Professor Mark Davis, known as “a friend to aliens,” is one of a growing number of academic scientists who are critical of invasion biology. His research has exonerated some of the introduced plants that are accused of harming native plants. Garlic mustard is an example. Professor Davis has briefly summarized his research of garlic mustard for publication on Million Trees.
The Nature Conservancy says, “…garlic mustard [is] one of the ten most destructive invasive species in Indiana today… It…displaces native or other desired plants in a relatively short period of time.” Although the Nature Conservancy description of the lifecycle of garlic mustard is identical to Professor Davis’s description, it reaches the opposite conclusion about its effect on native plants. The Nature Conservancy describes laborious methods of eradication and concludes that herbicides and prescribed burns may be necessary.
It is precisely that accusation that non-native plants “displace” native plants that is at the heart of the debate about so-called “invasive species.” Nativists do not believe that native and non-native plants can co-exist. In fact, when empirical studies test that hypothesis, they rarely find evidence of such displacement. The fact is, the environmental conditions that support native plants also support non-native plants and the result is usually more biodiversity, rather than less.
If native plants are not doing well, compared to non-natives, it is usually for other reasons, such as changes in the climate or available water or the arrival of a new insect or pathogen. It is rarely because of the arrival of new plants. Killing non-native plants with herbicide is simultaneously killing native plants and damaging the soil they live in.
There is room for everyone in the human world and in the natural world. Nativism in the human realm is as unnecessary as it is in the natural world. Fear and anger about human immigrants to America is strongest in places with few immigrants. I am fortunate to live in a place with many immigrants. I can see first-hand how they enrich my community. I also see first-hand how introduced plants are not doing any harm, but the futile efforts to kill them with herbicides IS doing a great deal of harm to every living thing in the environment.
Garlic mustard, Alliaria petiolata (Brassicaceae), is a European biennial plant (completing its life cycle over 2 years) now common in many Midwestern and eastern North American forests. It is self-pollinating, has abundant seeds that are viable in the soil for long periods of time, and can grow in a variety of forest environments. These characteristics allow garlic mustard to spread easily. Many consider it to be an invasive species that displaces native herbs and inhibits tree seedling growth and survival.
For the past six years I have worked with colleagues and students to determine to what extent garlic mustard is negatively affecting native herbs and tree seedlings in an oak forest in east-central Minnesota. Based on our monitoring of garlic mustard and the other plant species, garlic mustard appears to be acting similarly to other species.
Specifically, garlic mustard and the other common native herbs seem to be changing in abundance largely independent of one another. In other words, changes in abundance of one species is unrelated to changes in other. In six years of study, we have not been able to document any substantial effects by garlic mustard on other plant species, positive or negative.
In fact, the best predictor of garlic mustard presence is high diversity of native plants. The most likely explanation for this fact is that all the species, garlic mustard included, are simply establishing in microsites favorable to plants in general. The same conditions that benefit native plant species also probably benefit non-native plant species.
Overall, our findings are not consistent with the common claim that garlic mustard is a noxious invasive species responsible for the decline of many North American native forest herbs. Rather, our findings are more consistent with other recent studies and reviews that have concluded that garlic mustard is primarily responding to ecological changes in North American forests.
In other words, at many sites garlic mustard is not a significant driver of change but rather is a passenger of change. What might be the drivers of change, the real causes of declines of many native wildflowers? Evidence points to both earthworms and white-tailed deer.
Earthworms, which have been introduced from Europe and Asia, have drastically reduced the abundance of litter in these forests. The near absence of litter has been shown to negatively impact many of the native species. However, eradicating earthworms would be impossible and attempting to do so would cause more damage to the environment.
Due to the abundance of habitat, secondary forests, and lack of predators, white-tailed deer populations have exploded throughout many of the Midwestern and eastern forests. And, native plants are a common food of the deer. The deer tend to avoid garlic mustard due to its garlicy oils.
Mark Davis, Macalester College
Professor Davis’s studies of garlic mustard:
Davis MA, Anderson MD, Bock-Brownstein L, Staudenmaier A, Suliteanu M, Wareham A, and Dosch JJ. 2015. Little evidence of native and non-native species influencing one another’s abundance and distribution in the herb layer of an oak woodland. Journal of Vegetation Science 26:105-112. PDF
Davis MA, MacMillen C, LeFevre-Lefy M, Dallavalle C, Kriegel N, Tyndel S, Martinez Y, Anderson MD, and Dosch JJ. 2014. Population and plant community dynamics involving garlic
mustard (Alliaria petiolata) in a Minnesota Oak Woodland: a four year study. Journal of the Torrey Botanical Society 141: 205–216. PDF
Davis MA, Colehour A, Daney J, Foster E, MacMillen C, Merrill E, O’Neil J, Pearson M, Whitney M, Anderson MD, and Dosch JJ. 2012. The population dynamics and ecological effects of garlic mustard, Alliaria petiolata, in a Minnesota oak woodland. American Midland Naturalist 168: 364-374. PDF