Musing About Pests (Part II)
Invasive and Introduced Species
By John Reilly
There is a lot of confusion among gardeners between invasive plants and those that are simply aggressive in a garden setting. Invasives (not just plants) are non-native, that is ‘introduced’ species that have been determined by a state environmental agency as causing harm or potential harm to the natural flora and fauna of the state. Once they are designated invasives, it may be illegal to bring them into that jurisdiction. A species that is invasive in the southern part of the USA may be no problem for us here in Maine because it cannot survive the winter. Where gardeners seem to get confused is by thinking that a plant that is aggressive in their garden is ‘invasive’ and that plants that behave well in a garden setting are no problem, even though they have been labeled invasive.
“What seemed to be black and white in defining invasive species appears to include shades of gray.”
I love goldenrod (Solidago sp.), a beautiful native. Of the nearly 100 different species, some are tamer, but the most common are bullies in the garden. Physostegia virginiana’s common name, obedient plant, derives from the fact that you can bend the flowers around the stem, and they will stay that way, but in all other respects it is a rascal, spreading by underground runners and seeds. Dis ‘obedient plant’ that it is, a small root section detached from its mother, spreads even more rapaciously (maybe, or just my not-completely-clear-headed observation). In the right place, it is a beautiful native and attractive to pollinators. To make matters worse for me, Japanese beetles (a true invasive) preferentially enjoyed my plot of obedient plants, consuming all the petals before they opened.
Japanese knotweed is an invasive, introduced because of its blossoms and handsome foliage, and can be hard to eradicate from a garden, though mowing around a small plot will keep it under control. But it will spread widely by seeds and shoots that get carried away in flooding/heavy rain conditions. Purple loosestrife (Lythrum salicaria) and bush honeysuckle (Lonicera tatarica) and similar Lonicera sp. behave well in a garden, but the seeds spread far and wide making them menaces to broader native ecosystems.
But is it that simple? There are some descriptions of plants now endemic in North America as being non-native (and invasive?) that experts believe originated in Asia and managed to cross into North America (with humans) via the land bridge that formed in the last ice age some 10,000 years ago. Distribution that occurred tens of thousands of years ago is not typically considered ‘invasive.’ Trace back far enough, and all species originated from some spark of life somewhere and have since spread. Plants have developed clever mechanisms for spreading their populations broadly—seeds/fruits that birds or other animals eat and spread. Stickers that attach to fur or feathers spread far and wide. Some species have been clever enough to make themselves attractive to homo sapiens, using us as their vehicle to populate the world. Ecologists have, in recent years, wondered whether ‘assisted’ migration might help ‘preserve’ ecosystems and biodiversity in the face of climate change. What seemed to be black and white in defining invasive species appears to include shades of gray.
We have moved a lot of species around, for lots of reasons, intentionally and accidentally. Many ‘introduced’ species are not invasive (or so we think) because they appear unable to survive and self-propagate in the wild under the conditions present in their new environment. But species have a tremendous ability to evolve and change. For example, plant breeders, recognizing purple loosestrife as an invasive, developed what was sold as a sterile version of the plant, but over time at least some of the plants developed seeds, and so its invasive properties were slowed but not stopped.
As I was writing this, I was reading a new edition of a garden magazine, extolling the virtues of a dozen or so little-known hydrangeas (all native to Asia). The magazine is sensitive to the invasive species problem, but I can’t help wondering…if any catch on here, what could happen?
Next edition: Humans as pests.
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