Science meets the “restoration” industry

I was encouraged to hear a presentation by an academic scientist at the recent Beyond Pesticides Forum that was another indication of the paradigm shift in invasive species management toward a less destructive approach.  Dr. Bernd Blossey is a Professor at Cornell University, where he directs the Ecology and Management of Invasive Plants Program in the Department of Natural Resources.  His many years of studying invasive plants, such as purple loosestrife, garlic mustard, water chestnut, Japanese knotweed, and phragmites have convinced him that there are often “multiple stressors” that contribute to such invasions.  Some factors such as the presence of earthworms and deer can be more important factors in the Northeast than the non-native plants themselves. 

Based on his research experience, Dr. Blossey delivered wise advice to land managers at the Beyond Pesticide Forum.  The featured photo at the top of this article was his introductory slide. 

Before a restoration project begins, these questions should be asked and answered:

Source: Dr. Blossey’s presentation to Beyond Pesticides Forum on June 8, 2021

If the project seems worthwhile after such analysis is done, this is Dr. Blossey’s advice about monitoring the project and measuring its success:

Source: Dr. Blossey’s presentation to Beyond Pesticides Forum on June 8, 2021

Practicing what he preaches

Dr. Blossey used these principles in his study of garlic mustard in the forests of the northeast. (1) Over a period of more than 10 years, Dr. Blossey and his collaborators measured the abundance of garlic mustard in 16 plots from New Jersey to Illinois where no attempt had been made to control or eradicate it.  They found that growth rates initially increased, but decreased over time and eventually the population started to decline.  Dr. Blossey explained their findings in a recent webinar that is available HERE:

Garlic mustard was first recorded in North America in 1868 on Long Island, New York.  It spread west from there and is now found from southern Canada to Georgia and from New York and Quebec to Oregon, British Columbia and Alaska.  Because land managers believed that garlic mustard suppresses populations of native plants, they have been trying to eradicate garlic mustard in northern forests for decades, with little long term success.  Dr. Blossey addressed that concern in his webinar. 

Source: Dr. Blossey’s webinar about garlic mustard

Earthworms are the prerequisite for garlic mustard invasion.  Earthworms in northern forests are also considered alien invaders because they were killed, along with forests, by advancing glaciers during the Ice Age.  When forests returned after the Ice Age over 10,000 years ago, they evolved without earthworms that were reintroduced by European settlers less than 500 years ago. 

When deer are excluded from areas by fencing plots with garlic mustard populations, abundance of native vegetation does not decline.  Deer have a strong preference for native vegetation.  Absent deer, garlic mustard does not seem to suppress the growth of native plants in northern forests.

In other words, garlic mustard is not guilty as charged.  Dr. Blossey explains the disadvantages of attempting to eradicate it.  The decline of garlic mustard abundance over time is attributed to negative soil feedback that builds over time as the soil microbial community responds to the new plant. Removing garlic mustard episodically prolongs the process of building that negative soil feedback.  When groups of well-meaning young people are sent into the forest to pull garlic mustard, they trample the very native plants they are trying to save. 

Are there lessons for land managers in the Bay Area?

Because garlic mustard doesn’t exist in California and our native earthworms are considered beneficial to soil health, you might wonder if this study is relevant here.  California was not glaciated during the Ice Age.  Our earthworms survived the Ice Age and they evolved with our forests. 

So, what can we learn from this study?  The pattern of initial growth and eventual decline of populations of introduced plants is not unique to garlic mustard“A phenomenon that has received increased attention is whether introduced species go through boom and bust cycles, ultimately becoming non-threatening members of local communities.” (1)  One recently published study was based on nearly 5,000 vegetation inventories collected in 49 National Parks in the eastern United States.  It reported that non-native plants appeared to decline after 100-200 years: 

Residence time appears a core part of invasion that interacts with other mechanisms, such as climate matching, propagule pressure and empty niche. Initially, time appears to benefit non-native species as they establish in a novel range. They likely face low enemy loads, and any successful dispersal increases their populations and invaded range. As they spread, initial barriers, such as distance or suboptimal habitat, were overcome, as was resistance from native relatives. However, their biggest challenge appeared to be time, as they all declined after ~1 to 2 centuries, suggesting that pathogens and herbivores caught up with them.” (2)

The message for land managers everywhere is that patience is needed to judge the impact of introduced species.  Most will fit into ecosystems eventually and attempts to speed up that process often do more harm than good.  We can’t judge changes in nature by the short-term perspective of human lifetimes because the evolution of nature is a continual process that began long before humans existed and is likely to persist long after we are gone. 

Applying Dr. Blossey’s “Core Knowledge” to local projects

What if Dr. Blossey’s “Core Knowledge” had been applied to projects in the San Francisco Bay Area?  Here are examples of local eradication projects that might have benefitted:

  • San Francisco has been trying to eradicate oxalis in its parks for over 20 years by spraying a selective herbicide (Garlon).  There seems to be more oxalis now than there was 20 years ago.  Oxalis is visible only about 2 months of the year.  When it dies back in the spring it leaves behind the native plants with which it co-exists.  If a control plot had been set aside before they started eradicating oxalis perhaps we would know the answer to these important questions:  Does oxalis suppress the growth of native plants?  Does attempting to eradicate oxalis produce more or less oxalis?
  • California, Oregon, and Washington have been trying to eradicate non-native spartina marsh grass along the entire West Coast for over 20 years.  Here in the Bay Area, non-native species of spartina have been 99% eradicated, but a hybrid of the native and the non-native remains and is poisoned with imazapyr annually.  According to a recent presentation by the Invasive Spartina Project, the hybrid is visually indistinguishable from the native and it occupies the same elevation of the marsh.  Over 500 genetic tests are needed every year to distinguish the hybrid from the native in order to poison the hybrid.  Dr. Blossey’s approach might ask these important questions:  What harm is hybrid spartina doing?  Do more or fewer animals live in hybrid spartina?  What effect has 20 years of spraying imazapyr had on the soil and the microbes that live in it?  Is the eradication project doing more harm than good? 
oxalis bloom, February 2021

We don’t know the answers to these important questions because projects were initiated and implemented without the analysis and monitoring metrics needed to answer the questions.  The projects continue without being accountable for the damage they are doing.  Public money is funding these projects without requiring the projects to be accountable for the consequences. 

California has made a commitment to spend billions of dollars on “nature based solutions” and achieving “biodiversity goals.”  This is an opportunity to start new projects off on the right foot by:

  • Requiring the analysis needed to determine the impacts and causes of perceived problems in the environment.
  • Requiring control plots so that the effects of the project can be compared with the option of not doing the project.
  • Requiring that projects be monitored, using established metrics so that the success of the project can be measured.

  1. Bernd Blossey, et. al., “Residence time determines invasiveness and performance of garlic mustard (Alliaria petiolota) in North America, Ecology Letters, February 2021.  
  2. Robert Warren, et. al., “Multiple mechanisms in woodland plant species invasion,”  Plant Ecology, April 2019.

Another innocent introduced plant with a bad rap

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. 

The first sighting of the garlic mustard herb in the U.S.A. was in 1868 in Long Island, New York. Garlic mustard was intentionally introduced into the northeastern United States for food, erosion control, and medicine. Pretty isn’t it? It’s tasty as well.

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.

Million Trees

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