Native species garden.
Douglas W. Tallamy, the author of Bringing Nature Home: How You Can Sustain Wildlife with Native Plants, recently wrote a piece for the New York Times, titled “The Chickadee’s Guide to Gardening: In Your Garden, Choose Plants That Help the Environment,” which puts his thesis into a nutshell.
Last summer, as an experiment to demonstrate his point, Tallamy counted the number of caterpillars at head height on a white oak, a native species, and a Bradford pear, an ornamental from Asia, of the same size. He counted 410 caterpillars on the white oak and 1 caterpillar on the Bradford pear. So that’s settled, the Bradford pear is better because it is more pest free.
Maybe not. Privileging the Bradford pear, an ornamental or alien species, because it is pest free, is a product of the way we are accustomed to thinking of suburban landscaping, thinking of it as a cultural product rather than a natural one.
The ornamental nursery business is pretty much responsible for this state of mind. A large part of their business is finding new exotic, alien plants that will be the latest rage in suburban plants. With the guarantee of pest-free plants being high on their list of selling points.
If suburban yards and gardens were a minuscule percentage of our natural surroundings, as they were in, say, the nineteenth century, that way of thinking would be relatively harmless–and it was relatively harmless back then.
But reality today is different. What with ever-increasing development and the advent of invasive alien species in our remaining wild places, we have come to a point where suburban yards and gardens may be the last best hope for the insect world that underpins our natural life.
Say what? When was the last time you gave a thought to a caterpillar? A passing sorrow at the plight of the monarch butterfly, probably. Oh, well, what’s one butterfly species more or less.
But wait before you discount insects entirely. Let’s back up to understand what Tallamy is getting at. What is it that makes native plant species important to native insect species, and to us.
Plants don’t like to be eaten, so over evolutionary time, they develop defenses against being eaten. Why do onions make you cry? Because they don’t like being eaten, by insects or any other form of life, like you. Have you ever noticed that onions don’t make you cry until you cut into them? So over evolutionary time, onions have devised chemical methods of discouraging predators from eating them. Good for them. But just as you learn to put on goggles, or cut onions under running water, or turn on a fan to keep from crying while you are cutting them up to make a delicious stew, so certain insects build up a resistance to the chemicals that plants like onions develop to avoid being eaten.
Monarch butterflies (Danaus plexippus) are an excellent example. According to Monarch Watch, when monarch caterpillars eat milkweed, their larval host plant, they sequester poisonous cardenolides, or cardiac glycosides, found in the milkweed. The sequestered poison protects adult, flying monarchs from vertebrates, such as birds. Birds may eat 1 monarch butterfly, become sick, and learn to never eat another one. Or they may take a bite out of a monarch’s wing and learn the same lesson, leaving the monarch butterfly still capable of carrying on. Even more, the bright colors of the adult flying monarch butterfly warn predators like birds that the monarch is poisonous and should be left alone.
So the poisonous cardenolides protect milkweed from most insects, but some insects, such as monarchs, have become immune to the poison in milkweed and even use the poison for their own defense. Without milkweed as a larval host plant, monarchs would not survive. They don’t have that evolutionary relationship with any other plant. Certainly not with recently introduced or alien species.
In a native species environment, plants and insects achieve a balance. Some plants get eaten by some insects but all manage to survive and even thrive. And more than that. To go up the food chain, or into the food web, higher species are also able to survive and thrive. Birds, for example, like the chickadees that Tallamy uses as an example. When insects thrive because they have plants to eat, chickadees survive and thrive because they have insects to eat. No one wants chickadees to starve, I think, because who doesn’t like chickadees, but also because if chickadees starve to death, who will starve next. How long before we will starve.
When ornamental, or alien, plants gain a certain foothold in their new environment, there’s a good chance they will thrive beyond our wildest expectations. When ornamentals escape the suburban yard, usually through birds, or deer, or other living creatures, eating their fruit and defecating their seeds, they can become invasive in a native environment. Without natural predators, which they had in their native environment, nothing can stop them from using the ecological resources that native species depend on but which alien species usurp. Native insects are not equipped to eat alien plants. That takes evolutionary time. Millions of years. And so the balance of nature is upset. Alien invasive plants that use ecological resources–sun, water, nutrients in soil, growing room–but don’t give back by providing food to other life forms, mostly insects, create an imbalance in the food web, with a resulting decrease in insect life, thus bird life, etc.
It has been an historical attribute of our country that we have sought out and encouraged exotic, alien species. Thomas Jefferson made an agricultural life’s work of bringing seeds and cuttings from other countries to this country. He was particularly interested in finding new agricultural industries. He wanted to find olive trees that would produce in this country and start a new industry in olive oil. He wanted to introduce upland rice to southern states to replace swamp rice that was associated with disease. He once smuggled a pocketful of rice seeds out of Italy against Italian law. He wanted to introduce cotton, which he thought was easier for manufacturing cloth than flax or hemp. His interest in successful agricultural industries for this new country was unflagging. His correspondence in this regard is awe-inspiring.
Jefferson had the best interests of his country at heart. He wrote of collecting seeds and cuttings as a patriotic duty. But his efforts and those of many other patriots at that time had unintended consequences.
Tallamy uses the Norway maple (Acer platanoides) as an example of an alien species that was imported in 1756 and has become invasive, still not interacting with native species after 3 centuries, a blink of an eye in evolutionary time after all. According to Tallamy, the Norway maple was imported by John Bartram, a nurseryman in Philadelphia. Although Tallamy doesn’t mention this, Bartram was well acquainted with Jefferson. They often corresponded regarding plants that Jefferson wished to acquire, and Bartram transported seeds, cuttings, and plants to and from Europe at Jefferson’s request.
I can find no suggestion that Jefferson ordered Norway maples from Bartram or asked Bartram to acquire the maple for him. He was more interested in expanding the growth of sugar maples in order to supplant cane sugar imports. However, these historical examples demonstrate the extent to which the importation of exotics to this country is an historical habit–perceived as a good thing to do.
The Norway maple, now the most common shade tree in America, according to Tallamy, produces seeds with a boomerang-type of dispersal mechanism, sending them whirling in the breeze to a destination beyond the parent tree. As mature trees, Norway maples produce a deep shade that precludes native species. They may become a monoculture in parks and other natural areas. Regardless of the Norway maple’s official status as an invasive species, it is still widely sold by nurseries. Beware.
Michael Pollan, in Second Nature, tells of planting a real tree, as he puts it, a tree with gravitas, a potential big tree on a property he owned in Connecticut. Remember that Pollan published Second Nature in 1991. I admire all of Pollan’s books, but I particularly enjoy Second Nature. I think of it as his author-as-a-young-man book. One good thing about gardening books is that they never cease to be useful. A good gardening book from 100 years ago may offer insights not found in the most recent title.
Back to Pollan. After deciding to expend time and money on a real tree, he does his research and settles on a sugar maple as the tree he wishes to pass on to future generations. A tree that will endure to enrich the lives of those who come after him. He goes to his local nursery and talks to John, the nursery manager. When Pollan expresses his desire to plant a sugar maple, John frowns, shakes his head, and warns Pollan against sugar maples because of the pear thrip, an insect that had infested sugar maples in New England.
Instead, John steers Pollan toward the Norway maple, a pest-free variety from Europe that was thriving in their area. So, Pollan buys and plants a Norway maple, not learning about its invasive nature until much later. Once time and money have been expended, once a plant has taken root and grown, settling into the environment, most of us, including Pollan, may rue our decision to buy an invasive species but still allow it to stand.
I had a big old Norway maple as a street tree but had it taken down because it was dying and might have fallen on the house in a windstorm. This was before hurricanes Irene and Sandy, so I thanked my lucky stars the tree was gone before these storms wreaked havoc on our region.
Otherwise, there are young Norway maples in my neighbors’ backyards that lean over my back fence and shade the vegetable garden, along with some white walnuts that are even more annoying. As I’ve mentioned before, this is a situation that I need to deal with but have not thus far. Maybe this year.
I have only recently started thinking about native species. I don’t plan to tear out any plant precipitously, only to choose wisely in future. As can be seen from Pollan’s experience, this isn’t easy. Impulse buying at the nursery is definitely out.
Tallamy has lists of native species for each region of the country in Bringing Nature Home, so that’s a good place to start. The Native Plant Society of New Jersey has lists of native species by county online. What a treasure that is. So, take pencil and pad to the nursery on the first trip. Write down information about the plants and do some research before buying on the second or third visit. Nurseries are fun places. Visit often. Don’t be in a hurry to buy.
I also want to inventory my gardens and determine what native and what alien species I now have, and to what extent are the aliens invasive. Also spend more time getting acquainted with the insects in my gardens. More about insects later. Much to do. Much to learn.