The California Invasive Plant Council held their 27th annual conference in Monterey in November. It was their biggest conference, with about 400 attendees and more sponsors than ever before. Clearly the industry that promotes the eradication of non-native plants is alive and well. However, a closer look at the conference presentations suggests otherwise. Eradication efforts are growing, but eradication success is not and establishing a native landscape after eradication is proving elusive.
A few common themes emerged from the presentations:
- Eradication cannot be accomplished without using pesticides.
- When eradication is achieved with pesticides, non-natives are rarely replaced by native plants.
- Planting natives after non-natives are eradicated reduces re-invasion, but secondary invasions of different non-native plants are common.
- “Managing” forests with prescribed burns did not result in more biodiversity than leaving the forest alone.
Goals of these eradication projects have shifted in response to these failures to achieve original goals:
- Replacement plantings after eradication are sometimes a mix of natives and non-natives.
- Inability to establish native grassland has given way to different goals.
- Language used to describe the projects are evolving to be more appealing to potential volunteers.
Here are a few examples of presentations that illustrate these themes:
Eradicating beach grass in Point Reyes National Seashore
About 60% of sand dunes in the Point Reyes National Seashore were covered in European beach grass when the eradication effort began in 2000. The goal of the project was to restore native dune plants and increase the population of endangered snowy plovers that nest on bare sand.
The project began by manually pulling beach grass from 30 acres of dunes at Abbott’s Lagoon. The grass grew back within one year, presumably because the roots of the beach grass are about 10 feet long. Manually pulling the grass from the surface does not destroy the roots.
A new method was devised that was more successful with respect to eradicating the beach grass. The grass and its roots were plowed up by bulldozers and buried deep in the sand. The cost of that method was prohibitively expensive at $25,000 to $30,000 per acre and the barren sand caused other problems.
The barren dunes were mobile in the wind. Sand blew into adjacent ranches and residential areas, causing neighbors of the park to object to the project. The sand also encroached into areas where there were native plants, burying them. The bare sand was eventually colonized by “secondary invaders.” Different non-native plants replaced the beach grass because they were more competitive than the desired native plants.
In 2011, the National Park Services adopted a third strategy for converting beach grass to native dune plants. They sprayed the beach grass with a mixture of glyphosate and imazapyr. At $2,500 to $3,000 per acre, this eradication method was significantly cheaper than the mechanical method.
However, it resulted in different problems that prevented the establishment of native dune plants. The poisoned thatch of dead beach grass was a physical barrier to successful seed germination and establishment of a new landscape. Where secondary invaders were capable of penetrating the dead thatch, the resulting vegetation does not resemble native dunes.
The concluding slides of this presentation were stunning. They said it is a “Restoration fallacy that killing an invader will result in native vegetation.” My 20 years of watching these futile efforts confirm this reality. However, I never expected to hear that said by someone actually engaged in this effort. The presenter mused that such projects are like Sisyphus trying to roll a boulder up hill.
Attempting to plant Douglas fir after eradication of broom
Over a period of 5.5 years, broom was eradicated in plots in Oregon by spraying glyphosate. The plots were then planted with Douglas fir seedlings that soon died. They were replanted the following year and died in the second year.
There were two theories about why the plantings failed, both broadly described as “legacy” effects in the soil left by the broom. One theory is that nitrogen levels were too high for successful growth of Douglas fir. That theory is consistent with the fact that broom is a nitrogen fixer. That is, broom—like all legumes—have the ability to transfer nitrogen in the atmosphere to nitrogen in the soil with the help of bacteria that facilitate that transfer. Nitrogen generally benefits plant growth, but there can also be too much nitrogen.
The second theory is that Douglas fir requires a specific suite of mycorrhizal fungi for successful growth. Mycorrhizal fungi live in roots of plants and trees. They transfer moisture and nutrients from the soil to the plants. Plants with a healthy suite of mycorrhizal fungi are more drought tolerant because they extract more moisture from the soil.
Neither of these theories has been successfully proven by this project. They remain unanswered questions. We were struck that the researchers had not considered the possibility that the repeated use of glyphosate could have been a factor in the failure of the Douglas fir. Glyphosate is known to kill bacteria in the soil. Could it also kill mycorrhizal fungi? (We know that triclopyr kills mycorrhizal fungi.) That possibility was not considered by this project. Did the project consider that glyphosate also changes the consistency of the soil by binding certain minerals together? It is more difficult for roots and water to penetrate the hard soil. Were soil samples taken before and after repeated applications of glyphosate to determine how the soil had been changed by pesticide applications?
The published abstract for this project made this observation: “It is typically assumed that once an invasive species is successfully removed, the impact of that species on the community is also eliminated. However, invasive species may change the environment in ways that persist, as legacy effects, long after the species itself is gone.” In fact, it seems likely that the pesticides used to eradicate the “invasive” species could also be the source of the “legacy effects.”
Does “managing” a forest result in greater biodiversity in the understory?
California State Parks tested that hypothesis by conducting prescribed burns in some of their forests in the Sierra Nevada 20 years ago, while leaving other portions of the forest “unmanaged.”
The abstract for this presentation describes the goals and expectations for the prescribed burns: “Prescribed fire is a tool used to reduce fuels in the forests in the Sierra Nevada and mimic the low and moderate severity wildfires that burned before the onset of fire suppression. A manager’s hope is that prescribed fire will create the disturbance necessary to stimulate the development of species rich understory communities and increase species richness, compared to unburned forests, which are often viewed as species depauperate.”
Twenty years after the burns, abundance and species composition of the understory in the burned areas were compared to the unburned areas. They found little difference in the biodiversity of the understory of burned areas compared to unmanaged forests:
- “Species richness was highly variable within burned and passively managed areas but was not statistically different.”
- “Passively managed areas did not appear to be depauperate in understory species diversity compared to areas managed with prescribed fire.”
- “Fire did not appear to reduce or enhance species richness numbers in burned areas, as compared to passively managed areas.”
No fires occurred in either the burned areas or the unmanaged areas during the 20-year period. Therefore, this study did not test the theory that prescribed burning reduces fire hazards in forests. This study found no significant differences in diversity of forest understory resulting from prescribed burns.
There are significant risks associated with prescribed burns. They cause air pollution and they frequently escape the controlled perimeter of the fire, becoming wildfires that destroy far more than intended. This study does not provide evidence that would justify taking those risks. In fact, available evidence supports the “leave-it-alone” approach to land management.
Moving the goal posts
If at first you don’t succeed, you have the option of redefining success. Here are a few of the projects presented at the conference that seemed to take that approach.
Make projects so small that success can be achieved
Eric Wrubel introduced himself as the National Park Service staff who is responsible for prioritizing invasive plants for removal in the National Parks in the Bay Area (GGNRA, PRNS, Muir Woods, and Pinnacles). His work is based on the premise that the most successful eradications are those that are small. The bigger the infestation, the greater the investment of time and resources it takes to eradicate it and the smaller the likelihood of success. This is illustrated by a graph showing this inverse relationship between the size of the invasive population and the success of eradication.
The process of prioritizing eradication projects began over 10 years ago with a survey of over 100 species of plants considered invasive. Cal-IPC’s “watch list” was used to identify the plants that are not yet widely spread in California, but considered a potential problem in the future. Cal-IPC’s risk assessment was the third element in the analysis. Plants with “High” risk ratings by Cal-IPC were put higher on the priority list than those with “Moderate” or “Limited” ratings. Plants that did not exist elsewhere in the region or watershed were also given higher priority, based on the assumption that re-invasion was less likely.
The priority list showed that the highest priority eradication projects were quite small. Some were just a few acres. Buddleia jumped out as the 7th highest priority on only 13 acres. Buddleia was recently added to a new category of plants on Cal-IPC’s “invasive” inventory. It is not considered invasive in California, although it is considered invasive elsewhere.
In placing buddleia on its “hit list,” Cal-IPC illustrates one of the fundamental weaknesses of its evaluation method. Cal-IPC does not evaluate pros and cons of non-native plants. Only traits considered negative are taken into consideration.
Buddleia is one of the most useful nectar plants for pollinators in California. We took the time to visit the monarch butterfly sanctuary in Monterey while attending the conference. The monarchs are arriving now to begin their winter roost in the eucalyptus, Monterey pine and cypress in this small grove. At the entrance to the sanctuary a sign instructs visitors to plant only native milkweed as the monarch’s host plant and only native flowers for nectar. Fortunately whoever planted the flowering shrubs in the sanctuary didn’t follow the advice of the sign-makers. They planted buddleia and other flowering non-natives such as bottle-brush. Several species of butterflies and hummingbirds were enjoying those plants in the Sanctuary. Strict adherence to the native plant agenda is not beneficial to wildlife because animals do not share our prejudices.
Acknowledging the difficulties of converting non-native annual grass to native perennial grass
Pinnacles National Park acquired 2000 acres of former ranchland in 2006. The park wanted to convert the non-native annual grasses and yellow-star thistle on the former ranch to perennial bunch grasses and oak woodland. They were able to reduce the amount of yellow-star thistle by burning and spraying with herbicide, but cover of native species remained low. Conversion of grasses from non-native annuals to native perennial grass has been tried many times, in many places, and for long periods of time. These projects were notoriously unsuccessful.
The project at Pinnacles has changed its goal to plant forbs (herbaceous flower plants) instead of grasses and they report that they are having some success. They justify that shift in goal on soil analysis that suggests forbs were more prevalent than perennial grasses in inland valleys in California than previously thought.
This change in goal could be described as “adaptive management,” which adjusts methods and goals in response to observable outcomes of existing methods. You could also call it “trial and error.” We would like to see more land managers make such adjustments to their strategies, rather than doing the same thing over and over again and expecting a different outcome.
Recruiting volunteers with appealing messages
There were several presentations about effective methods of recruiting volunteers to participate in restoration projects. Some of their messages seem to acknowledge that the language used in the past may have alienated some potential volunteers. Speaking from personal experience, I can confirm that observation. Here are just a few of the cringe-worthy native plant mottos that I hope have been abandoned in favor of a more positive message:
- “That plant doesn’t belong here.”
- “That is a good plant and the other is a bad plant.”
- “The invasive landscape is sick and requires chemotherapy.” (to justify the use of pesticides)
- “That’s a trash bird.” (said of common, introduced birds, such as starlings and house sparrows)
The speaker advised those who work with volunteers to focus on why an unwanted plant is a problem rather than where it comes from. Unfortunately, the list of problems is heavily influenced by the preferences of native plant advocates. If their criticisms are not accurate, or they don’t acknowledge the advantages of the plant, little has been achieved by using euphemisms. Here are a few of the inaccurate criticisms made of eucalyptus:
- “Nothing grows under eucalyptus.” This is observably untrue and no evidence of allelopathy has been found.
- “Eucalyptus kills birds.” This was one of the most ridiculous accusations, but is still occasionally heard among native plant advocates.
- “Eucalyptus is very invasive.” Cal-IPC rates invasiveness of eucalyptus as “Limited.” They spread only when planted beside streams or swales that carry their seeds downstream and in very foggy coastal locations with a lot of wind to carry the seeds.
What was missing?
Ecological restoration is a major industry. Thousands of people are employed by the industry, which is funded by many different sources of public money. Whether individual projects are successful or not, the industry will survive and thrive as long as it is funded. Greater care should be taken to design and implement projects that will be successful.
Stepping back from the conference presentations of specific restoration projects, here are a few issues that were conspicuously absent from the conference.
- Pesticides are being widely used by the restoration industry. When projects don’t achieve desired outcomes, pesticides should be considered as a factor. Did pesticides alter the soil? Were beneficial microbes and fungi killed? How persistent was the pesticide in the soil? How mobile was the pesticide in the soil? Was pesticide applied in the right manner? Could aerial drift account for death of non-target plants? There are many other useful questions that could be asked.
Update: The California Invasive Plant Council has published “Land Manager’s Guide to Developing an Invasive Plant Management Plan.” It says very little about the disadvantages of using herbicides to eradicate plants they consider “invasive” other than a vague reference to “unintended consequences,” without discussion of what they are or how to avoid them.
However, it does give us another clue about why eradication efforts are often unsuccessful. When herbicides are used repeatedly, as they have been in the past 20 years, weeds develop resistance to them: “The International Survey of Herbicide Resistant Weeds (2018) reports there are currently 496 unique cases (species x site of action) of herbicide-resistant weeds globally, with 255 species…Further, weeds have evolved resistance to 23 of the 26 known herbicide sites of action and to 163 different herbicides.” The Guide therefore recommends that land managers rotate herbicides so that the “invasive” plants do not develop resistance to any particular herbicide. The Guide gives only generic advice to use “herbicide X” initially and “herbicide Y or Z” for subsequent applications.
In other words, the California Invasive Plant Council continues to promote the use of herbicides to kill plants they consider “invasive.” They give advice about ensuring the effectiveness of herbicides, but they do not give advice about how to avoid damaging the soil, killing insects, and harming the health of the public and the workers who apply the herbicides. May 20, 2019
- Are workers who apply pesticides being adequately trained and supervised by certified applicators? The safety of workers should be one of many goals of restoration projects.
- When non-native plants are eradicated, serious thought should be given in advance to the probable outcome. Will native plants return? Will wildlife be harmed? Will the risks of failure outweigh the potential benefits of success?
- Is climate change taken into consideration when planning the replacement landscape? Are the plants that grew in the project location 200 years ago still adapted to that location? Is there enough available water?
- If new plantings require irrigation to be established, what is the water source? Is it recycled water with high salt content that will kill many plants, including redwoods?
- Are the new plantings vulnerable to new infectious diseases, such as phytopthera or infestations of new insects such as shot-hole borer?
- Does the project team have sufficient horticultural knowledge to choose plants that can survive in current conditions? Does the project team know the horticultural needs of the plants they are planting? Is there enough sunlight, water and wind protection for the trees they are planting?
The public is investing heavily in the “restoration” of ecosystems. We can only hope that our investment is being used wisely and that projects will not do more harm than good. Cal-IPC can play a role in raising the questions that have the potential to improve projects and enable them to succeed. The long-term survival of the “restoration” industry depends on it.
Most quotes are from abstracts of presentations published in the conference program.