Garlic mustard pulling season is nearly upon us here in the Upper Midwestern United States, where attacking the pest plant has become a spring ritual. As winter slowly recedes and spring drifts in on cold misty rains, dormant second-year garlic mustard (Alliaria petiolata) plants will resume growth. Growth is an understatement, for in a spectacular surge of photosynthetic prowess with a startling rapidity the plant goes from emergence, to flower, to seed in the eight weeks of April and May.
Garlic mustard is a weed of restored and remnant woodlands and savannas, woodlots, backyards and unmanaged vacant spaces. Originally from Europe, it first appeared in the United States in the late 1800’s and has since spread across the eastern United States and is now everywhere you can look here in Madison. This rapid spread is a concern for local restoration ecologists, natural area managers, and stewards of public lands who believe that the weed is a fundamental cause of woodland deterioration because it crowds out native wildflowers and tree and shrub seedlings. Eradication, control, or containment is the recommended solution.
This “war” against garlic mustard has become an environmental rallying cry to which hundreds, if not thousands, of volunteer citizens—myself included—have responded. You can see hordes of us on our hands and knees each spring, lending a few hours on weekends and evenings to pull bags full of garlic mustard from infested public or private lands. It is very satisfying work, and when we’re done we even use some of the aromatic and tasty leaves in pesto sauces, breads, and jams/jellies.
I’ll be out there in the garlic mustard fields again this year but while pulling I’ll be wondering if the effort is worth it and questioning if all this volunteer work is aimed at the right target. I’ll be thinking about two recent research reports, the results of which say that white-tailed deer (Odocoileus virginianus)—and not garlic mustard—is a fundamental cause of the disruption of a forest’s natural growth and loss of native plant biodiversity and are that deer are ultimately responsible for the garlic mustard invasions themselves that we place the blame on.
In a study by DiTommaso, et al (2014) “Researchers have discovered that a burgeoning deer population forever alters the progression of a forest’s natural future by creating environmental havoc in the soil and disrupting the soil’s natural seed banks.”
The authors are saying that the impacts of an overpopulation of deer on the soil and seed banks are a major disruption to forested ecosystems.
In another study (Kalisz, et al 2014) the researchers say that deer density in the U.S. is about four to 10 times what it was prior to European settlement of North America and make a link between this and the declining plant biodiversity in some regions.”
“To study the effect of rampant deer on trillium and garlic mustard populations, the researchers established multiple 196-square-meter plots in the forest. Half were fenced to exclude deer. Years of observation and hours of statistical analysis later, the team found that in plots where deer were excluded, the trillium population is increasing, and the garlic mustard population is trending toward zero.”
“There is a link between disruption of the native animal community and invasion by non-native plant species according to this research and a co-author of the study suggests that “similar links maybe found in other ecosystems between disrupted fauna and declining diversity of flora.”
The authors go on to say that the success of garlic mustard invasion is dependent upon high deer populations.
Earthworms and Garlic Mustard
And that’s not all. Garlic mustard might not be having the impacts we think and white-tailed deer might have an unseen co-conspirator. In a 2009 research summary (Blossy, et al, 2009) found that non-native earthworms may be implicated in the garlic mustard takeover of our woodlands. Blossy and colleagues were “unable to find negative ecosystem effects of garlic mustard” on the forest organisms they studied in the Finger Lakes region of New York State. The report goes on: “There is strong evidence that garlic mustard is a symptom of larger, but less visible, invasion by non-native earthworms.”
Implications for Restoration Practice
These findings are consistent with my experience and observation of the association of garlic mustard populations with established deer trails and feeding areas. It seems to me that these studies have several implications for practice:
- Garlic mustard is a symptom—and not the cause—of the illness of forest degradation. Garlic mustard as a symptom tells us that there is a more fundamental cause (or causes) of environmental degradation at work.
- White-tailed deer may be the proximate cause of loss of woodland biodiversity but are deer the fundamental issue? Perhaps it is land use changes and the human impact on urban and rural landscapes that is closer to being the fundamental cause of loss of woodland biodiversity and the disruption in the natural growth of forests.
- Ridding an area of garlic mustard (or another pest species) may be necessary but is probably not sufficient to achieve restoration goals. Pest species management must be accompanied by other actions called for in the restoration plan that correct the underlying site conditions that contribute to the pest infestation.
- Garlic mustard “pulls” may stimulate civic engagement and environmental education but by themselves they are in many cases an inefficient and ineffective use of public energy and time. Efforts might be better spent on planting native species.
- A white-tailed deer is a pest species too. Like many pest species deer are a social and cultural problem that society—and not just land managers—must deal with.
- A focus on garlic mustard control while ignoring an ever-expanding urban deer population or the non-native earthworm problem takes our eye off the ball, ignores a fundamental problem, and may be a missed opportunity to educate, inform, and restore.
This is not to suggest that land managers abandon efforts to eradicate, control, or contain garlic mustard. And this is not to suggest that efforts to manage the pest plant are inappropriate or unneeded. However, new knowledge about, the invasion complex of garlic mustard-nonnative earthworms-white-tailed deer-European buckthorn does suggest several things: 1) that the restoration problems we face are more complex than we first thought; 2) land managers need a full tool box to deal with difficult challenges because; 3)there is no silver-bullet, one-step approach to restoring our degraded woodlands; and 4) each restoration site and situation will need an equally complex, nuanced, and balanced restoration management plan.
Blossy, B. and V. Nuzzo, J. Maerz, and A. Davalos. 2009. Ecosystem Impacts of Alliaria petiolata (Garlic mustard) IN Proceedings Symposium on the Biology, Ecology and Management of Garlic Mustard (Alliaria petiolata) and European buckthorn (Rhamnus cathartica), L. Skinner, Editor. Forest Heath Technology Enterprise Team. US Forest Service Publication, FHTET-2005-09
Antonio DiTommaso, Scott H. Morris, John D. Parker, Caitlin L. Cone, Anurag A. Agrawal. Deer Browsing Delays Succession by Altering Aboveground Vegetation and Belowground Seed Banks. PLoS ONE, 2014; 9 (3): e91155 DOI: 10.1371/journal.pone.0091155
S. Kalisz, R. B. Spigler, C. C. Horvitz. In a long-term experimental demography study, excluding ungulates reversed invader’s explosive population growth rate and restored natives. Proceedings of the National Academy of Sciences, 2014; DOI: 10.1073/pnas.1310121111