It is difficult to know where to begin a response to Marlene Condon’s article titled “Ecologists Recognizing Value of Alien Plants” in the February 2019 Crozet Gazette. Her bias and manipulative language displays itself in every paragraph as she portrays “plant nativists” and invasion biologists as extremists with an evil agenda that will cost you, the taxpayer, millions of dollars in unnecessary and wasted spending. She engages in blanket and baseless accusations and name-calling against people engaged in valuable environmental work and cherry-picks her observations and citations. She even claims her casual observations are more valid than those of scientists performing rigorous research.
Condon’s article begins with: “Scientists are either waking up to what I’ve been saying for years, or finally becoming brave enough to speak out against the widespread invasive-plant movement.” So, she thinks scientists are timid and half asleep and that people concerned about invasive plants are a threatening “movement”? Next, she cites as a scientific reference an eight-year-old Comment in the journal Nature by Mark A. Davis and others. What she does not mention is that the Comment elicited immediate responses that Nature published in a subsequent issue and that Davis did not answer those letters. (Full disclosure: one of us, MTL, co-authored one of these responses, volume 475, pp. 36-37). Nor does she mention, as we discuss below, that Davis’ et al. conclusion goes against her own argument.
Condon appears to ignore and does not cite the vast amount of peer-reviewed literature on the damaging effects of nonnative plants. For example, recent research published by Narango et al., in the October 22, 2018, issue of the Proceedings of the National Academy of Science demonstrates that native plants are best for birds. The research showed that yards landscaped with the usual garden center plants, which are mostly nonnative ornamentals, could not support a stable population of chickadees. Yards where native plants composed at least 70 percent of the plantings were able to do so. This is because native plants host more insects than non-natives and therefore provide the necessary high-protein food that birds need to feed their chicks.
Condon mistakenly asserts that “nativists” (a derogatory term she uses for people who encourage planting only native plants in their gardens and in our natural areas) insist that all alien plants are problems and must go. On the contrary, we so-called “nativists,” who are actually informed naturalists concerned for the integrity of the natural world, understand that many garden plants from Asia and elsewhere stay right where they are planted and don’t invade a natural landscape. The “aliens” we worry about are those nonnatives that can become invasive because they:
- Grow fast.
- Have few pests or pathogens to control their numbers.
- Produce many seeds.
- Can disperse their seeds great distances.
- Tolerate a wide range of growing conditions.
- Often resprout from extensive roots.
- Out-compete natives for water, nutrients, and sunlight.
These characteristics allow invasive aliens to kill or displace native plants because they grow so fast they can form monotypic stands and because they lack pests or pathogens that can control them. Forest invaders, such as Japanese stiltgrass and garlic mustard, to name only two, out-compete forest floor wildflowers and prevent tree regeneration. Invasive vines, such as Japanese honeysuckle and Oriental bittersweet, twist around tree trunks, strangling them and cutting off the flow of water and nutrients, and ultimately killing them. Vine foliage also blocks sunlight to the tree’s leaves, while the sheer weight of a large vine can topple a tree. Oriental bittersweet, whose orange-and-red berries are beloved for decorative fall wreaths, can invade and kill forests; it grows faster than native grape species according to Invasive Plants Field and Reference Guide: An Ecological Perspective of Plant Invaders of Forests and Woodlands, USDA. Bittersweet can be observed strangling trees along many roadsides in the Blue Ridge.
By and large, these nonnative invasive plants, which are not members of our local ecosystems and plant communities, play little to no role in the complex food web of insects, mammals, amphibians, reptiles and birds, because they did not evolve with them. For instance, entomologists have shown that approximately 90 percent of all insect species must feed on a particular plant species or group of related plants to complete their life cycles. (Reminder: monarch caterpillars must feed on milkweed to complete their lifecycle and pupate into the showy butterflies we all love.) When nonnative, invasive plants replace native plants, there are fewer vegetation-feeding insects and fewer appropriate fruits and nuts to serve as food for birds and other wildlife.
Condon writes in her earlier articles that she has observed birds eating the autumn olive fruits from shrubs in her backyard garden and then concludes that this invasive shrub is good for wildlife. But she errs in equating observation with real research. Birds find most nonnative fruits unattractive and do not eat them; but they do feast on autumn olive. (And that feast, by the way, contributes to the spread of this nasty invasive shrub because the birds poop and “plant” the seeds far and wide.) Scientists have determined that our migrating birds require high-fat foods to fuel their southward flights in autumn. However, autumn olive berries are sugary sweet treats, the junk food of the bird diet. Thus, the fruits of this Asian shrub that Condon observes being eaten by “her birds,” may be contributing to the documented decline in bird populations. Smith et al. demonstrated that autumn olive fruits provide about half the nutrition of several types of native fruits, such as dogwood, at the time of year when North American migrating songbirds need fat, not carbohydrates, to fuel their long flights. (“Fruit Quality and Consumption by Songbirds during Autumn Migration,” Wilson Journal of Ornithology, March, 2007).
Condon claims she has not seen autumn olive spreading from her garden and therefore it is not a problem invasive. But has she walked her neighbors’ properties? Has she spoken to any farmers lately, who curse the invasiveness of this difficult-to-eradicate pest? She wrote that she has driven along Route 81 in April and observed hillsides covered with this silvery-leaved shrub, but she believes this is a good thing because native plants (so she says) cannot grow under such circumstances and this invasive shrub offers food and shelter. This is simply not true. There is much documentary evidence about this and many other invasive shrubs ruining farmers’ fields and invading and overtaking natural meadows and even forming a dense understory in woodland settings. (By the way, the Massachusetts Audubon Society provides instructions on its website for controlling this invasive shrub.)
Condon states in her February article: “Read about virtually any “invasive” species and you will find that these plants are typically growing in disturbed areas where man or a weather event destroyed the original soil profile. As a result, the plants that had been growing there previously did not come back because they could not handle the altered physical conditions of the site. It’s why you see so-called invasive plants mainly along roadways, in parks, and along river trails—all areas easily seen by people who then mistakenly believe the exotic plants pushed out native species.” (Italics ours.) Colonies of invasives on disturbed sites pose a special sort of threat because they can (and often do) spread from there into undisturbed areas because they grow faster and out-compete the natives. For instance, Japanese stiltgrass may first appear along a trail, its seed having been brought in on hikers’ boots. The stiltgrass does not remain only along the trails for long. From there, the invasive’s copious seeds wash down water courses and the invasion spreads to make carpets of stiltgrass in the forest, where it out-competes native wildflowers and prevents tree regeneration.
Condon states in this, and in many of her articles, that nonnative plants benefit wildlife by providing food and shelter in disturbed areas where native plants cannot grow. That statement is grossly exaggerated. Native plants, particularly those termed pioneer species, can and do grow in disturbed sites, although they may not come back immediately if soil disturbance is extensive. With proper management, native species can almost always grow in any disturbed area. Those sites should not be left unattended and might need to be revegetated with natives adapted to those particular conditions. Nonnative, invasive plants are not better than natives for wildlife; they do thrive in disturbed sites and once established will invade undisturbed areas nearby.
While it appears that many nonnatives, such as Japanese knotweed, which Condon mentions in her recent article, provide shelter for animals, that shelter is not always useful and appropriate for many species. Take the bobwhite quail, for instance. Baby quail need shelter that allows them to move about quickly. They require clump-forming grasses whose structure offers ground-level spaces beneath dense, overhead cover to protect them from predators. Native grasses are mostly clump-formers. Most nonnative grasses, which abound in hayfields and meadows, form continuous mats that lack the needed spaces. The documented decrease in quail populations is due in part to habitat destruction both from development and from invasive plants displacing native plants.
In her articles about “so-called invasives,” Condon repeatedly asserts that land managers are driven by the “foolishness of blindly pushing an agenda without giving any thought to the real-world consequences of doing so.” And: “this deliberate destruction of habitat is taking place because of ideology, an illustration of the danger posed by people who have been led to believe they are part of an environmentally moral crusade.” Really? How does she know these motivations? She asserts that nonnatives offer benefits, without considering that those possible benefits are often illusory and are almost always out-weighed by the negative impacts of a species’ invasiveness, traits well-known to land managers.
Condon cherry picks her sources to drive her ill-informed, anti-natural agenda, which actually furthers the destruction of the natural world. For example, in the beginning of her February 2019 article, Condon cites the Davis et al. Nature Comment from 2011 to support her views, but she does not mention the conclusion, which follows here: “We are not suggesting that conservationists abandon their efforts to mitigate serious problems caused by some introduced species, or that governments should stop trying to prevent potentially harmful species from entering their countries.”
We ask that readers consider Condon’s diatribes with skepticism and use a discerning and critical eye before taking her words as gospel.
Susan A. Roth, William Hamersky, and Manuel T. Lerdau
Susan A. Roth holds a BS and an MS in Ornamental Horticulture from Cornell University and is the author of 10 gardening books. She is “doing penance” now for recommending in her books plants that escape the garden and are threatening our natural landscapes. William Hamersky holds a BS in Wildlife Biology from SUNY College of Environmental Science and Forestry and a MS in Biology from California State University, Hayward. Manuel T. Lerdau is a Professor of Environmental Science and of Biology at the University of Virginia and has a PhD in Biology from Stanford.