Roth, A. Lodge, N. Eisenhauer, L.E. Freich, P.B. Reich. 2014. Resident plant diversity and introduced earthworms have contrasting effects on the success of invasive plants. Biological Invasions 16: 2181–2193.

Holdsworth, A., L.E. Frelich, P.B. Reich. 2012. Leaf litter disappearance in earthworm-invaded northern hardwood forests: role of tree species and the chemistry and diversity of litter. Ecosystems DOI: 10.1007/s10021-012-9554-y

Knight, K., J. Oleksyn, A.M. Jagodzinski, P.B. Reich, M. Kasprowicz. 2008. Overstorey tree species regulate colonization by native and exotic plants: a source of positive relationships between understorey diversity and invasibility. Diversity and Distributions 14:666-675.

Knight, K.S., J. Kurylo , T. Endress, R. Stewart, P.B. Reich. 2007. Ecology and ecosystem impacts of Rhamnus cathartica: A Review. Biological Invasions 9(8): 925-937.

Knight, K.S., P.B. Reich. 2005. Opposite relationships between invasibility and native species richness at patch versus landscape scales. Oikos 109: 81-88.