(this article appears in the Fall 2012 issue of The Piedmont Virginian)
When I work with landowners on how they can better manage their properties for wildlife habitat, many common questions concern invasive plants. These questions include: “What is that weird, abundant vine/shrub/tree/pond muck? And how do I get rid of it?” Other times, it’s: “Do I need to worry about plant X?” or “Why bother?”
This last question is often the hardest to answer. In general there aren’t many easy answers to the issue of non-native invasive plants — species such as Asian bittersweet, autumn olive, and tree of heaven (often called “tree of hell,” jokingly). As any ecology professor will tell you, if you’re looking for cut and dry answers to hard questions, then you better change to an easier subject — such as physics. First, some definitions:
A. Native – A plant present in the Piedmont before Europeans arrived Examples: Tulip trees, blackberries, red bud, poison ivy, and horse nettle
B. Non-native – A plant brought over after Europeans arrived Examples: wheat, domestic apple, Callery pear (Bradford pear), boxwoods, and kudzu
1. Naturalized – A non-native that persists or re-seeds outside of cultivation. but not considered invasive Example: domestic apple, Excludes: wheat, boxwoods
2. Invasive – A non-native species whose introduction does or is likely to cause economic or environmental harm or harm to human health. (U.S. Executive Order 13112) Examples: Kudzu, Japanese honeysuckle, Callery Pear, and Autumn olive
C. Weed – A plant (native or non-native) that is not desirable in certain contexts Examples: Poison ivy (allergic reaction), kudzu (invasive), black cherry (“messy” to homeowners & landscapers; twigs and leaves poisonous to many animals)
As you can see, there are grey areas in these definitions. It’s often a value judgment, particularly when using the term “weed.” To put it in gardening terms: one man’s horse radish is another man’s weed. In the context of wildlife habitat, for example, our native grape species are excellent sources of food in the fall for literally hundreds of birds and mammals. Yet many people consider these grape vines to be “weeds” due to their aggressive growth habit.
The difference between “non-native naturalized” and “non-native invasive” is another confusing area. The challenge is that it’s hard to tell if or when a species will become invasive, since all invasive plants where at one time just “naturalized”. The classic example is Japanese honeysuckle, which was sold in the nursery trade for decades before people realized it was highly invasive. You can now find it in every corner of the Piedmont.
With this terminology, let’s consider the original question: “Why bother?” After all, we will not eradicate autumn olive (often called Russian olive), Asian bittersweet, English Ivy, Japanese barberry, and so on from the entire Piedmont landscape. It’s simply too late to eliminate these species across the vast acreages they now inhabit, and areas that are cleared may simply re-seed. In fact, in many cases we bred them to do this very thing — many of these species that are the most aggressive were intentionally selected by the horticulture industry for this very reason. They have abundant flowers and fruits, resistance to disease, prolific vegetative growth, and long growing seasons. It’s estimated that a whopping 82% of woody invasive plants were brought over by the nursery trade. So, should we just give up? As a wildlife ecologist, I say unequivocally no. Quite simply, most invasive plants do not perform the same ecological function that our natives do.
As an elegant example, consider the beautiful monarch butterfly, which migrates from Mexico to Canada. During its caterpillar stage, the monarch feeds exclusively on our native milkweed plants — no other plant genus will do. For nearly all other animals, milkweeds are highly poisonous, since the plant contains a heart attack-inducing chemical. This principle is known as co-evolution. Over millions of years, the monarch butterfly has co-evolved alongside the chemical defenses of the milkweed plant, with the plant and insect slowly developing new methods to outwit each other in a natural selection arms race. Contrast this to black swallowwort, an invasive plant related to milkweed now common in the northeast. Monarch caterpillars will use this plant, but research has shown that all the caterpillars die. There is an evolutionary mismatch between the native insect and the non-native plant.
Nature has countless co-evolutionary relationships. They are very common among insects, which are the primary food source for 97% of nesting birds — including bluebirds, eastern meadowlarks, and bobwhite quail. As invasive plants become more common in the landscape, our native fauna has less food to eat.
One of my favorite examples is white-tailed deer and invasive plants. While it is hard to generalize for all invasive plants, as a rule of thumb, deer avoid woody invasive plants (like Japanese barberry) because they have not co-evolved with the plant’s defenses. Basically, invasive plants taste bad to deer. Deer avoid the invasive plants and seek out their preferred foods, which include native species like oak seedlings. Over time, this can drastically change the ecological balance of a forest — resulting in more invasives and less native tree regeneration.
We will not eliminate species such as autumn olive across the landscape, but we can pick our battles by managing invasives to create native plant “beachheads”. I recommend starting in some of the least invaded parts of your property perhaps where you have interesting native plants you’d like to promote.. The wildlife response can be dramatic. By encouraging or planting natives, and controlling invasives, insect diversity will increase. From this larger food base, there will be more mammals, birds, amphibians, etc.
For more information about this important topic, Dr. Doug Tallamy’s book, Bringing Nature Home, is required reading. If you’re landscaping, purchase native plants whenever possible and always avoid plants listed as invasive by the Virginia Natural Heritage Program. You can find this list at their website: www.dcr.virginia.gov/natural_heritage
Sustainable Habitat Program Manager here at PEC