EVERY NOW AND THEN, I hear someone say, "Every plant is native somewhere." Of course, this is true in the literal sense. But as far as I can figure out, what people usually mean by this statement is something along the lines of, "All plants are native, so it's ok for me to plant anything I want." That's not so true, and I'd like to explain why.
First let's talk about how the word native is defined by people like me, who say that we "garden with native plants." Typically those of us who live in North America use this word as shorthand for "plants that were present in a specific place before the arrival of European settlers."
Why do we place the dividing line at the arrival of Europeans? Because Europeans brought a lot of plants with them when they came. So in a very short period of time, the ecosystems that had existed in North America for thousands of years, evolving very slowly, were suddenly invaded by plants that had never been seen on this continent before. Now, that wasn't always bad. I happen to be rather fond of dandelions (one of the "newcomers") myself. But often the new plants have spread to and invaded the original ecosystems, crowding out the plants that have been here for thousands of years. When those plants are gone, the animals that depend on them for food and shelter often become threatened too.
As a matter of fact, I got interested in native plant gardening primarily because I like birds and other wildlife. I started out as a wildlife-friendly gardener, just trying to plant things that animals would like, without paying any attention to whether the plants were native or not. But the more I learned about wildlife and plants, the more I came to realize how much wild birds and other animals depend on the plants that have been in their environment for a very long time. I began to realize that newer introductions just can't replace the plants that animals have evolved with. That's when I started adding native plants to my garden.
Here's one of my favorite examples of why it's so important to make a distinction between plants that are native "somewhere" and plants that are native in our local area: In 1999, researchers from Illinois and Tennessee reported on the results of a study of an Illinois forest preserve that had been overrun by non-native honeysuckle and buckthorn. Honeysuckle had replaced the arrowwood that was native there, and buckthorn had replaced the native hawthorn. Both the honeysuckle and the buckthorn were probably introduced (at least in part) by gardeners. Birds visit gardens, eat the berries from these plants, fly into the forests, and then "deposit" the plants; the plants grow extremely well in forested areas, typically displacing the native plants.
Buckthorn, for example, tends "to form dense, even-aged thickets, crowding and shading out native shrubs and herbs, often completely obliterating them." (Plant Conservation Alliance Alien Plant Working Group).
Japanese honeysuckle "has few natural enemies which allows it to spread widely and out-compete native plant species. Its evergreen to semi-evergreen nature gives it an added advantage over native species in many areas. Shrubs and young trees can be killed by girdling when vines twist tightly around stems and trunks, cutting off the flow of water through the plant. Dense growths of honeysuckle covering vegetation can gradually kill plants by blocking sunlight from reaching their leaves. Vigorous root competition also helps Japanese honeysuckle spread and displace neighboring native vegetation." (PCA Alien Plant Working Group)
But you could say, So what? If the birds like the berries, why does it matter whether the forest is composed of non-native plants or native plants? Aren't they all plants?
Well, that brings us to the research that was conducted in Illinois. What the researchers found was that nest predation of American robins and wood thrushes was higher in the non-native shrubs than in the native shrubs and trees. According to the press release that was issued at the time, they thought this was "partly due to physical differences between the native and non-native shrubs. Buckthorn lacks hawthorn's sharp thorns, which could deter mammalian predators. Honeysuckle has sturdier branches, which could both help predators climb higher and support nests closer to theground, where they are more accessible to predators."
So in other words, even though these plants appeal to birds, who like to eat the berries, in the long term the plants are hurting birds that are already at serious risk: "In recent years, the Wood Thrush, like many other Neotropical migrants, has undergone an alarming population decline." (Cornell Laboratory of Ornithology)
This is just one example, but I think it highlights the risks that are associated with introducing non-native invasive plants: These plants can and all too often do replace native plants when they compete with them in natural areas. When the native plants are replaced by non-native plants, the effects on wildlife are unpredictable but often damaging.
And that's why it's so important to try to protect the plants that are native right here, not "somewhere."
Fortunately, all non-native plants are not equally dangerous. There are plenty of non-natives that are not invasive and are beneficial for wildlife, which is why I do discuss non-natives and I also use them in my garden. Ideally each non-native plant would be given a fair hearing, and we would draw conclusions on a case-by-case basis. Unfortunately, there isn't always time to study every plant before someone introduces it. And we already have far too many examples of invasive non-native plants that have been introduced by gardeners. Also, many of the plants that are native to specific regions are being lost at an alarming rate, along with the native wildlife they support. That's why I think it's so important for gardeners to make an effort to use more natives in their gardens, and why I encourage gardeners to be very cautious about introducing new non-native plants.